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<div class="section" id="what-s-new-in-python-2-5">
<h1>What’s New in Python 2.5<a class="headerlink" href="#what-s-new-in-python-2-5" title="Permalink to this headline">¶</a></h1>
<dl class="field-list simple">
<dt class="field-odd">Author</dt>
<dd class="field-odd"><p>A.M. Kuchling</p>
</dd>
</dl>
<p>This article explains the new features in Python 2.5. The final release of
Python 2.5 is scheduled for August 2006; <span class="target" id="index-0"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0356"><strong>PEP 356</strong></a> describes the planned
release schedule.</p>
<p>The changes in Python 2.5 are an interesting mix of language and library
improvements. The library enhancements will be more important to Python’s user
community, I think, because several widely-useful packages were added. New
modules include ElementTree for XML processing (<code class="xref py py-mod docutils literal notranslate"><span class="pre">xml.etree</span></code>),
the SQLite database module (<code class="xref py py-mod docutils literal notranslate"><span class="pre">sqlite</span></code>), and the <a class="reference internal" href="../library/ctypes.html#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate"><span class="pre">ctypes</span></code></a>
module for calling C functions.</p>
<p>The language changes are of middling significance. Some pleasant new features
were added, but most of them aren’t features that you’ll use every day.
Conditional expressions were finally added to the language using a novel syntax;
see section <a class="reference internal" href="#pep-308"><span class="std std-ref">PEP 308: Conditional Expressions</span></a>. The new ‘<a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>’ statement will make
writing cleanup code easier (section <a class="reference internal" href="#pep-343"><span class="std std-ref">PEP 343: The ‘with’ statement</span></a>). Values can now be passed
into generators (section <a class="reference internal" href="#pep-342"><span class="std std-ref">PEP 342: New Generator Features</span></a>). Imports are now visible as either
absolute or relative (section <a class="reference internal" href="#pep-328"><span class="std std-ref">PEP 328: Absolute and Relative Imports</span></a>). Some corner cases of exception
handling are handled better (section <a class="reference internal" href="#pep-341"><span class="std std-ref">PEP 341: Unified try/except/finally</span></a>). All these improvements
are worthwhile, but they’re improvements to one specific language feature or
another; none of them are broad modifications to Python’s semantics.</p>
<p>As well as the language and library additions, other improvements and bugfixes
were made throughout the source tree. A search through the SVN change logs
finds there were 353 patches applied and 458 bugs fixed between Python 2.4 and
2.5. (Both figures are likely to be underestimates.)</p>
<p>This article doesn’t try to be a complete specification of the new features;
instead changes are briefly introduced using helpful examples. For full
details, you should always refer to the documentation for Python 2.5 at
<a class="reference external" href="https://docs.python.org">https://docs.python.org</a>. If you want to understand the complete implementation
and design rationale, refer to the PEP for a particular new feature.</p>
<p>Comments, suggestions, and error reports for this document are welcome; please
e-mail them to the author or open a bug in the Python bug tracker.</p>
<div class="section" id="pep-308-conditional-expressions">
<span id="pep-308"></span><h2>PEP 308: Conditional Expressions<a class="headerlink" href="#pep-308-conditional-expressions" title="Permalink to this headline">¶</a></h2>
<p>For a long time, people have been requesting a way to write conditional
expressions, which are expressions that return value A or value B depending on
whether a Boolean value is true or false. A conditional expression lets you
write a single assignment statement that has the same effect as the following:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">condition</span><span class="p">:</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">true_value</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">false_value</span>
</pre></div>
</div>
<p>There have been endless tedious discussions of syntax on both python-dev and
comp.lang.python. A vote was even held that found the majority of voters wanted
conditional expressions in some form, but there was no syntax that was preferred
by a clear majority. Candidates included C’s <code class="docutils literal notranslate"><span class="pre">cond</span> <span class="pre">?</span> <span class="pre">true_v</span> <span class="pre">:</span> <span class="pre">false_v</span></code>, <code class="docutils literal notranslate"><span class="pre">if</span>
<span class="pre">cond</span> <span class="pre">then</span> <span class="pre">true_v</span> <span class="pre">else</span> <span class="pre">false_v</span></code>, and 16 other variations.</p>
<p>Guido van Rossum eventually chose a surprising syntax:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">x</span> <span class="o">=</span> <span class="n">true_value</span> <span class="k">if</span> <span class="n">condition</span> <span class="k">else</span> <span class="n">false_value</span>
</pre></div>
</div>
<p>Evaluation is still lazy as in existing Boolean expressions, so the order of
evaluation jumps around a bit. The <em>condition</em> expression in the middle is
evaluated first, and the <em>true_value</em> expression is evaluated only if the
condition was true. Similarly, the <em>false_value</em> expression is only evaluated
when the condition is false.</p>
<p>This syntax may seem strange and backwards; why does the condition go in the
<em>middle</em> of the expression, and not in the front as in C’s <code class="docutils literal notranslate"><span class="pre">c</span> <span class="pre">?</span> <span class="pre">x</span> <span class="pre">:</span> <span class="pre">y</span></code>? The
decision was checked by applying the new syntax to the modules in the standard
library and seeing how the resulting code read. In many cases where a
conditional expression is used, one value seems to be the ‘common case’ and one
value is an ‘exceptional case’, used only on rarer occasions when the condition
isn’t met. The conditional syntax makes this pattern a bit more obvious:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">contents</span> <span class="o">=</span> <span class="p">((</span><span class="n">doc</span> <span class="o">+</span> <span class="s1">'</span><span class="se">\n</span><span class="s1">'</span><span class="p">)</span> <span class="k">if</span> <span class="n">doc</span> <span class="k">else</span> <span class="s1">''</span><span class="p">)</span>
</pre></div>
</div>
<p>I read the above statement as meaning “here <em>contents</em> is usually assigned a
value of <code class="docutils literal notranslate"><span class="pre">doc+'\n'</span></code>; sometimes <em>doc</em> is empty, in which special case an empty
string is returned.” I doubt I will use conditional expressions very often
where there isn’t a clear common and uncommon case.</p>
<p>There was some discussion of whether the language should require surrounding
conditional expressions with parentheses. The decision was made to <em>not</em>
require parentheses in the Python language’s grammar, but as a matter of style I
think you should always use them. Consider these two statements:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># First version -- no parens</span>
<span class="n">level</span> <span class="o">=</span> <span class="mi">1</span> <span class="k">if</span> <span class="n">logging</span> <span class="k">else</span> <span class="mi">0</span>
<span class="c1"># Second version -- with parens</span>
<span class="n">level</span> <span class="o">=</span> <span class="p">(</span><span class="mi">1</span> <span class="k">if</span> <span class="n">logging</span> <span class="k">else</span> <span class="mi">0</span><span class="p">)</span>
</pre></div>
</div>
<p>In the first version, I think a reader’s eye might group the statement into
‘level = 1’, ‘if logging’, ‘else 0’, and think that the condition decides
whether the assignment to <em>level</em> is performed. The second version reads
better, in my opinion, because it makes it clear that the assignment is always
performed and the choice is being made between two values.</p>
<p>Another reason for including the brackets: a few odd combinations of list
comprehensions and lambdas could look like incorrect conditional expressions.
See <span class="target" id="index-1"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0308"><strong>PEP 308</strong></a> for some examples. If you put parentheses around your
conditional expressions, you won’t run into this case.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><span class="target" id="index-2"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0308"><strong>PEP 308</strong></a> - Conditional Expressions</dt><dd><p>PEP written by Guido van Rossum and Raymond D. Hettinger; implemented by Thomas
Wouters.</p>
</dd>
</dl>
</div>
</div>
<div class="section" id="pep-309-partial-function-application">
<span id="pep-309"></span><h2>PEP 309: Partial Function Application<a class="headerlink" href="#pep-309-partial-function-application" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference internal" href="../library/functools.html#module-functools" title="functools: Higher-order functions and operations on callable objects."><code class="xref py py-mod docutils literal notranslate"><span class="pre">functools</span></code></a> module is intended to contain tools for functional-style
programming.</p>
<p>One useful tool in this module is the <code class="xref py py-func docutils literal notranslate"><span class="pre">partial()</span></code> function. For programs
written in a functional style, you’ll sometimes want to construct variants of
existing functions that have some of the parameters filled in. Consider a
Python function <code class="docutils literal notranslate"><span class="pre">f(a,</span> <span class="pre">b,</span> <span class="pre">c)</span></code>; you could create a new function <code class="docutils literal notranslate"><span class="pre">g(b,</span> <span class="pre">c)</span></code> that
was equivalent to <code class="docutils literal notranslate"><span class="pre">f(1,</span> <span class="pre">b,</span> <span class="pre">c)</span></code>. This is called “partial function
application”.</p>
<p><code class="xref py py-func docutils literal notranslate"><span class="pre">partial()</span></code> takes the arguments <code class="docutils literal notranslate"><span class="pre">(function,</span> <span class="pre">arg1,</span> <span class="pre">arg2,</span> <span class="pre">...</span> <span class="pre">kwarg1=value1,</span>
<span class="pre">kwarg2=value2)</span></code>. The resulting object is callable, so you can just call it to
invoke <em>function</em> with the filled-in arguments.</p>
<p>Here’s a small but realistic example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">functools</span>
<span class="k">def</span> <span class="nf">log</span> <span class="p">(</span><span class="n">message</span><span class="p">,</span> <span class="n">subsystem</span><span class="p">):</span>
<span class="s2">"Write the contents of 'message' to the specified subsystem."</span>
<span class="nb">print</span> <span class="s1">'</span><span class="si">%s</span><span class="s1">: </span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="p">(</span><span class="n">subsystem</span><span class="p">,</span> <span class="n">message</span><span class="p">)</span>
<span class="o">...</span>
<span class="n">server_log</span> <span class="o">=</span> <span class="n">functools</span><span class="o">.</span><span class="n">partial</span><span class="p">(</span><span class="n">log</span><span class="p">,</span> <span class="n">subsystem</span><span class="o">=</span><span class="s1">'server'</span><span class="p">)</span>
<span class="n">server_log</span><span class="p">(</span><span class="s1">'Unable to open socket'</span><span class="p">)</span>
</pre></div>
</div>
<p>Here’s another example, from a program that uses PyGTK. Here a context-sensitive
pop-up menu is being constructed dynamically. The callback provided
for the menu option is a partially applied version of the <code class="xref py py-meth docutils literal notranslate"><span class="pre">open_item()</span></code>
method, where the first argument has been provided.</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="o">...</span>
<span class="k">class</span> <span class="nc">Application</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">open_item</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">path</span><span class="p">):</span>
<span class="o">...</span>
<span class="k">def</span> <span class="nf">init</span> <span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">open_func</span> <span class="o">=</span> <span class="n">functools</span><span class="o">.</span><span class="n">partial</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">open_item</span><span class="p">,</span> <span class="n">item_path</span><span class="p">)</span>
<span class="n">popup_menu</span><span class="o">.</span><span class="n">append</span><span class="p">(</span> <span class="p">(</span><span class="s2">"Open"</span><span class="p">,</span> <span class="n">open_func</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span> <span class="p">)</span>
</pre></div>
</div>
<p>Another function in the <a class="reference internal" href="../library/functools.html#module-functools" title="functools: Higher-order functions and operations on callable objects."><code class="xref py py-mod docutils literal notranslate"><span class="pre">functools</span></code></a> module is the
<code class="docutils literal notranslate"><span class="pre">update_wrapper(wrapper,</span> <span class="pre">wrapped)</span></code> function that helps you write
well-behaved decorators. <code class="xref py py-func docutils literal notranslate"><span class="pre">update_wrapper()</span></code> copies the name, module, and
docstring attribute to a wrapper function so that tracebacks inside the wrapped
function are easier to understand. For example, you might write:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">my_decorator</span><span class="p">(</span><span class="n">f</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">wrapper</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">):</span>
<span class="nb">print</span> <span class="s1">'Calling decorated function'</span>
<span class="k">return</span> <span class="n">f</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">)</span>
<span class="n">functools</span><span class="o">.</span><span class="n">update_wrapper</span><span class="p">(</span><span class="n">wrapper</span><span class="p">,</span> <span class="n">f</span><span class="p">)</span>
<span class="k">return</span> <span class="n">wrapper</span>
</pre></div>
</div>
<p><code class="xref py py-func docutils literal notranslate"><span class="pre">wraps()</span></code> is a decorator that can be used inside your own decorators to copy
the wrapped function’s information. An alternate version of the previous
example would be:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">my_decorator</span><span class="p">(</span><span class="n">f</span><span class="p">):</span>
<span class="nd">@functools</span><span class="o">.</span><span class="n">wraps</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">wrapper</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">):</span>
<span class="nb">print</span> <span class="s1">'Calling decorated function'</span>
<span class="k">return</span> <span class="n">f</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwds</span><span class="p">)</span>
<span class="k">return</span> <span class="n">wrapper</span>
</pre></div>
</div>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><span class="target" id="index-3"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0309"><strong>PEP 309</strong></a> - Partial Function Application</dt><dd><p>PEP proposed and written by Peter Harris; implemented by Hye-Shik Chang and Nick
Coghlan, with adaptations by Raymond Hettinger.</p>
</dd>
</dl>
</div>
</div>
<div class="section" id="pep-314-metadata-for-python-software-packages-v1-1">
<span id="pep-314"></span><h2>PEP 314: Metadata for Python Software Packages v1.1<a class="headerlink" href="#pep-314-metadata-for-python-software-packages-v1-1" title="Permalink to this headline">¶</a></h2>
<p>Some simple dependency support was added to Distutils. The <code class="xref py py-func docutils literal notranslate"><span class="pre">setup()</span></code>
function now has <code class="docutils literal notranslate"><span class="pre">requires</span></code>, <code class="docutils literal notranslate"><span class="pre">provides</span></code>, and <code class="docutils literal notranslate"><span class="pre">obsoletes</span></code> keyword
parameters. When you build a source distribution using the <code class="docutils literal notranslate"><span class="pre">sdist</span></code> command,
the dependency information will be recorded in the <code class="file docutils literal notranslate"><span class="pre">PKG-INFO</span></code> file.</p>
<p>Another new keyword parameter is <code class="docutils literal notranslate"><span class="pre">download_url</span></code>, which should be set to a URL
for the package’s source code. This means it’s now possible to look up an entry
in the package index, determine the dependencies for a package, and download the
required packages.</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">VERSION</span> <span class="o">=</span> <span class="s1">'1.0'</span>
<span class="n">setup</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s1">'PyPackage'</span><span class="p">,</span>
<span class="n">version</span><span class="o">=</span><span class="n">VERSION</span><span class="p">,</span>
<span class="n">requires</span><span class="o">=</span><span class="p">[</span><span class="s1">'numarray'</span><span class="p">,</span> <span class="s1">'zlib (>=1.1.4)'</span><span class="p">],</span>
<span class="n">obsoletes</span><span class="o">=</span><span class="p">[</span><span class="s1">'OldPackage'</span><span class="p">]</span>
<span class="n">download_url</span><span class="o">=</span><span class="p">(</span><span class="s1">'http://www.example.com/pypackage/dist/pkg-</span><span class="si">%s</span><span class="s1">.tar.gz'</span>
<span class="o">%</span> <span class="n">VERSION</span><span class="p">),</span>
<span class="p">)</span>
</pre></div>
</div>
<p>Another new enhancement to the Python package index at
<a class="reference external" href="https://pypi.org">https://pypi.org</a> is storing source and binary archives for a
package. The new <strong class="command">upload</strong> Distutils command will upload a package to
the repository.</p>
<p>Before a package can be uploaded, you must be able to build a distribution using
the <strong class="command">sdist</strong> Distutils command. Once that works, you can run <code class="docutils literal notranslate"><span class="pre">python</span>
<span class="pre">setup.py</span> <span class="pre">upload</span></code> to add your package to the PyPI archive. Optionally you can
GPG-sign the package by supplying the <code class="xref std std-option docutils literal notranslate"><span class="pre">--sign</span></code> and <code class="xref std std-option docutils literal notranslate"><span class="pre">--identity</span></code>
options.</p>
<p>Package uploading was implemented by Martin von Löwis and Richard Jones.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><span class="target" id="index-4"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0314"><strong>PEP 314</strong></a> - Metadata for Python Software Packages v1.1</dt><dd><p>PEP proposed and written by A.M. Kuchling, Richard Jones, and Fred Drake;
implemented by Richard Jones and Fred Drake.</p>
</dd>
</dl>
</div>
</div>
<div class="section" id="pep-328-absolute-and-relative-imports">
<span id="pep-328"></span><h2>PEP 328: Absolute and Relative Imports<a class="headerlink" href="#pep-328-absolute-and-relative-imports" title="Permalink to this headline">¶</a></h2>
<p>The simpler part of PEP 328 was implemented in Python 2.4: parentheses could now
be used to enclose the names imported from a module using the <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">...</span> <span class="pre">import</span>
<span class="pre">...</span></code> statement, making it easier to import many different names.</p>
<p>The more complicated part has been implemented in Python 2.5: importing a module
can be specified to use absolute or package-relative imports. The plan is to
move toward making absolute imports the default in future versions of Python.</p>
<p>Let’s say you have a package directory like this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">pkg</span><span class="o">/</span>
<span class="n">pkg</span><span class="o">/</span><span class="fm">__init__</span><span class="o">.</span><span class="n">py</span>
<span class="n">pkg</span><span class="o">/</span><span class="n">main</span><span class="o">.</span><span class="n">py</span>
<span class="n">pkg</span><span class="o">/</span><span class="n">string</span><span class="o">.</span><span class="n">py</span>
</pre></div>
</div>
<p>This defines a package named <code class="xref py py-mod docutils literal notranslate"><span class="pre">pkg</span></code> containing the <code class="xref py py-mod docutils literal notranslate"><span class="pre">pkg.main</span></code> and
<code class="xref py py-mod docutils literal notranslate"><span class="pre">pkg.string</span></code> submodules.</p>
<p>Consider the code in the <code class="file docutils literal notranslate"><span class="pre">main.py</span></code> module. What happens if it executes
the statement <code class="docutils literal notranslate"><span class="pre">import</span> <span class="pre">string</span></code>? In Python 2.4 and earlier, it will first look
in the package’s directory to perform a relative import, finds
<code class="file docutils literal notranslate"><span class="pre">pkg/string.py</span></code>, imports the contents of that file as the
<code class="xref py py-mod docutils literal notranslate"><span class="pre">pkg.string</span></code> module, and that module is bound to the name <code class="docutils literal notranslate"><span class="pre">string</span></code> in the
<code class="xref py py-mod docutils literal notranslate"><span class="pre">pkg.main</span></code> module’s namespace.</p>
<p>That’s fine if <code class="xref py py-mod docutils literal notranslate"><span class="pre">pkg.string</span></code> was what you wanted. But what if you wanted
Python’s standard <a class="reference internal" href="../library/string.html#module-string" title="string: Common string operations."><code class="xref py py-mod docutils literal notranslate"><span class="pre">string</span></code></a> module? There’s no clean way to ignore
<code class="xref py py-mod docutils literal notranslate"><span class="pre">pkg.string</span></code> and look for the standard module; generally you had to look at
the contents of <code class="docutils literal notranslate"><span class="pre">sys.modules</span></code>, which is slightly unclean. Holger Krekel’s
<code class="xref py py-mod docutils literal notranslate"><span class="pre">py.std</span></code> package provides a tidier way to perform imports from the standard
library, <code class="docutils literal notranslate"><span class="pre">import</span> <span class="pre">py;</span> <span class="pre">py.std.string.join()</span></code>, but that package isn’t available
on all Python installations.</p>
<p>Reading code which relies on relative imports is also less clear, because a
reader may be confused about which module, <a class="reference internal" href="../library/string.html#module-string" title="string: Common string operations."><code class="xref py py-mod docutils literal notranslate"><span class="pre">string</span></code></a> or <code class="xref py py-mod docutils literal notranslate"><span class="pre">pkg.string</span></code>,
is intended to be used. Python users soon learned not to duplicate the names of
standard library modules in the names of their packages’ submodules, but you
can’t protect against having your submodule’s name being used for a new module
added in a future version of Python.</p>
<p>In Python 2.5, you can switch <a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a>’s behaviour to absolute imports
using a <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">__future__</span> <span class="pre">import</span> <span class="pre">absolute_import</span></code> directive. This absolute-import
behaviour will become the default in a future version (probably Python
2.7). Once absolute imports are the default, <code class="docutils literal notranslate"><span class="pre">import</span> <span class="pre">string</span></code> will always
find the standard library’s version. It’s suggested that users should begin
using absolute imports as much as possible, so it’s preferable to begin writing
<code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">pkg</span> <span class="pre">import</span> <span class="pre">string</span></code> in your code.</p>
<p>Relative imports are still possible by adding a leading period to the module
name when using the <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">...</span> <span class="pre">import</span></code> form:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># Import names from pkg.string</span>
<span class="kn">from</span> <span class="nn">.string</span> <span class="kn">import</span> <span class="n">name1</span><span class="p">,</span> <span class="n">name2</span>
<span class="c1"># Import pkg.string</span>
<span class="kn">from</span> <span class="nn">.</span> <span class="kn">import</span> <span class="n">string</span>
</pre></div>
</div>
<p>This imports the <a class="reference internal" href="../library/string.html#module-string" title="string: Common string operations."><code class="xref py py-mod docutils literal notranslate"><span class="pre">string</span></code></a> module relative to the current package, so in
<code class="xref py py-mod docutils literal notranslate"><span class="pre">pkg.main</span></code> this will import <em>name1</em> and <em>name2</em> from <code class="xref py py-mod docutils literal notranslate"><span class="pre">pkg.string</span></code>.
Additional leading periods perform the relative import starting from the parent
of the current package. For example, code in the <code class="xref py py-mod docutils literal notranslate"><span class="pre">A.B.C</span></code> module can do:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">.</span> <span class="kn">import</span> <span class="n">D</span> <span class="c1"># Imports A.B.D</span>
<span class="kn">from</span> <span class="nn">..</span> <span class="kn">import</span> <span class="n">E</span> <span class="c1"># Imports A.E</span>
<span class="kn">from</span> <span class="nn">..F</span> <span class="kn">import</span> <span class="n">G</span> <span class="c1"># Imports A.F.G</span>
</pre></div>
</div>
<p>Leading periods cannot be used with the <code class="docutils literal notranslate"><span class="pre">import</span> <span class="pre">modname</span></code> form of the import
statement, only the <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">...</span> <span class="pre">import</span></code> form.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><span class="target" id="index-5"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0328"><strong>PEP 328</strong></a> - Imports: Multi-Line and Absolute/Relative</dt><dd><p>PEP written by Aahz; implemented by Thomas Wouters.</p>
</dd>
<dt><a class="reference external" href="https://pylib.readthedocs.io/">https://pylib.readthedocs.io/</a></dt><dd><p>The py library by Holger Krekel, which contains the <code class="xref py py-mod docutils literal notranslate"><span class="pre">py.std</span></code> package.</p>
</dd>
</dl>
</div>
</div>
<div class="section" id="pep-338-executing-modules-as-scripts">
<span id="pep-338"></span><h2>PEP 338: Executing Modules as Scripts<a class="headerlink" href="#pep-338-executing-modules-as-scripts" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference internal" href="../using/cmdline.html#cmdoption-m"><code class="xref std std-option docutils literal notranslate"><span class="pre">-m</span></code></a> switch added in Python 2.4 to execute a module as a script
gained a few more abilities. Instead of being implemented in C code inside the
Python interpreter, the switch now uses an implementation in a new module,
<a class="reference internal" href="../library/runpy.html#module-runpy" title="runpy: Locate and run Python modules without importing them first."><code class="xref py py-mod docutils literal notranslate"><span class="pre">runpy</span></code></a>.</p>
<p>The <a class="reference internal" href="../library/runpy.html#module-runpy" title="runpy: Locate and run Python modules without importing them first."><code class="xref py py-mod docutils literal notranslate"><span class="pre">runpy</span></code></a> module implements a more sophisticated import mechanism so that
it’s now possible to run modules in a package such as <code class="xref py py-mod docutils literal notranslate"><span class="pre">pychecker.checker</span></code>.
The module also supports alternative import mechanisms such as the
<a class="reference internal" href="../library/zipimport.html#module-zipimport" title="zipimport: Support for importing Python modules from ZIP archives."><code class="xref py py-mod docutils literal notranslate"><span class="pre">zipimport</span></code></a> module. This means you can add a .zip archive’s path to
<code class="docutils literal notranslate"><span class="pre">sys.path</span></code> and then use the <a class="reference internal" href="../using/cmdline.html#cmdoption-m"><code class="xref std std-option docutils literal notranslate"><span class="pre">-m</span></code></a> switch to execute code from the
archive.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><span class="target" id="index-6"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0338"><strong>PEP 338</strong></a> - Executing modules as scripts</dt><dd><p>PEP written and implemented by Nick Coghlan.</p>
</dd>
</dl>
</div>
</div>
<div class="section" id="pep-341-unified-try-except-finally">
<span id="pep-341"></span><h2>PEP 341: Unified try/except/finally<a class="headerlink" href="#pep-341-unified-try-except-finally" title="Permalink to this headline">¶</a></h2>
<p>Until Python 2.5, the <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a> statement came in two flavours. You could
use a <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> block to ensure that code is always executed, or one or
more <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> blocks to catch specific exceptions. You couldn’t
combine both <code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code> blocks and a <code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code> block, because
generating the right bytecode for the combined version was complicated and it
wasn’t clear what the semantics of the combined statement should be.</p>
<p>Guido van Rossum spent some time working with Java, which does support the
equivalent of combining <a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> blocks and a <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> block,
and this clarified what the statement should mean. In Python 2.5, you can now
write:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span>
<span class="n">block</span><span class="o">-</span><span class="mi">1</span> <span class="o">...</span>
<span class="k">except</span> <span class="n">Exception1</span><span class="p">:</span>
<span class="n">handler</span><span class="o">-</span><span class="mi">1</span> <span class="o">...</span>
<span class="k">except</span> <span class="n">Exception2</span><span class="p">:</span>
<span class="n">handler</span><span class="o">-</span><span class="mi">2</span> <span class="o">...</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">else</span><span class="o">-</span><span class="n">block</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">final</span><span class="o">-</span><span class="n">block</span>
</pre></div>
</div>
<p>The code in <em>block-1</em> is executed. If the code raises an exception, the various
<a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> blocks are tested: if the exception is of class
<code class="xref py py-class docutils literal notranslate"><span class="pre">Exception1</span></code>, <em>handler-1</em> is executed; otherwise if it’s of class
<code class="xref py py-class docutils literal notranslate"><span class="pre">Exception2</span></code>, <em>handler-2</em> is executed, and so forth. If no exception is
raised, the <em>else-block</em> is executed.</p>
<p>No matter what happened previously, the <em>final-block</em> is executed once the code
block is complete and any raised exceptions handled. Even if there’s an error in
an exception handler or the <em>else-block</em> and a new exception is raised, the code
in the <em>final-block</em> is still run.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><span class="target" id="index-7"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0341"><strong>PEP 341</strong></a> - Unifying try-except and try-finally</dt><dd><p>PEP written by Georg Brandl; implementation by Thomas Lee.</p>
</dd>
</dl>
</div>
</div>
<div class="section" id="pep-342-new-generator-features">
<span id="pep-342"></span><h2>PEP 342: New Generator Features<a class="headerlink" href="#pep-342-new-generator-features" title="Permalink to this headline">¶</a></h2>
<p>Python 2.5 adds a simple way to pass values <em>into</em> a generator. As introduced in
Python 2.3, generators only produce output; once a generator’s code was invoked
to create an iterator, there was no way to pass any new information into the
function when its execution is resumed. Sometimes the ability to pass in some
information would be useful. Hackish solutions to this include making the
generator’s code look at a global variable and then changing the global
variable’s value, or passing in some mutable object that callers then modify.</p>
<p>To refresh your memory of basic generators, here’s a simple example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">counter</span> <span class="p">(</span><span class="n">maximum</span><span class="p">):</span>
<span class="n">i</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">while</span> <span class="n">i</span> <span class="o"><</span> <span class="n">maximum</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">i</span>
<span class="n">i</span> <span class="o">+=</span> <span class="mi">1</span>
</pre></div>
</div>
<p>When you call <code class="docutils literal notranslate"><span class="pre">counter(10)</span></code>, the result is an iterator that returns the values
from 0 up to 9. On encountering the <a class="reference internal" href="../reference/simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statement, the iterator
returns the provided value and suspends the function’s execution, preserving the
local variables. Execution resumes on the following call to the iterator’s
<a class="reference internal" href="../library/functions.html#next" title="next"><code class="xref py py-meth docutils literal notranslate"><span class="pre">next()</span></code></a> method, picking up after the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code> statement.</p>
<p>In Python 2.3, <a class="reference internal" href="../reference/simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> was a statement; it didn’t return any value. In
2.5, <code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code> is now an expression, returning a value that can be
assigned to a variable or otherwise operated on:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">val</span> <span class="o">=</span> <span class="p">(</span><span class="k">yield</span> <span class="n">i</span><span class="p">)</span>
</pre></div>
</div>
<p>I recommend that you always put parentheses around a <a class="reference internal" href="../reference/simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> expression
when you’re doing something with the returned value, as in the above example.
The parentheses aren’t always necessary, but it’s easier to always add them
instead of having to remember when they’re needed.</p>
<p>(<span class="target" id="index-8"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0342"><strong>PEP 342</strong></a> explains the exact rules, which are that a
<a class="reference internal" href="../reference/simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a>-expression must always be parenthesized except when it
occurs at the top-level
expression on the right-hand side of an assignment. This means you can write
<code class="docutils literal notranslate"><span class="pre">val</span> <span class="pre">=</span> <span class="pre">yield</span> <span class="pre">i</span></code> but have to use parentheses when there’s an operation, as in
<code class="docutils literal notranslate"><span class="pre">val</span> <span class="pre">=</span> <span class="pre">(yield</span> <span class="pre">i)</span> <span class="pre">+</span> <span class="pre">12</span></code>.)</p>
<p>Values are sent into a generator by calling its <code class="docutils literal notranslate"><span class="pre">send(value)</span></code> method. The
generator’s code is then resumed and the <a class="reference internal" href="../reference/simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> expression returns the
specified <em>value</em>. If the regular <a class="reference internal" href="../library/functions.html#next" title="next"><code class="xref py py-meth docutils literal notranslate"><span class="pre">next()</span></code></a> method is called, the
<code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code> returns <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a>.</p>
<p>Here’s the previous example, modified to allow changing the value of the
internal counter.</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">counter</span> <span class="p">(</span><span class="n">maximum</span><span class="p">):</span>
<span class="n">i</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">while</span> <span class="n">i</span> <span class="o"><</span> <span class="n">maximum</span><span class="p">:</span>
<span class="n">val</span> <span class="o">=</span> <span class="p">(</span><span class="k">yield</span> <span class="n">i</span><span class="p">)</span>
<span class="c1"># If value provided, change counter</span>
<span class="k">if</span> <span class="n">val</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
<span class="n">i</span> <span class="o">=</span> <span class="n">val</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">i</span> <span class="o">+=</span> <span class="mi">1</span>
</pre></div>
</div>
<p>And here’s an example of changing the counter:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">it</span> <span class="o">=</span> <span class="n">counter</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span>
<span class="gp">>>> </span><span class="nb">print</span> <span class="n">it</span><span class="o">.</span><span class="n">next</span><span class="p">()</span>
<span class="go">0</span>
<span class="gp">>>> </span><span class="nb">print</span> <span class="n">it</span><span class="o">.</span><span class="n">next</span><span class="p">()</span>
<span class="go">1</span>
<span class="gp">>>> </span><span class="nb">print</span> <span class="n">it</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="mi">8</span><span class="p">)</span>
<span class="go">8</span>
<span class="gp">>>> </span><span class="nb">print</span> <span class="n">it</span><span class="o">.</span><span class="n">next</span><span class="p">()</span>
<span class="go">9</span>
<span class="gp">>>> </span><span class="nb">print</span> <span class="n">it</span><span class="o">.</span><span class="n">next</span><span class="p">()</span>
<span class="gt">Traceback (most recent call last):</span>
File <span class="nb">"t.py"</span>, line <span class="m">15</span>, in <span class="n">?</span>
<span class="nb">print</span> <span class="n">it</span><span class="o">.</span><span class="n">next</span><span class="p">()</span>
<span class="gr">StopIteration</span>
</pre></div>
</div>
<p><a class="reference internal" href="../reference/simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> will usually return <a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a>, so you should always check
for this case. Don’t just use its value in expressions unless you’re sure that
the <code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code> method will be the only method used to resume your generator
function.</p>
<p>In addition to <code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code>, there are two other new methods on generators:</p>
<ul>
<li><p><code class="docutils literal notranslate"><span class="pre">throw(type,</span> <span class="pre">value=None,</span> <span class="pre">traceback=None)</span></code> is used to raise an exception
inside the generator; the exception is raised by the <a class="reference internal" href="../reference/simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> expression
where the generator’s execution is paused.</p></li>
<li><p><code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code> raises a new <a class="reference internal" href="../library/exceptions.html#GeneratorExit" title="GeneratorExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">GeneratorExit</span></code></a> exception inside the generator
to terminate the iteration. On receiving this exception, the generator’s code
must either raise <a class="reference internal" href="../library/exceptions.html#GeneratorExit" title="GeneratorExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">GeneratorExit</span></code></a> or <a class="reference internal" href="../library/exceptions.html#StopIteration" title="StopIteration"><code class="xref py py-exc docutils literal notranslate"><span class="pre">StopIteration</span></code></a>. Catching the
<a class="reference internal" href="../library/exceptions.html#GeneratorExit" title="GeneratorExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">GeneratorExit</span></code></a> exception and returning a value is illegal and will trigger
a <a class="reference internal" href="../library/exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RuntimeError</span></code></a>; if the function raises some other exception, that
exception is propagated to the caller. <code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code> will also be called by
Python’s garbage collector when the generator is garbage-collected.</p>
<p>If you need to run cleanup code when a <a class="reference internal" href="../library/exceptions.html#GeneratorExit" title="GeneratorExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">GeneratorExit</span></code></a> occurs, I suggest
using a <code class="docutils literal notranslate"><span class="pre">try:</span> <span class="pre">...</span> <span class="pre">finally:</span></code> suite instead of catching <a class="reference internal" href="../library/exceptions.html#GeneratorExit" title="GeneratorExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">GeneratorExit</span></code></a>.</p>
</li>
</ul>
<p>The cumulative effect of these changes is to turn generators from one-way
producers of information into both producers and consumers.</p>
<p>Generators also become <em>coroutines</em>, a more generalized form of subroutines.
Subroutines are entered at one point and exited at another point (the top of the
function, and a <a class="reference internal" href="../reference/simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> statement), but coroutines can be entered,
exited, and resumed at many different points (the <a class="reference internal" href="../reference/simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> statements).
We’ll have to figure out patterns for using coroutines effectively in Python.</p>
<p>The addition of the <code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code> method has one side effect that isn’t obvious.
<code class="xref py py-meth docutils literal notranslate"><span class="pre">close()</span></code> is called when a generator is garbage-collected, so this means the
generator’s code gets one last chance to run before the generator is destroyed.
This last chance means that <code class="docutils literal notranslate"><span class="pre">try...finally</span></code> statements in generators can now
be guaranteed to work; the <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">finally</span></code></a> clause will now always get a
chance to run. The syntactic restriction that you couldn’t mix <a class="reference internal" href="../reference/simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a>
statements with a <code class="docutils literal notranslate"><span class="pre">try...finally</span></code> suite has therefore been removed. This
seems like a minor bit of language trivia, but using generators and
<code class="docutils literal notranslate"><span class="pre">try...finally</span></code> is actually necessary in order to implement the
<a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement described by PEP 343. I’ll look at this new statement
in the following section.</p>
<p>Another even more esoteric effect of this change: previously, the
<code class="xref py py-attr docutils literal notranslate"><span class="pre">gi_frame</span></code> attribute of a generator was always a frame object. It’s now
possible for <code class="xref py py-attr docutils literal notranslate"><span class="pre">gi_frame</span></code> to be <code class="docutils literal notranslate"><span class="pre">None</span></code> once the generator has been
exhausted.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl>
<dt><span class="target" id="index-9"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0342"><strong>PEP 342</strong></a> - Coroutines via Enhanced Generators</dt><dd><p>PEP written by Guido van Rossum and Phillip J. Eby; implemented by Phillip J.
Eby. Includes examples of some fancier uses of generators as coroutines.</p>
<p>Earlier versions of these features were proposed in <span class="target" id="index-10"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0288"><strong>PEP 288</strong></a> by Raymond
Hettinger and <span class="target" id="index-11"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0325"><strong>PEP 325</strong></a> by Samuele Pedroni.</p>
</dd>
<dt><a class="reference external" href="https://en.wikipedia.org/wiki/Coroutine">https://en.wikipedia.org/wiki/Coroutine</a></dt><dd><p>The Wikipedia entry for coroutines.</p>
</dd>
<dt><a class="reference external" href="http://www.sidhe.org/~dan/blog/archives/000178.html">http://www.sidhe.org/~dan/blog/archives/000178.html</a></dt><dd><p>An explanation of coroutines from a Perl point of view, written by Dan Sugalski.</p>
</dd>
</dl>
</div>
</div>
<div class="section" id="pep-343-the-with-statement">
<span id="pep-343"></span><h2>PEP 343: The ‘with’ statement<a class="headerlink" href="#pep-343-the-with-statement" title="Permalink to this headline">¶</a></h2>
<p>The ‘<a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>’ statement clarifies code that previously would use
<code class="docutils literal notranslate"><span class="pre">try...finally</span></code> blocks to ensure that clean-up code is executed. In this
section, I’ll discuss the statement as it will commonly be used. In the next
section, I’ll examine the implementation details and show how to write objects
for use with this statement.</p>
<p>The ‘<a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>’ statement is a new control-flow structure whose basic
structure is:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">expression</span> <span class="p">[</span><span class="k">as</span> <span class="n">variable</span><span class="p">]:</span>
<span class="k">with</span><span class="o">-</span><span class="n">block</span>
</pre></div>
</div>
<p>The expression is evaluated, and it should result in an object that supports the
context management protocol (that is, has <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> and <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a>
methods.</p>
<p>The object’s <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> is called before <em>with-block</em> is executed and
therefore can run set-up code. It also may return a value that is bound to the
name <em>variable</em>, if given. (Note carefully that <em>variable</em> is <em>not</em> assigned
the result of <em>expression</em>.)</p>
<p>After execution of the <em>with-block</em> is finished, the object’s <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a>
method is called, even if the block raised an exception, and can therefore run
clean-up code.</p>
<p>To enable the statement in Python 2.5, you need to add the following directive
to your module:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">__future__</span> <span class="kn">import</span> <span class="n">with_statement</span>
</pre></div>
</div>
<p>The statement will always be enabled in Python 2.6.</p>
<p>Some standard Python objects now support the context management protocol and can
be used with the ‘<a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>’ statement. File objects are one example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">'/etc/passwd'</span><span class="p">,</span> <span class="s1">'r'</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
<span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="n">f</span><span class="p">:</span>
<span class="nb">print</span> <span class="n">line</span>
<span class="o">...</span> <span class="n">more</span> <span class="n">processing</span> <span class="n">code</span> <span class="o">...</span>
</pre></div>
</div>
<p>After this statement has executed, the file object in <em>f</em> will have been
automatically closed, even if the <a class="reference internal" href="../reference/compound_stmts.html#for"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">for</span></code></a> loop raised an exception
part-way through the block.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>In this case, <em>f</em> is the same object created by <a class="reference internal" href="../library/functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate"><span class="pre">open()</span></code></a>, because
<code class="xref py py-meth docutils literal notranslate"><span class="pre">file.__enter__()</span></code> returns <em>self</em>.</p>
</div>
<p>The <a class="reference internal" href="../library/threading.html#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a> module’s locks and condition variables also support the
‘<a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>’ statement:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">lock</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Lock</span><span class="p">()</span>
<span class="k">with</span> <span class="n">lock</span><span class="p">:</span>
<span class="c1"># Critical section of code</span>
<span class="o">...</span>
</pre></div>
</div>
<p>The lock is acquired before the block is executed and always released once the
block is complete.</p>
<p>The new <code class="xref py py-func docutils literal notranslate"><span class="pre">localcontext()</span></code> function in the <a class="reference internal" href="../library/decimal.html#module-decimal" title="decimal: Implementation of the General Decimal Arithmetic Specification."><code class="xref py py-mod docutils literal notranslate"><span class="pre">decimal</span></code></a> module makes it easy
to save and restore the current decimal context, which encapsulates the desired
precision and rounding characteristics for computations:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">decimal</span> <span class="kn">import</span> <span class="n">Decimal</span><span class="p">,</span> <span class="n">Context</span><span class="p">,</span> <span class="n">localcontext</span>
<span class="c1"># Displays with default precision of 28 digits</span>
<span class="n">v</span> <span class="o">=</span> <span class="n">Decimal</span><span class="p">(</span><span class="s1">'578'</span><span class="p">)</span>
<span class="nb">print</span> <span class="n">v</span><span class="o">.</span><span class="n">sqrt</span><span class="p">()</span>
<span class="k">with</span> <span class="n">localcontext</span><span class="p">(</span><span class="n">Context</span><span class="p">(</span><span class="n">prec</span><span class="o">=</span><span class="mi">16</span><span class="p">)):</span>
<span class="c1"># All code in this block uses a precision of 16 digits.</span>
<span class="c1"># The original context is restored on exiting the block.</span>
<span class="nb">print</span> <span class="n">v</span><span class="o">.</span><span class="n">sqrt</span><span class="p">()</span>
</pre></div>
</div>
<div class="section" id="writing-context-managers">
<span id="new-25-context-managers"></span><h3>Writing Context Managers<a class="headerlink" href="#writing-context-managers" title="Permalink to this headline">¶</a></h3>
<p>Under the hood, the ‘<a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>’ statement is fairly complicated. Most
people will only use ‘<code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code>’ in company with existing objects and
don’t need to know these details, so you can skip the rest of this section if
you like. Authors of new objects will need to understand the details of the
underlying implementation and should keep reading.</p>
<p>A high-level explanation of the context management protocol is:</p>
<ul class="simple">
<li><p>The expression is evaluated and should result in an object called a “context
manager”. The context manager must have <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> and <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a>
methods.</p></li>
<li><p>The context manager’s <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> method is called. The value returned
is assigned to <em>VAR</em>. If no <code class="docutils literal notranslate"><span class="pre">'as</span> <span class="pre">VAR'</span></code> clause is present, the value is simply
discarded.</p></li>
<li><p>The code in <em>BLOCK</em> is executed.</p></li>
<li><p>If <em>BLOCK</em> raises an exception, the <code class="docutils literal notranslate"><span class="pre">__exit__(type,</span> <span class="pre">value,</span> <span class="pre">traceback)</span></code>
is called with the exception details, the same values returned by
<a class="reference internal" href="../library/sys.html#sys.exc_info" title="sys.exc_info"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.exc_info()</span></code></a>. The method’s return value controls whether the exception
is re-raised: any false value re-raises the exception, and <code class="docutils literal notranslate"><span class="pre">True</span></code> will result
in suppressing it. You’ll only rarely want to suppress the exception, because
if you do the author of the code containing the ‘<a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>’ statement will
never realize anything went wrong.</p></li>
<li><p>If <em>BLOCK</em> didn’t raise an exception, the <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> method is still
called, but <em>type</em>, <em>value</em>, and <em>traceback</em> are all <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p></li>
</ul>
<p>Let’s think through an example. I won’t present detailed code but will only
sketch the methods necessary for a database that supports transactions.</p>
<p>(For people unfamiliar with database terminology: a set of changes to the
database are grouped into a transaction. Transactions can be either committed,
meaning that all the changes are written into the database, or rolled back,
meaning that the changes are all discarded and the database is unchanged. See
any database textbook for more information.)</p>
<p>Let’s assume there’s an object representing a database connection. Our goal will
be to let the user write code like this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">db_connection</span> <span class="o">=</span> <span class="n">DatabaseConnection</span><span class="p">()</span>
<span class="k">with</span> <span class="n">db_connection</span> <span class="k">as</span> <span class="n">cursor</span><span class="p">:</span>
<span class="n">cursor</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s1">'insert into ...'</span><span class="p">)</span>
<span class="n">cursor</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s1">'delete from ...'</span><span class="p">)</span>
<span class="c1"># ... more operations ...</span>
</pre></div>
</div>
<p>The transaction should be committed if the code in the block runs flawlessly or
rolled back if there’s an exception. Here’s the basic interface for
<code class="xref py py-class docutils literal notranslate"><span class="pre">DatabaseConnection</span></code> that I’ll assume:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">DatabaseConnection</span><span class="p">:</span>
<span class="c1"># Database interface</span>
<span class="k">def</span> <span class="nf">cursor</span> <span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="s2">"Returns a cursor object and starts a new transaction"</span>
<span class="k">def</span> <span class="nf">commit</span> <span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="s2">"Commits current transaction"</span>
<span class="k">def</span> <span class="nf">rollback</span> <span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="s2">"Rolls back current transaction"</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> method is pretty easy, having only to start a new
transaction. For this application the resulting cursor object would be a useful
result, so the method will return it. The user can then add <code class="docutils literal notranslate"><span class="pre">as</span> <span class="pre">cursor</span></code> to
their ‘<a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>’ statement to bind the cursor to a variable name.</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">DatabaseConnection</span><span class="p">:</span>
<span class="o">...</span>
<span class="k">def</span> <span class="fm">__enter__</span> <span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="c1"># Code to start a new transaction</span>
<span class="n">cursor</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">cursor</span><span class="p">()</span>
<span class="k">return</span> <span class="n">cursor</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> method is the most complicated because it’s where most of
the work has to be done. The method has to check if an exception occurred. If
there was no exception, the transaction is committed. The transaction is rolled
back if there was an exception.</p>
<p>In the code below, execution will just fall off the end of the function,
returning the default value of <code class="docutils literal notranslate"><span class="pre">None</span></code>. <code class="docutils literal notranslate"><span class="pre">None</span></code> is false, so the exception
will be re-raised automatically. If you wished, you could be more explicit and
add a <a class="reference internal" href="../reference/simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">return</span></code></a> statement at the marked location.</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">DatabaseConnection</span><span class="p">:</span>
<span class="o">...</span>
<span class="k">def</span> <span class="fm">__exit__</span> <span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="nb">type</span><span class="p">,</span> <span class="n">value</span><span class="p">,</span> <span class="n">tb</span><span class="p">):</span>
<span class="k">if</span> <span class="n">tb</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="c1"># No exception, so commit</span>
<span class="bp">self</span><span class="o">.</span><span class="n">commit</span><span class="p">()</span>
<span class="k">else</span><span class="p">:</span>
<span class="c1"># Exception occurred, so rollback.</span>
<span class="bp">self</span><span class="o">.</span><span class="n">rollback</span><span class="p">()</span>
<span class="c1"># return False</span>
</pre></div>
</div>
</div>
<div class="section" id="the-contextlib-module">
<span id="contextlibmod"></span><h3>The contextlib module<a class="headerlink" href="#the-contextlib-module" title="Permalink to this headline">¶</a></h3>
<p>The new <a class="reference internal" href="../library/contextlib.html#module-contextlib" title="contextlib: Utilities for with-statement contexts."><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextlib</span></code></a> module provides some functions and a decorator that
are useful for writing objects for use with the ‘<a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>’ statement.</p>
<p>The decorator is called <code class="xref py py-func docutils literal notranslate"><span class="pre">contextmanager()</span></code>, and lets you write a single
generator function instead of defining a new class. The generator should yield
exactly one value. The code up to the <a class="reference internal" href="../reference/simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> will be executed as the
<a class="reference internal" href="../reference/datamodel.html#object.__enter__" title="object.__enter__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__enter__()</span></code></a> method, and the value yielded will be the method’s return
value that will get bound to the variable in the ‘<a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>’ statement’s
<code class="xref std std-keyword docutils literal notranslate"><span class="pre">as</span></code> clause, if any. The code after the <a class="reference internal" href="../reference/simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code></a> will be
executed in the <a class="reference internal" href="../reference/datamodel.html#object.__exit__" title="object.__exit__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__exit__()</span></code></a> method. Any exception raised in the block will
be raised by the <code class="xref std std-keyword docutils literal notranslate"><span class="pre">yield</span></code> statement.</p>
<p>Our database example from the previous section could be written using this
decorator as:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">contextmanager</span>
<span class="nd">@contextmanager</span>
<span class="k">def</span> <span class="nf">db_transaction</span> <span class="p">(</span><span class="n">connection</span><span class="p">):</span>
<span class="n">cursor</span> <span class="o">=</span> <span class="n">connection</span><span class="o">.</span><span class="n">cursor</span><span class="p">()</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">yield</span> <span class="n">cursor</span>
<span class="k">except</span><span class="p">:</span>
<span class="n">connection</span><span class="o">.</span><span class="n">rollback</span><span class="p">()</span>
<span class="k">raise</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">connection</span><span class="o">.</span><span class="n">commit</span><span class="p">()</span>
<span class="n">db</span> <span class="o">=</span> <span class="n">DatabaseConnection</span><span class="p">()</span>
<span class="k">with</span> <span class="n">db_transaction</span><span class="p">(</span><span class="n">db</span><span class="p">)</span> <span class="k">as</span> <span class="n">cursor</span><span class="p">:</span>
<span class="o">...</span>
</pre></div>
</div>
<p>The <a class="reference internal" href="../library/contextlib.html#module-contextlib" title="contextlib: Utilities for with-statement contexts."><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextlib</span></code></a> module also has a <code class="docutils literal notranslate"><span class="pre">nested(mgr1,</span> <span class="pre">mgr2,</span> <span class="pre">...)</span></code> function
that combines a number of context managers so you don’t need to write nested
‘<a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>’ statements. In this example, the single ‘<code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code>’
statement both starts a database transaction and acquires a thread lock:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">lock</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Lock</span><span class="p">()</span>
<span class="k">with</span> <span class="n">nested</span> <span class="p">(</span><span class="n">db_transaction</span><span class="p">(</span><span class="n">db</span><span class="p">),</span> <span class="n">lock</span><span class="p">)</span> <span class="k">as</span> <span class="p">(</span><span class="n">cursor</span><span class="p">,</span> <span class="n">locked</span><span class="p">):</span>
<span class="o">...</span>
</pre></div>
</div>
<p>Finally, the <code class="docutils literal notranslate"><span class="pre">closing(object)</span></code> function returns <em>object</em> so that it can be
bound to a variable, and calls <code class="docutils literal notranslate"><span class="pre">object.close</span></code> at the end of the block.</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">urllib</span><span class="o">,</span> <span class="nn">sys</span>
<span class="kn">from</span> <span class="nn">contextlib</span> <span class="kn">import</span> <span class="n">closing</span>
<span class="k">with</span> <span class="n">closing</span><span class="p">(</span><span class="n">urllib</span><span class="o">.</span><span class="n">urlopen</span><span class="p">(</span><span class="s1">'http://www.yahoo.com'</span><span class="p">))</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
<span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="n">f</span><span class="p">:</span>
<span class="n">sys</span><span class="o">.</span><span class="n">stdout</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">line</span><span class="p">)</span>
</pre></div>
</div>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><span class="target" id="index-12"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0343"><strong>PEP 343</strong></a> - The “with” statement</dt><dd><p>PEP written by Guido van Rossum and Nick Coghlan; implemented by Mike Bland,
Guido van Rossum, and Neal Norwitz. The PEP shows the code generated for a
‘<a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>’ statement, which can be helpful in learning how the statement
works.</p>
</dd>
</dl>
<p>The documentation for the <a class="reference internal" href="../library/contextlib.html#module-contextlib" title="contextlib: Utilities for with-statement contexts."><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextlib</span></code></a> module.</p>
</div>
</div>
</div>
<div class="section" id="pep-352-exceptions-as-new-style-classes">
<span id="pep-352"></span><h2>PEP 352: Exceptions as New-Style Classes<a class="headerlink" href="#pep-352-exceptions-as-new-style-classes" title="Permalink to this headline">¶</a></h2>
<p>Exception classes can now be new-style classes, not just classic classes, and
the built-in <a class="reference internal" href="../library/exceptions.html#Exception" title="Exception"><code class="xref py py-exc docutils literal notranslate"><span class="pre">Exception</span></code></a> class and all the standard built-in exceptions
(<a class="reference internal" href="../library/exceptions.html#NameError" title="NameError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">NameError</span></code></a>, <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a>, etc.) are now new-style classes.</p>
<p>The inheritance hierarchy for exceptions has been rearranged a bit. In 2.5, the
inheritance relationships are:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="ne">BaseException</span> <span class="c1"># New in Python 2.5</span>
<span class="o">|-</span> <span class="ne">KeyboardInterrupt</span>
<span class="o">|-</span> <span class="ne">SystemExit</span>
<span class="o">|-</span> <span class="ne">Exception</span>
<span class="o">|-</span> <span class="p">(</span><span class="nb">all</span> <span class="n">other</span> <span class="n">current</span> <span class="n">built</span><span class="o">-</span><span class="ow">in</span> <span class="n">exceptions</span><span class="p">)</span>
</pre></div>
</div>
<p>This rearrangement was done because people often want to catch all exceptions
that indicate program errors. <a class="reference internal" href="../library/exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code></a> and <a class="reference internal" href="../library/exceptions.html#SystemExit" title="SystemExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SystemExit</span></code></a>
aren’t errors, though, and usually represent an explicit action such as the user
hitting <kbd class="kbd docutils literal notranslate">Control-C</kbd> or code calling <a class="reference internal" href="../library/sys.html#sys.exit" title="sys.exit"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.exit()</span></code></a>. A bare <code class="docutils literal notranslate"><span class="pre">except:</span></code> will
catch all exceptions, so you commonly need to list <a class="reference internal" href="../library/exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code></a> and
<a class="reference internal" href="../library/exceptions.html#SystemExit" title="SystemExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SystemExit</span></code></a> in order to re-raise them. The usual pattern is:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">try</span><span class="p">:</span>
<span class="o">...</span>
<span class="k">except</span> <span class="p">(</span><span class="ne">KeyboardInterrupt</span><span class="p">,</span> <span class="ne">SystemExit</span><span class="p">):</span>
<span class="k">raise</span>
<span class="k">except</span><span class="p">:</span>
<span class="c1"># Log error...</span>
<span class="c1"># Continue running program...</span>
</pre></div>
</div>
<p>In Python 2.5, you can now write <code class="docutils literal notranslate"><span class="pre">except</span> <span class="pre">Exception</span></code> to achieve the same
result, catching all the exceptions that usually indicate errors but leaving
<a class="reference internal" href="../library/exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate"><span class="pre">KeyboardInterrupt</span></code></a> and <a class="reference internal" href="../library/exceptions.html#SystemExit" title="SystemExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SystemExit</span></code></a> alone. As in previous versions,
a bare <code class="docutils literal notranslate"><span class="pre">except:</span></code> still catches all exceptions.</p>
<p>The goal for Python 3.0 is to require any class raised as an exception to derive
from <a class="reference internal" href="../library/exceptions.html#BaseException" title="BaseException"><code class="xref py py-exc docutils literal notranslate"><span class="pre">BaseException</span></code></a> or some descendant of <a class="reference internal" href="../library/exceptions.html#BaseException" title="BaseException"><code class="xref py py-exc docutils literal notranslate"><span class="pre">BaseException</span></code></a>, and future
releases in the Python 2.x series may begin to enforce this constraint.
Therefore, I suggest you begin making all your exception classes derive from
<a class="reference internal" href="../library/exceptions.html#Exception" title="Exception"><code class="xref py py-exc docutils literal notranslate"><span class="pre">Exception</span></code></a> now. It’s been suggested that the bare <code class="docutils literal notranslate"><span class="pre">except:</span></code> form should
be removed in Python 3.0, but Guido van Rossum hasn’t decided whether to do this
or not.</p>
<p>Raising of strings as exceptions, as in the statement <code class="docutils literal notranslate"><span class="pre">raise</span> <span class="pre">"Error</span>
<span class="pre">occurred"</span></code>, is deprecated in Python 2.5 and will trigger a warning. The aim is
to be able to remove the string-exception feature in a few releases.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><span class="target" id="index-13"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0352"><strong>PEP 352</strong></a> - Required Superclass for Exceptions</dt><dd><p>PEP written by Brett Cannon and Guido van Rossum; implemented by Brett Cannon.</p>
</dd>
</dl>
</div>
</div>
<div class="section" id="pep-353-using-ssize-t-as-the-index-type">
<span id="pep-353"></span><h2>PEP 353: Using ssize_t as the index type<a class="headerlink" href="#pep-353-using-ssize-t-as-the-index-type" title="Permalink to this headline">¶</a></h2>
<p>A wide-ranging change to Python’s C API, using a new <code class="xref c c-type docutils literal notranslate"><span class="pre">Py_ssize_t</span></code> type
definition instead of <code class="xref c c-type docutils literal notranslate"><span class="pre">int</span></code>, will permit the interpreter to handle more
data on 64-bit platforms. This change doesn’t affect Python’s capacity on 32-bit
platforms.</p>
<p>Various pieces of the Python interpreter used C’s <code class="xref c c-type docutils literal notranslate"><span class="pre">int</span></code> type to store
sizes or counts; for example, the number of items in a list or tuple were stored
in an <code class="xref c c-type docutils literal notranslate"><span class="pre">int</span></code>. The C compilers for most 64-bit platforms still define
<code class="xref c c-type docutils literal notranslate"><span class="pre">int</span></code> as a 32-bit type, so that meant that lists could only hold up to
<code class="docutils literal notranslate"><span class="pre">2**31</span> <span class="pre">-</span> <span class="pre">1</span></code> = 2147483647 items. (There are actually a few different
programming models that 64-bit C compilers can use – see
<a class="reference external" href="http://www.unix.org/version2/whatsnew/lp64_wp.html">http://www.unix.org/version2/whatsnew/lp64_wp.html</a> for a discussion – but the
most commonly available model leaves <code class="xref c c-type docutils literal notranslate"><span class="pre">int</span></code> as 32 bits.)</p>
<p>A limit of 2147483647 items doesn’t really matter on a 32-bit platform because
you’ll run out of memory before hitting the length limit. Each list item
requires space for a pointer, which is 4 bytes, plus space for a
<a class="reference internal" href="../c-api/structures.html#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal notranslate"><span class="pre">PyObject</span></code></a> representing the item. 2147483647*4 is already more bytes
than a 32-bit address space can contain.</p>
<p>It’s possible to address that much memory on a 64-bit platform, however. The
pointers for a list that size would only require 16 GiB of space, so it’s not
unreasonable that Python programmers might construct lists that large.
Therefore, the Python interpreter had to be changed to use some type other than
<code class="xref c c-type docutils literal notranslate"><span class="pre">int</span></code>, and this will be a 64-bit type on 64-bit platforms. The change
will cause incompatibilities on 64-bit machines, so it was deemed worth making
the transition now, while the number of 64-bit users is still relatively small.
(In 5 or 10 years, we may <em>all</em> be on 64-bit machines, and the transition would
be more painful then.)</p>
<p>This change most strongly affects authors of C extension modules. Python
strings and container types such as lists and tuples now use
<code class="xref c c-type docutils literal notranslate"><span class="pre">Py_ssize_t</span></code> to store their size. Functions such as
<a class="reference internal" href="../c-api/list.html#c.PyList_Size" title="PyList_Size"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyList_Size()</span></code></a> now return <code class="xref c c-type docutils literal notranslate"><span class="pre">Py_ssize_t</span></code>. Code in extension modules
may therefore need to have some variables changed to <code class="xref c c-type docutils literal notranslate"><span class="pre">Py_ssize_t</span></code>.</p>
<p>The <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a> and <a class="reference internal" href="../c-api/arg.html#c.Py_BuildValue" title="Py_BuildValue"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_BuildValue()</span></code></a> functions have a new
conversion code, <code class="docutils literal notranslate"><span class="pre">n</span></code>, for <code class="xref c c-type docutils literal notranslate"><span class="pre">Py_ssize_t</span></code>. <a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyArg_ParseTuple()</span></code></a>’s
<code class="docutils literal notranslate"><span class="pre">s#</span></code> and <code class="docutils literal notranslate"><span class="pre">t#</span></code> still output <code class="xref c c-type docutils literal notranslate"><span class="pre">int</span></code> by default, but you can define the
macro <code class="xref c c-macro docutils literal notranslate"><span class="pre">PY_SSIZE_T_CLEAN</span></code> before including <code class="file docutils literal notranslate"><span class="pre">Python.h</span></code> to make
them return <code class="xref c c-type docutils literal notranslate"><span class="pre">Py_ssize_t</span></code>.</p>
<p><span class="target" id="index-14"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0353"><strong>PEP 353</strong></a> has a section on conversion guidelines that extension authors should
read to learn about supporting 64-bit platforms.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><span class="target" id="index-15"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0353"><strong>PEP 353</strong></a> - Using ssize_t as the index type</dt><dd><p>PEP written and implemented by Martin von Löwis.</p>
</dd>
</dl>
</div>
</div>
<div class="section" id="pep-357-the-index-method">
<span id="pep-357"></span><h2>PEP 357: The ‘__index__’ method<a class="headerlink" href="#pep-357-the-index-method" title="Permalink to this headline">¶</a></h2>
<p>The NumPy developers had a problem that could only be solved by adding a new
special method, <a class="reference internal" href="../reference/datamodel.html#object.__index__" title="object.__index__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__index__()</span></code></a>. When using slice notation, as in
<code class="docutils literal notranslate"><span class="pre">[start:stop:step]</span></code>, the values of the <em>start</em>, <em>stop</em>, and <em>step</em> indexes
must all be either integers or long integers. NumPy defines a variety of
specialized integer types corresponding to unsigned and signed integers of 8,
16, 32, and 64 bits, but there was no way to signal that these types could be
used as slice indexes.</p>
<p>Slicing can’t just use the existing <a class="reference internal" href="../reference/datamodel.html#object.__int__" title="object.__int__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__int__()</span></code></a> method because that method
is also used to implement coercion to integers. If slicing used
<a class="reference internal" href="../reference/datamodel.html#object.__int__" title="object.__int__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__int__()</span></code></a>, floating-point numbers would also become legal slice indexes
and that’s clearly an undesirable behaviour.</p>
<p>Instead, a new special method called <a class="reference internal" href="../reference/datamodel.html#object.__index__" title="object.__index__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__index__()</span></code></a> was added. It takes no
arguments and returns an integer giving the slice index to use. For example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span>
<span class="k">def</span> <span class="fm">__index__</span> <span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">value</span>
</pre></div>
</div>
<p>The return value must be either a Python integer or long integer. The
interpreter will check that the type returned is correct, and raises a
<a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> if this requirement isn’t met.</p>
<p>A corresponding <code class="xref py py-attr docutils literal notranslate"><span class="pre">nb_index</span></code> slot was added to the C-level
<a class="reference internal" href="../c-api/typeobj.html#c.PyNumberMethods" title="PyNumberMethods"><code class="xref c c-type docutils literal notranslate"><span class="pre">PyNumberMethods</span></code></a> structure to let C extensions implement this protocol.
<code class="docutils literal notranslate"><span class="pre">PyNumber_Index(obj)</span></code> can be used in extension code to call the
<a class="reference internal" href="../reference/datamodel.html#object.__index__" title="object.__index__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__index__()</span></code></a> function and retrieve its result.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><span class="target" id="index-16"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0357"><strong>PEP 357</strong></a> - Allowing Any Object to be Used for Slicing</dt><dd><p>PEP written and implemented by Travis Oliphant.</p>
</dd>
</dl>
</div>
</div>
<div class="section" id="other-language-changes">
<span id="other-lang"></span><h2>Other Language Changes<a class="headerlink" href="#other-language-changes" title="Permalink to this headline">¶</a></h2>
<p>Here are all of the changes that Python 2.5 makes to the core Python language.</p>
<ul>
<li><p>The <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate"><span class="pre">dict</span></code></a> type has a new hook for letting subclasses provide a default
value when a key isn’t contained in the dictionary. When a key isn’t found, the
dictionary’s <code class="docutils literal notranslate"><span class="pre">__missing__(key)</span></code> method will be called. This hook is used
to implement the new <code class="xref py py-class docutils literal notranslate"><span class="pre">defaultdict</span></code> class in the <a class="reference internal" href="../library/collections.html#module-collections" title="collections: Container datatypes"><code class="xref py py-mod docutils literal notranslate"><span class="pre">collections</span></code></a>
module. The following example defines a dictionary that returns zero for any
missing key:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">zerodict</span> <span class="p">(</span><span class="nb">dict</span><span class="p">):</span>
<span class="k">def</span> <span class="fm">__missing__</span> <span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">key</span><span class="p">):</span>
<span class="k">return</span> <span class="mi">0</span>
<span class="n">d</span> <span class="o">=</span> <span class="n">zerodict</span><span class="p">({</span><span class="mi">1</span><span class="p">:</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">:</span><span class="mi">2</span><span class="p">})</span>
<span class="nb">print</span> <span class="n">d</span><span class="p">[</span><span class="mi">1</span><span class="p">],</span> <span class="n">d</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="c1"># Prints 1, 2</span>
<span class="nb">print</span> <span class="n">d</span><span class="p">[</span><span class="mi">3</span><span class="p">],</span> <span class="n">d</span><span class="p">[</span><span class="mi">4</span><span class="p">]</span> <span class="c1"># Prints 0, 0</span>
</pre></div>
</div>
</li>
<li><p>Both 8-bit and Unicode strings have new <code class="docutils literal notranslate"><span class="pre">partition(sep)</span></code> and
<code class="docutils literal notranslate"><span class="pre">rpartition(sep)</span></code> methods that simplify a common use case.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">find(S)</span></code> method is often used to get an index which is then used to
slice the string and obtain the pieces that are before and after the separator.
<code class="docutils literal notranslate"><span class="pre">partition(sep)</span></code> condenses this pattern into a single method call that
returns a 3-tuple containing the substring before the separator, the separator
itself, and the substring after the separator. If the separator isn’t found,
the first element of the tuple is the entire string and the other two elements
are empty. <code class="docutils literal notranslate"><span class="pre">rpartition(sep)</span></code> also returns a 3-tuple but starts searching
from the end of the string; the <code class="docutils literal notranslate"><span class="pre">r</span></code> stands for ‘reverse’.</p>
<p>Some examples:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="p">(</span><span class="s1">'http://www.python.org'</span><span class="p">)</span><span class="o">.</span><span class="n">partition</span><span class="p">(</span><span class="s1">'://'</span><span class="p">)</span>
<span class="go">('http', '://', 'www.python.org')</span>
<span class="gp">>>> </span><span class="p">(</span><span class="s1">'file:/usr/share/doc/index.html'</span><span class="p">)</span><span class="o">.</span><span class="n">partition</span><span class="p">(</span><span class="s1">'://'</span><span class="p">)</span>
<span class="go">('file:/usr/share/doc/index.html', '', '')</span>
<span class="gp">>>> </span><span class="p">(</span><span class="sa">u</span><span class="s1">'Subject: a quick question'</span><span class="p">)</span><span class="o">.</span><span class="n">partition</span><span class="p">(</span><span class="s1">':'</span><span class="p">)</span>
<span class="go">(u'Subject', u':', u' a quick question')</span>
<span class="gp">>>> </span><span class="s1">'www.python.org'</span><span class="o">.</span><span class="n">rpartition</span><span class="p">(</span><span class="s1">'.'</span><span class="p">)</span>
<span class="go">('www.python', '.', 'org')</span>
<span class="gp">>>> </span><span class="s1">'www.python.org'</span><span class="o">.</span><span class="n">rpartition</span><span class="p">(</span><span class="s1">':'</span><span class="p">)</span>
<span class="go">('', '', 'www.python.org')</span>
</pre></div>
</div>
<p>(Implemented by Fredrik Lundh following a suggestion by Raymond Hettinger.)</p>
</li>
<li><p>The <code class="xref py py-meth docutils literal notranslate"><span class="pre">startswith()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">endswith()</span></code> methods of string types now accept
tuples of strings to check for.</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">is_image_file</span> <span class="p">(</span><span class="n">filename</span><span class="p">):</span>
<span class="k">return</span> <span class="n">filename</span><span class="o">.</span><span class="n">endswith</span><span class="p">((</span><span class="s1">'.gif'</span><span class="p">,</span> <span class="s1">'.jpg'</span><span class="p">,</span> <span class="s1">'.tiff'</span><span class="p">))</span>
</pre></div>
</div>
<p>(Implemented by Georg Brandl following a suggestion by Tom Lynn.)</p>
</li>
<li><p>The <a class="reference internal" href="../library/functions.html#min" title="min"><code class="xref py py-func docutils literal notranslate"><span class="pre">min()</span></code></a> and <a class="reference internal" href="../library/functions.html#max" title="max"><code class="xref py py-func docutils literal notranslate"><span class="pre">max()</span></code></a> built-in functions gained a <code class="docutils literal notranslate"><span class="pre">key</span></code> keyword
parameter analogous to the <code class="docutils literal notranslate"><span class="pre">key</span></code> argument for <code class="xref py py-meth docutils literal notranslate"><span class="pre">sort()</span></code>. This parameter
supplies a function that takes a single argument and is called for every value
in the list; <a class="reference internal" href="../library/functions.html#min" title="min"><code class="xref py py-func docutils literal notranslate"><span class="pre">min()</span></code></a>/<a class="reference internal" href="../library/functions.html#max" title="max"><code class="xref py py-func docutils literal notranslate"><span class="pre">max()</span></code></a> will return the element with the
smallest/largest return value from this function. For example, to find the
longest string in a list, you can do:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">L</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'medium'</span><span class="p">,</span> <span class="s1">'longest'</span><span class="p">,</span> <span class="s1">'short'</span><span class="p">]</span>
<span class="c1"># Prints 'longest'</span>
<span class="nb">print</span> <span class="nb">max</span><span class="p">(</span><span class="n">L</span><span class="p">,</span> <span class="n">key</span><span class="o">=</span><span class="nb">len</span><span class="p">)</span>
<span class="c1"># Prints 'short', because lexicographically 'short' has the largest value</span>
<span class="nb">print</span> <span class="nb">max</span><span class="p">(</span><span class="n">L</span><span class="p">)</span>
</pre></div>
</div>
<p>(Contributed by Steven Bethard and Raymond Hettinger.)</p>
</li>
<li><p>Two new built-in functions, <a class="reference internal" href="../library/functions.html#any" title="any"><code class="xref py py-func docutils literal notranslate"><span class="pre">any()</span></code></a> and <a class="reference internal" href="../library/functions.html#all" title="all"><code class="xref py py-func docutils literal notranslate"><span class="pre">all()</span></code></a>, evaluate whether an
iterator contains any true or false values. <a class="reference internal" href="../library/functions.html#any" title="any"><code class="xref py py-func docutils literal notranslate"><span class="pre">any()</span></code></a> returns <a class="reference internal" href="../library/constants.html#True" title="True"><code class="xref py py-const docutils literal notranslate"><span class="pre">True</span></code></a>
if any value returned by the iterator is true; otherwise it will return
<a class="reference internal" href="../library/constants.html#False" title="False"><code class="xref py py-const docutils literal notranslate"><span class="pre">False</span></code></a>. <a class="reference internal" href="../library/functions.html#all" title="all"><code class="xref py py-func docutils literal notranslate"><span class="pre">all()</span></code></a> returns <a class="reference internal" href="../library/constants.html#True" title="True"><code class="xref py py-const docutils literal notranslate"><span class="pre">True</span></code></a> only if all of the values
returned by the iterator evaluate as true. (Suggested by Guido van Rossum, and
implemented by Raymond Hettinger.)</p></li>
<li><p>The result of a class’s <a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> method can now be either a long
integer or a regular integer. If a long integer is returned, the hash of that
value is taken. In earlier versions the hash value was required to be a
regular integer, but in 2.5 the <a class="reference internal" href="../library/functions.html#id" title="id"><code class="xref py py-func docutils literal notranslate"><span class="pre">id()</span></code></a> built-in was changed to always
return non-negative numbers, and users often seem to use <code class="docutils literal notranslate"><span class="pre">id(self)</span></code> in
<a class="reference internal" href="../reference/datamodel.html#object.__hash__" title="object.__hash__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code></a> methods (though this is discouraged).</p>
</li>
<li><p>ASCII is now the default encoding for modules. It’s now a syntax error if a
module contains string literals with 8-bit characters but doesn’t have an
encoding declaration. In Python 2.4 this triggered a warning, not a syntax
error. See <span class="target" id="index-17"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0263"><strong>PEP 263</strong></a> for how to declare a module’s encoding; for example, you
might add a line like this near the top of the source file:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># -*- coding: latin1 -*-</span>
</pre></div>
</div>
</li>
<li><p>A new warning, <a class="reference internal" href="../library/exceptions.html#UnicodeWarning" title="UnicodeWarning"><code class="xref py py-class docutils literal notranslate"><span class="pre">UnicodeWarning</span></code></a>, is triggered when you attempt to
compare a Unicode string and an 8-bit string that can’t be converted to Unicode
using the default ASCII encoding. The result of the comparison is false:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">chr</span><span class="p">(</span><span class="mi">128</span><span class="p">)</span> <span class="o">==</span> <span class="n">unichr</span><span class="p">(</span><span class="mi">128</span><span class="p">)</span> <span class="c1"># Can't convert chr(128) to Unicode</span>
<span class="go">__main__:1: UnicodeWarning: Unicode equal comparison failed</span>
<span class="go"> to convert both arguments to Unicode - interpreting them</span>
<span class="go"> as being unequal</span>
<span class="go">False</span>
<span class="gp">>>> </span><span class="nb">chr</span><span class="p">(</span><span class="mi">127</span><span class="p">)</span> <span class="o">==</span> <span class="n">unichr</span><span class="p">(</span><span class="mi">127</span><span class="p">)</span> <span class="c1"># chr(127) can be converted</span>
<span class="go">True</span>
</pre></div>
</div>
<p>Previously this would raise a <a class="reference internal" href="../library/exceptions.html#UnicodeDecodeError" title="UnicodeDecodeError"><code class="xref py py-class docutils literal notranslate"><span class="pre">UnicodeDecodeError</span></code></a> exception, but in 2.5
this could result in puzzling problems when accessing a dictionary. If you
looked up <code class="docutils literal notranslate"><span class="pre">unichr(128)</span></code> and <code class="docutils literal notranslate"><span class="pre">chr(128)</span></code> was being used as a key, you’d get a
<a class="reference internal" href="../library/exceptions.html#UnicodeDecodeError" title="UnicodeDecodeError"><code class="xref py py-class docutils literal notranslate"><span class="pre">UnicodeDecodeError</span></code></a> exception. Other changes in 2.5 resulted in this
exception being raised instead of suppressed by the code in <code class="file docutils literal notranslate"><span class="pre">dictobject.c</span></code>
that implements dictionaries.</p>
<p>Raising an exception for such a comparison is strictly correct, but the change
might have broken code, so instead <a class="reference internal" href="../library/exceptions.html#UnicodeWarning" title="UnicodeWarning"><code class="xref py py-class docutils literal notranslate"><span class="pre">UnicodeWarning</span></code></a> was introduced.</p>
<p>(Implemented by Marc-André Lemburg.)</p>
</li>
<li><p>One error that Python programmers sometimes make is forgetting to include an
<code class="file docutils literal notranslate"><span class="pre">__init__.py</span></code> module in a package directory. Debugging this mistake can be
confusing, and usually requires running Python with the <a class="reference internal" href="../using/cmdline.html#id4"><code class="xref std std-option docutils literal notranslate"><span class="pre">-v</span></code></a> switch to
log all the paths searched. In Python 2.5, a new <a class="reference internal" href="../library/exceptions.html#ImportWarning" title="ImportWarning"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ImportWarning</span></code></a> warning is
triggered when an import would have picked up a directory as a package but no
<code class="file docutils literal notranslate"><span class="pre">__init__.py</span></code> was found. This warning is silently ignored by default;
provide the <a class="reference internal" href="../using/cmdline.html#cmdoption-w"><code class="xref std std-option docutils literal notranslate"><span class="pre">-Wd</span></code></a> option when running the Python executable to display
the warning message. (Implemented by Thomas Wouters.)</p></li>
<li><p>The list of base classes in a class definition can now be empty. As an
example, this is now legal:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">C</span><span class="p">():</span>
<span class="k">pass</span>
</pre></div>
</div>
<p>(Implemented by Brett Cannon.)</p>
</li>
</ul>
<div class="section" id="interactive-interpreter-changes">
<span id="interactive"></span><h3>Interactive Interpreter Changes<a class="headerlink" href="#interactive-interpreter-changes" title="Permalink to this headline">¶</a></h3>
<p>In the interactive interpreter, <code class="docutils literal notranslate"><span class="pre">quit</span></code> and <code class="docutils literal notranslate"><span class="pre">exit</span></code> have long been strings so
that new users get a somewhat helpful message when they try to quit:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">quit</span>
<span class="go">'Use Ctrl-D (i.e. EOF) to exit.'</span>
</pre></div>
</div>
<p>In Python 2.5, <code class="docutils literal notranslate"><span class="pre">quit</span></code> and <code class="docutils literal notranslate"><span class="pre">exit</span></code> are now objects that still produce string
representations of themselves, but are also callable. Newbies who try <code class="docutils literal notranslate"><span class="pre">quit()</span></code>
or <code class="docutils literal notranslate"><span class="pre">exit()</span></code> will now exit the interpreter as they expect. (Implemented by
Georg Brandl.)</p>
<p>The Python executable now accepts the standard long options <a class="reference internal" href="../using/cmdline.html#cmdoption-help"><code class="xref std std-option docutils literal notranslate"><span class="pre">--help</span></code></a>
and <a class="reference internal" href="../using/cmdline.html#cmdoption-version"><code class="xref std std-option docutils literal notranslate"><span class="pre">--version</span></code></a>; on Windows, it also accepts the <a class="reference internal" href="../using/cmdline.html#cmdoption"><code class="xref std std-option docutils literal notranslate"><span class="pre">/?</span></code></a> option
for displaying a help message. (Implemented by Georg Brandl.)</p>
</div>
<div class="section" id="optimizations">
<span id="opts"></span><h3>Optimizations<a class="headerlink" href="#optimizations" title="Permalink to this headline">¶</a></h3>
<p>Several of the optimizations were developed at the NeedForSpeed sprint, an event
held in Reykjavik, Iceland, from May 21–28 2006. The sprint focused on speed
enhancements to the CPython implementation and was funded by EWT LLC with local
support from CCP Games. Those optimizations added at this sprint are specially
marked in the following list.</p>
<ul class="simple">
<li><p>When they were introduced in Python 2.4, the built-in <a class="reference internal" href="../library/stdtypes.html#set" title="set"><code class="xref py py-class docutils literal notranslate"><span class="pre">set</span></code></a> and
<a class="reference internal" href="../library/stdtypes.html#frozenset" title="frozenset"><code class="xref py py-class docutils literal notranslate"><span class="pre">frozenset</span></code></a> types were built on top of Python’s dictionary type. In 2.5
the internal data structure has been customized for implementing sets, and as a
result sets will use a third less memory and are somewhat faster. (Implemented
by Raymond Hettinger.)</p></li>
<li><p>The speed of some Unicode operations, such as finding substrings, string
splitting, and character map encoding and decoding, has been improved.
(Substring search and splitting improvements were added by Fredrik Lundh and
Andrew Dalke at the NeedForSpeed sprint. Character maps were improved by Walter
Dörwald and Martin von Löwis.)</p>
</li>
<li><p>The <code class="docutils literal notranslate"><span class="pre">long(str,</span> <span class="pre">base)</span></code> function is now faster on long digit strings
because fewer intermediate results are calculated. The peak is for strings of
around 800–1000 digits where the function is 6 times faster. (Contributed by
Alan McIntyre and committed at the NeedForSpeed sprint.)</p>
</li>
<li><p>It’s now illegal to mix iterating over a file with <code class="docutils literal notranslate"><span class="pre">for</span> <span class="pre">line</span> <span class="pre">in</span> <span class="pre">file</span></code> and
calling the file object’s <code class="xref py py-meth docutils literal notranslate"><span class="pre">read()</span></code>/<a class="reference internal" href="../library/readline.html#module-readline" title="readline: GNU readline support for Python. (Unix)"><code class="xref py py-meth docutils literal notranslate"><span class="pre">readline()</span></code></a>/<code class="xref py py-meth docutils literal notranslate"><span class="pre">readlines()</span></code>
methods. Iteration uses an internal buffer and the <code class="xref py py-meth docutils literal notranslate"><span class="pre">read*()</span></code> methods
don’t use that buffer. Instead they would return the data following the
buffer, causing the data to appear out of order. Mixing iteration and these
methods will now trigger a <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> from the <code class="xref py py-meth docutils literal notranslate"><span class="pre">read*()</span></code> method.
(Implemented by Thomas Wouters.)</p>
</li>
<li><p>The <a class="reference internal" href="../library/struct.html#module-struct" title="struct: Interpret bytes as packed binary data."><code class="xref py py-mod docutils literal notranslate"><span class="pre">struct</span></code></a> module now compiles structure format strings into an
internal representation and caches this representation, yielding a 20% speedup.
(Contributed by Bob Ippolito at the NeedForSpeed sprint.)</p></li>
<li><p>The <a class="reference internal" href="../library/re.html#module-re" title="re: Regular expression operations."><code class="xref py py-mod docutils literal notranslate"><span class="pre">re</span></code></a> module got a 1 or 2% speedup by switching to Python’s allocator
functions instead of the system’s <code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code> and <code class="xref c c-func docutils literal notranslate"><span class="pre">free()</span></code>.
(Contributed by Jack Diederich at the NeedForSpeed sprint.)</p></li>
<li><p>The code generator’s peephole optimizer now performs simple constant folding
in expressions. If you write something like <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre">=</span> <span class="pre">2+3</span></code>, the code generator
will do the arithmetic and produce code corresponding to <code class="docutils literal notranslate"><span class="pre">a</span> <span class="pre">=</span> <span class="pre">5</span></code>. (Proposed
and implemented by Raymond Hettinger.)</p></li>
<li><p>Function calls are now faster because code objects now keep the most recently
finished frame (a “zombie frame”) in an internal field of the code object,
reusing it the next time the code object is invoked. (Original patch by Michael
Hudson, modified by Armin Rigo and Richard Jones; committed at the NeedForSpeed
sprint.) Frame objects are also slightly smaller, which may improve cache
locality and reduce memory usage a bit. (Contributed by Neal Norwitz.)</p>
</li>
<li><p>Python’s built-in exceptions are now new-style classes, a change that speeds
up instantiation considerably. Exception handling in Python 2.5 is therefore
about 30% faster than in 2.4. (Contributed by Richard Jones, Georg Brandl and
Sean Reifschneider at the NeedForSpeed sprint.)</p></li>
<li><p>Importing now caches the paths tried, recording whether they exist or not so
that the interpreter makes fewer <code class="xref c c-func docutils literal notranslate"><span class="pre">open()</span></code> and <code class="xref c c-func docutils literal notranslate"><span class="pre">stat()</span></code> calls on
startup. (Contributed by Martin von Löwis and Georg Brandl.)</p>
</li>
</ul>
</div>
</div>
<div class="section" id="new-improved-and-removed-modules">
<span id="modules"></span><h2>New, Improved, and Removed Modules<a class="headerlink" href="#new-improved-and-removed-modules" title="Permalink to this headline">¶</a></h2>
<p>The standard library received many enhancements and bug fixes in Python 2.5.
Here’s a partial list of the most notable changes, sorted alphabetically by
module name. Consult the <code class="file docutils literal notranslate"><span class="pre">Misc/NEWS</span></code> file in the source tree for a more
complete list of changes, or look through the SVN logs for all the details.</p>
<ul>
<li><p>The <a class="reference internal" href="../library/audioop.html#module-audioop" title="audioop: Manipulate raw audio data."><code class="xref py py-mod docutils literal notranslate"><span class="pre">audioop</span></code></a> module now supports the a-LAW encoding, and the code for
u-LAW encoding has been improved. (Contributed by Lars Immisch.)</p></li>
<li><p>The <a class="reference internal" href="../library/codecs.html#module-codecs" title="codecs: Encode and decode data and streams."><code class="xref py py-mod docutils literal notranslate"><span class="pre">codecs</span></code></a> module gained support for incremental codecs. The
<code class="xref py py-func docutils literal notranslate"><span class="pre">codec.lookup()</span></code> function now returns a <code class="xref py py-class docutils literal notranslate"><span class="pre">CodecInfo</span></code> instance instead
of a tuple. <code class="xref py py-class docutils literal notranslate"><span class="pre">CodecInfo</span></code> instances behave like a 4-tuple to preserve
backward compatibility but also have the attributes <code class="xref py py-attr docutils literal notranslate"><span class="pre">encode</span></code>,
<code class="xref py py-attr docutils literal notranslate"><span class="pre">decode</span></code>, <code class="xref py py-attr docutils literal notranslate"><span class="pre">incrementalencoder</span></code>, <code class="xref py py-attr docutils literal notranslate"><span class="pre">incrementaldecoder</span></code>,
<code class="xref py py-attr docutils literal notranslate"><span class="pre">streamwriter</span></code>, and <code class="xref py py-attr docutils literal notranslate"><span class="pre">streamreader</span></code>. Incremental codecs can receive
input and produce output in multiple chunks; the output is the same as if the
entire input was fed to the non-incremental codec. See the <a class="reference internal" href="../library/codecs.html#module-codecs" title="codecs: Encode and decode data and streams."><code class="xref py py-mod docutils literal notranslate"><span class="pre">codecs</span></code></a> module
documentation for details. (Designed and implemented by Walter Dörwald.)</p>
</li>
<li><p>The <a class="reference internal" href="../library/collections.html#module-collections" title="collections: Container datatypes"><code class="xref py py-mod docutils literal notranslate"><span class="pre">collections</span></code></a> module gained a new type, <code class="xref py py-class docutils literal notranslate"><span class="pre">defaultdict</span></code>, that
subclasses the standard <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate"><span class="pre">dict</span></code></a> type. The new type mostly behaves like a
dictionary but constructs a default value when a key isn’t present,
automatically adding it to the dictionary for the requested key value.</p>
<p>The first argument to <code class="xref py py-class docutils literal notranslate"><span class="pre">defaultdict</span></code>’s constructor is a factory function
that gets called whenever a key is requested but not found. This factory
function receives no arguments, so you can use built-in type constructors such
as <a class="reference internal" href="../library/stdtypes.html#list" title="list"><code class="xref py py-func docutils literal notranslate"><span class="pre">list()</span></code></a> or <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-func docutils literal notranslate"><span class="pre">int()</span></code></a>. For example, you can make an index of words
based on their initial letter like this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">words</span> <span class="o">=</span> <span class="s2">"""Nel mezzo del cammin di nostra vita</span>
<span class="s2">mi ritrovai per una selva oscura</span>
<span class="s2">che la diritta via era smarrita"""</span><span class="o">.</span><span class="n">lower</span><span class="p">()</span><span class="o">.</span><span class="n">split</span><span class="p">()</span>
<span class="n">index</span> <span class="o">=</span> <span class="n">defaultdict</span><span class="p">(</span><span class="nb">list</span><span class="p">)</span>
<span class="k">for</span> <span class="n">w</span> <span class="ow">in</span> <span class="n">words</span><span class="p">:</span>
<span class="n">init_letter</span> <span class="o">=</span> <span class="n">w</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
<span class="n">index</span><span class="p">[</span><span class="n">init_letter</span><span class="p">]</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">w</span><span class="p">)</span>
</pre></div>
</div>
<p>Printing <code class="docutils literal notranslate"><span class="pre">index</span></code> results in the following output:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">defaultdict</span><span class="p">(</span><span class="o"><</span><span class="nb">type</span> <span class="s1">'list'</span><span class="o">></span><span class="p">,</span> <span class="p">{</span><span class="s1">'c'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'cammin'</span><span class="p">,</span> <span class="s1">'che'</span><span class="p">],</span> <span class="s1">'e'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'era'</span><span class="p">],</span>
<span class="s1">'d'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'del'</span><span class="p">,</span> <span class="s1">'di'</span><span class="p">,</span> <span class="s1">'diritta'</span><span class="p">],</span> <span class="s1">'m'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'mezzo'</span><span class="p">,</span> <span class="s1">'mi'</span><span class="p">],</span>
<span class="s1">'l'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'la'</span><span class="p">],</span> <span class="s1">'o'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'oscura'</span><span class="p">],</span> <span class="s1">'n'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'nel'</span><span class="p">,</span> <span class="s1">'nostra'</span><span class="p">],</span>
<span class="s1">'p'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'per'</span><span class="p">],</span> <span class="s1">'s'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'selva'</span><span class="p">,</span> <span class="s1">'smarrita'</span><span class="p">],</span>
<span class="s1">'r'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'ritrovai'</span><span class="p">],</span> <span class="s1">'u'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'una'</span><span class="p">],</span> <span class="s1">'v'</span><span class="p">:</span> <span class="p">[</span><span class="s1">'vita'</span><span class="p">,</span> <span class="s1">'via'</span><span class="p">]}</span>
</pre></div>
</div>
<p>(Contributed by Guido van Rossum.)</p>
</li>
<li><p>The <code class="xref py py-class docutils literal notranslate"><span class="pre">deque</span></code> double-ended queue type supplied by the <a class="reference internal" href="../library/collections.html#module-collections" title="collections: Container datatypes"><code class="xref py py-mod docutils literal notranslate"><span class="pre">collections</span></code></a>
module now has a <code class="docutils literal notranslate"><span class="pre">remove(value)</span></code> method that removes the first occurrence
of <em>value</em> in the queue, raising <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> if the value isn’t found.
(Contributed by Raymond Hettinger.)</p></li>
<li><p>New module: The <a class="reference internal" href="../library/contextlib.html#module-contextlib" title="contextlib: Utilities for with-statement contexts."><code class="xref py py-mod docutils literal notranslate"><span class="pre">contextlib</span></code></a> module contains helper functions for use
with the new ‘<a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a>’ statement. See section <a class="reference internal" href="#contextlibmod"><span class="std std-ref">The contextlib module</span></a>
for more about this module.</p></li>
<li><p>New module: The <a class="reference internal" href="../library/profile.html#module-cProfile" title="cProfile"><code class="xref py py-mod docutils literal notranslate"><span class="pre">cProfile</span></code></a> module is a C implementation of the existing
<a class="reference internal" href="../library/profile.html#module-profile" title="profile: Python source profiler."><code class="xref py py-mod docutils literal notranslate"><span class="pre">profile</span></code></a> module that has much lower overhead. The module’s interface is
the same as <a class="reference internal" href="../library/profile.html#module-profile" title="profile: Python source profiler."><code class="xref py py-mod docutils literal notranslate"><span class="pre">profile</span></code></a>: you run <code class="docutils literal notranslate"><span class="pre">cProfile.run('main()')</span></code> to profile a
function, can save profile data to a file, etc. It’s not yet known if the
Hotshot profiler, which is also written in C but doesn’t match the
<a class="reference internal" href="../library/profile.html#module-profile" title="profile: Python source profiler."><code class="xref py py-mod docutils literal notranslate"><span class="pre">profile</span></code></a> module’s interface, will continue to be maintained in future
versions of Python. (Contributed by Armin Rigo.)</p>
<p>Also, the <a class="reference internal" href="../library/profile.html#module-pstats" title="pstats: Statistics object for use with the profiler."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pstats</span></code></a> module for analyzing the data measured by the profiler
now supports directing the output to any file object by supplying a <em>stream</em>
argument to the <code class="xref py py-class docutils literal notranslate"><span class="pre">Stats</span></code> constructor. (Contributed by Skip Montanaro.)</p>
</li>
<li><p>The <a class="reference internal" href="../library/csv.html#module-csv" title="csv: Write and read tabular data to and from delimited files."><code class="xref py py-mod docutils literal notranslate"><span class="pre">csv</span></code></a> module, which parses files in comma-separated value format,
received several enhancements and a number of bugfixes. You can now set the
maximum size in bytes of a field by calling the
<code class="docutils literal notranslate"><span class="pre">csv.field_size_limit(new_limit)</span></code> function; omitting the <em>new_limit</em>
argument will return the currently-set limit. The <code class="xref py py-class docutils literal notranslate"><span class="pre">reader</span></code> class now has
a <code class="xref py py-attr docutils literal notranslate"><span class="pre">line_num</span></code> attribute that counts the number of physical lines read from
the source; records can span multiple physical lines, so <code class="xref py py-attr docutils literal notranslate"><span class="pre">line_num</span></code> is not
the same as the number of records read.</p>
<p>The CSV parser is now stricter about multi-line quoted fields. Previously, if a
line ended within a quoted field without a terminating newline character, a
newline would be inserted into the returned field. This behavior caused problems
when reading files that contained carriage return characters within fields, so
the code was changed to return the field without inserting newlines. As a
consequence, if newlines embedded within fields are important, the input should
be split into lines in a manner that preserves the newline characters.</p>
<p>(Contributed by Skip Montanaro and Andrew McNamara.)</p>
</li>
<li><p>The <a class="reference internal" href="../library/datetime.html#datetime.datetime" title="datetime.datetime"><code class="xref py py-class docutils literal notranslate"><span class="pre">datetime</span></code></a> class in the <a class="reference internal" href="../library/datetime.html#module-datetime" title="datetime: Basic date and time types."><code class="xref py py-mod docutils literal notranslate"><span class="pre">datetime</span></code></a> module now has a
<code class="docutils literal notranslate"><span class="pre">strptime(string,</span> <span class="pre">format)</span></code> method for parsing date strings, contributed
by Josh Spoerri. It uses the same format characters as <a class="reference internal" href="../library/time.html#time.strptime" title="time.strptime"><code class="xref py py-func docutils literal notranslate"><span class="pre">time.strptime()</span></code></a> and
<a class="reference internal" href="../library/time.html#time.strftime" title="time.strftime"><code class="xref py py-func docutils literal notranslate"><span class="pre">time.strftime()</span></code></a>:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">datetime</span> <span class="kn">import</span> <span class="n">datetime</span>
<span class="n">ts</span> <span class="o">=</span> <span class="n">datetime</span><span class="o">.</span><span class="n">strptime</span><span class="p">(</span><span class="s1">'10:13:15 2006-03-07'</span><span class="p">,</span>
<span class="s1">'%H:%M:%S %Y-%m-</span><span class="si">%d</span><span class="s1">'</span><span class="p">)</span>
</pre></div>
</div>
</li>
<li><p>The <code class="xref py py-meth docutils literal notranslate"><span class="pre">SequenceMatcher.get_matching_blocks()</span></code> method in the <a class="reference internal" href="../library/difflib.html#module-difflib" title="difflib: Helpers for computing differences between objects."><code class="xref py py-mod docutils literal notranslate"><span class="pre">difflib</span></code></a>
module now guarantees to return a minimal list of blocks describing matching
subsequences. Previously, the algorithm would occasionally break a block of
matching elements into two list entries. (Enhancement by Tim Peters.)</p></li>
<li><p>The <a class="reference internal" href="../library/doctest.html#module-doctest" title="doctest: Test pieces of code within docstrings."><code class="xref py py-mod docutils literal notranslate"><span class="pre">doctest</span></code></a> module gained a <code class="docutils literal notranslate"><span class="pre">SKIP</span></code> option that keeps an example from
being executed at all. This is intended for code snippets that are usage
examples intended for the reader and aren’t actually test cases.</p>
<p>An <em>encoding</em> parameter was added to the <code class="xref py py-func docutils literal notranslate"><span class="pre">testfile()</span></code> function and the
<code class="xref py py-class docutils literal notranslate"><span class="pre">DocFileSuite</span></code> class to specify the file’s encoding. This makes it
easier to use non-ASCII characters in tests contained within a docstring.
(Contributed by Bjorn Tillenius.)</p>
</li>
<li><p>The <a class="reference internal" href="../library/email.html#module-email" title="email: Package supporting the parsing, manipulating, and generating email messages."><code class="xref py py-mod docutils literal notranslate"><span class="pre">email</span></code></a> package has been updated to version 4.0. (Contributed by
Barry Warsaw.)</p>
</li>
<li id="index-18"><p>The <a class="reference internal" href="../library/fileinput.html#module-fileinput" title="fileinput: Loop over standard input or a list of files."><code class="xref py py-mod docutils literal notranslate"><span class="pre">fileinput</span></code></a> module was made more flexible. Unicode filenames are now
supported, and a <em>mode</em> parameter that defaults to <code class="docutils literal notranslate"><span class="pre">"r"</span></code> was added to the
<a class="reference internal" href="../library/functions.html#input" title="input"><code class="xref py py-func docutils literal notranslate"><span class="pre">input()</span></code></a> function to allow opening files in binary or <a class="reference internal" href="../glossary.html#term-universal-newlines"><span class="xref std std-term">universal
newlines</span></a> mode. Another new parameter, <em>openhook</em>, lets you use a function
other than <a class="reference internal" href="../library/functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate"><span class="pre">open()</span></code></a> to open the input files. Once you’re iterating over
the set of files, the <code class="xref py py-class docutils literal notranslate"><span class="pre">FileInput</span></code> object’s new <code class="xref py py-meth docutils literal notranslate"><span class="pre">fileno()</span></code> returns
the file descriptor for the currently opened file. (Contributed by Georg
Brandl.)</p></li>
<li><p>In the <a class="reference internal" href="../library/gc.html#module-gc" title="gc: Interface to the cycle-detecting garbage collector."><code class="xref py py-mod docutils literal notranslate"><span class="pre">gc</span></code></a> module, the new <code class="xref py py-func docutils literal notranslate"><span class="pre">get_count()</span></code> function returns a 3-tuple
containing the current collection counts for the three GC generations. This is
accounting information for the garbage collector; when these counts reach a
specified threshold, a garbage collection sweep will be made. The existing
<a class="reference internal" href="../library/gc.html#gc.collect" title="gc.collect"><code class="xref py py-func docutils literal notranslate"><span class="pre">gc.collect()</span></code></a> function now takes an optional <em>generation</em> argument of 0, 1,
or 2 to specify which generation to collect. (Contributed by Barry Warsaw.)</p></li>
<li><p>The <code class="xref py py-func docutils literal notranslate"><span class="pre">nsmallest()</span></code> and <code class="xref py py-func docutils literal notranslate"><span class="pre">nlargest()</span></code> functions in the <a class="reference internal" href="../library/heapq.html#module-heapq" title="heapq: Heap queue algorithm (a.k.a. priority queue)."><code class="xref py py-mod docutils literal notranslate"><span class="pre">heapq</span></code></a>
module now support a <code class="docutils literal notranslate"><span class="pre">key</span></code> keyword parameter similar to the one provided by
the <a class="reference internal" href="../library/functions.html#min" title="min"><code class="xref py py-func docutils literal notranslate"><span class="pre">min()</span></code></a>/<a class="reference internal" href="../library/functions.html#max" title="max"><code class="xref py py-func docutils literal notranslate"><span class="pre">max()</span></code></a> functions and the <code class="xref py py-meth docutils literal notranslate"><span class="pre">sort()</span></code> methods. For
example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">heapq</span>
<span class="gp">>>> </span><span class="n">L</span> <span class="o">=</span> <span class="p">[</span><span class="s2">"short"</span><span class="p">,</span> <span class="s1">'medium'</span><span class="p">,</span> <span class="s1">'longest'</span><span class="p">,</span> <span class="s1">'longer still'</span><span class="p">]</span>
<span class="gp">>>> </span><span class="n">heapq</span><span class="o">.</span><span class="n">nsmallest</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="n">L</span><span class="p">)</span> <span class="c1"># Return two lowest elements, lexicographically</span>
<span class="go">['longer still', 'longest']</span>
<span class="gp">>>> </span><span class="n">heapq</span><span class="o">.</span><span class="n">nsmallest</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="n">L</span><span class="p">,</span> <span class="n">key</span><span class="o">=</span><span class="nb">len</span><span class="p">)</span> <span class="c1"># Return two shortest elements</span>
<span class="go">['short', 'medium']</span>
</pre></div>
</div>
<p>(Contributed by Raymond Hettinger.)</p>
</li>
<li><p>The <a class="reference internal" href="../library/itertools.html#itertools.islice" title="itertools.islice"><code class="xref py py-func docutils literal notranslate"><span class="pre">itertools.islice()</span></code></a> function now accepts <code class="docutils literal notranslate"><span class="pre">None</span></code> for the start and
step arguments. This makes it more compatible with the attributes of slice
objects, so that you can now write the following:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">s</span> <span class="o">=</span> <span class="nb">slice</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span> <span class="c1"># Create slice object</span>
<span class="n">itertools</span><span class="o">.</span><span class="n">islice</span><span class="p">(</span><span class="n">iterable</span><span class="p">,</span> <span class="n">s</span><span class="o">.</span><span class="n">start</span><span class="p">,</span> <span class="n">s</span><span class="o">.</span><span class="n">stop</span><span class="p">,</span> <span class="n">s</span><span class="o">.</span><span class="n">step</span><span class="p">)</span>
</pre></div>
</div>
<p>(Contributed by Raymond Hettinger.)</p>
</li>
<li><p>The <a class="reference internal" href="../library/functions.html#format" title="format"><code class="xref py py-func docutils literal notranslate"><span class="pre">format()</span></code></a> function in the <a class="reference internal" href="../library/locale.html#module-locale" title="locale: Internationalization services."><code class="xref py py-mod docutils literal notranslate"><span class="pre">locale</span></code></a> module has been modified and
two new functions were added, <code class="xref py py-func docutils literal notranslate"><span class="pre">format_string()</span></code> and <code class="xref py py-func docutils literal notranslate"><span class="pre">currency()</span></code>.</p>
<p>The <a class="reference internal" href="../library/functions.html#format" title="format"><code class="xref py py-func docutils literal notranslate"><span class="pre">format()</span></code></a> function’s <em>val</em> parameter could previously be a string as
long as no more than one %char specifier appeared; now the parameter must be
exactly one %char specifier with no surrounding text. An optional <em>monetary</em>
parameter was also added which, if <code class="docutils literal notranslate"><span class="pre">True</span></code>, will use the locale’s rules for
formatting currency in placing a separator between groups of three digits.</p>
<p>To format strings with multiple %char specifiers, use the new
<code class="xref py py-func docutils literal notranslate"><span class="pre">format_string()</span></code> function that works like <a class="reference internal" href="../library/functions.html#format" title="format"><code class="xref py py-func docutils literal notranslate"><span class="pre">format()</span></code></a> but also supports
mixing %char specifiers with arbitrary text.</p>
<p>A new <code class="xref py py-func docutils literal notranslate"><span class="pre">currency()</span></code> function was also added that formats a number according
to the current locale’s settings.</p>
<p>(Contributed by Georg Brandl.)</p>
</li>
<li><p>The <a class="reference internal" href="../library/mailbox.html#module-mailbox" title="mailbox: Manipulate mailboxes in various formats"><code class="xref py py-mod docutils literal notranslate"><span class="pre">mailbox</span></code></a> module underwent a massive rewrite to add the capability to
modify mailboxes in addition to reading them. A new set of classes that include
<code class="xref py py-class docutils literal notranslate"><span class="pre">mbox</span></code>, <code class="xref py py-class docutils literal notranslate"><span class="pre">MH</span></code>, and <code class="xref py py-class docutils literal notranslate"><span class="pre">Maildir</span></code> are used to read mailboxes, and
have an <code class="docutils literal notranslate"><span class="pre">add(message)</span></code> method to add messages, <code class="docutils literal notranslate"><span class="pre">remove(key)</span></code> to
remove messages, and <code class="xref py py-meth docutils literal notranslate"><span class="pre">lock()</span></code>/<code class="xref py py-meth docutils literal notranslate"><span class="pre">unlock()</span></code> to lock/unlock the mailbox.
The following example converts a maildir-format mailbox into an mbox-format
one:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">mailbox</span>
<span class="c1"># 'factory=None' uses email.Message.Message as the class representing</span>
<span class="c1"># individual messages.</span>
<span class="n">src</span> <span class="o">=</span> <span class="n">mailbox</span><span class="o">.</span><span class="n">Maildir</span><span class="p">(</span><span class="s1">'maildir'</span><span class="p">,</span> <span class="n">factory</span><span class="o">=</span><span class="kc">None</span><span class="p">)</span>
<span class="n">dest</span> <span class="o">=</span> <span class="n">mailbox</span><span class="o">.</span><span class="n">mbox</span><span class="p">(</span><span class="s1">'/tmp/mbox'</span><span class="p">)</span>
<span class="k">for</span> <span class="n">msg</span> <span class="ow">in</span> <span class="n">src</span><span class="p">:</span>
<span class="n">dest</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">msg</span><span class="p">)</span>
</pre></div>
</div>
<p>(Contributed by Gregory K. Johnson. Funding was provided by Google’s 2005
Summer of Code.)</p>
</li>
<li><p>New module: the <a class="reference internal" href="../library/msilib.html#module-msilib" title="msilib: Creation of Microsoft Installer files, and CAB files. (Windows)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">msilib</span></code></a> module allows creating Microsoft Installer
<code class="file docutils literal notranslate"><span class="pre">.msi</span></code> files and CAB files. Some support for reading the <code class="file docutils literal notranslate"><span class="pre">.msi</span></code>
database is also included. (Contributed by Martin von Löwis.)</p></li>
<li><p>The <a class="reference internal" href="../library/nis.html#module-nis" title="nis: Interface to Sun's NIS (Yellow Pages) library. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">nis</span></code></a> module now supports accessing domains other than the system
default domain by supplying a <em>domain</em> argument to the <a class="reference internal" href="../library/nis.html#nis.match" title="nis.match"><code class="xref py py-func docutils literal notranslate"><span class="pre">nis.match()</span></code></a> and
<a class="reference internal" href="../library/nis.html#nis.maps" title="nis.maps"><code class="xref py py-func docutils literal notranslate"><span class="pre">nis.maps()</span></code></a> functions. (Contributed by Ben Bell.)</p></li>
<li><p>The <a class="reference internal" href="../library/operator.html#module-operator" title="operator: Functions corresponding to the standard operators."><code class="xref py py-mod docutils literal notranslate"><span class="pre">operator</span></code></a> module’s <code class="xref py py-func docutils literal notranslate"><span class="pre">itemgetter()</span></code> and <code class="xref py py-func docutils literal notranslate"><span class="pre">attrgetter()</span></code>
functions now support multiple fields. A call such as
<code class="docutils literal notranslate"><span class="pre">operator.attrgetter('a',</span> <span class="pre">'b')</span></code> will return a function that retrieves the
<code class="xref py py-attr docutils literal notranslate"><span class="pre">a</span></code> and <code class="xref py py-attr docutils literal notranslate"><span class="pre">b</span></code> attributes. Combining this new feature with the
<code class="xref py py-meth docutils literal notranslate"><span class="pre">sort()</span></code> method’s <code class="docutils literal notranslate"><span class="pre">key</span></code> parameter lets you easily sort lists using
multiple fields. (Contributed by Raymond Hettinger.)</p></li>
<li><p>The <a class="reference internal" href="../library/optparse.html#module-optparse" title="optparse: Command-line option parsing library. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">optparse</span></code></a> module was updated to version 1.5.1 of the Optik library.
The <code class="xref py py-class docutils literal notranslate"><span class="pre">OptionParser</span></code> class gained an <code class="xref py py-attr docutils literal notranslate"><span class="pre">epilog</span></code> attribute, a string
that will be printed after the help message, and a <code class="xref py py-meth docutils literal notranslate"><span class="pre">destroy()</span></code> method to
break reference cycles created by the object. (Contributed by Greg Ward.)</p></li>
<li><p>The <a class="reference internal" href="../library/os.html#module-os" title="os: Miscellaneous operating system interfaces."><code class="xref py py-mod docutils literal notranslate"><span class="pre">os</span></code></a> module underwent several changes. The <code class="xref py py-attr docutils literal notranslate"><span class="pre">stat_float_times</span></code>
variable now defaults to true, meaning that <a class="reference internal" href="../library/os.html#os.stat" title="os.stat"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.stat()</span></code></a> will now return time
values as floats. (This doesn’t necessarily mean that <a class="reference internal" href="../library/os.html#os.stat" title="os.stat"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.stat()</span></code></a> will
return times that are precise to fractions of a second; not all systems support
such precision.)</p>
<p>Constants named <a class="reference internal" href="../library/os.html#os.SEEK_SET" title="os.SEEK_SET"><code class="xref py py-attr docutils literal notranslate"><span class="pre">os.SEEK_SET</span></code></a>, <a class="reference internal" href="../library/os.html#os.SEEK_CUR" title="os.SEEK_CUR"><code class="xref py py-attr docutils literal notranslate"><span class="pre">os.SEEK_CUR</span></code></a>, and
<a class="reference internal" href="../library/os.html#os.SEEK_END" title="os.SEEK_END"><code class="xref py py-attr docutils literal notranslate"><span class="pre">os.SEEK_END</span></code></a> have been added; these are the parameters to the
<a class="reference internal" href="../library/os.html#os.lseek" title="os.lseek"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.lseek()</span></code></a> function. Two new constants for locking are
<a class="reference internal" href="../library/os.html#os.O_SHLOCK" title="os.O_SHLOCK"><code class="xref py py-attr docutils literal notranslate"><span class="pre">os.O_SHLOCK</span></code></a> and <a class="reference internal" href="../library/os.html#os.O_EXLOCK" title="os.O_EXLOCK"><code class="xref py py-attr docutils literal notranslate"><span class="pre">os.O_EXLOCK</span></code></a>.</p>
<p>Two new functions, <code class="xref py py-func docutils literal notranslate"><span class="pre">wait3()</span></code> and <code class="xref py py-func docutils literal notranslate"><span class="pre">wait4()</span></code>, were added. They’re similar
the <code class="xref py py-func docutils literal notranslate"><span class="pre">waitpid()</span></code> function which waits for a child process to exit and returns
a tuple of the process ID and its exit status, but <code class="xref py py-func docutils literal notranslate"><span class="pre">wait3()</span></code> and
<code class="xref py py-func docutils literal notranslate"><span class="pre">wait4()</span></code> return additional information. <code class="xref py py-func docutils literal notranslate"><span class="pre">wait3()</span></code> doesn’t take a
process ID as input, so it waits for any child process to exit and returns a
3-tuple of <em>process-id</em>, <em>exit-status</em>, <em>resource-usage</em> as returned from the
<a class="reference internal" href="../library/resource.html#resource.getrusage" title="resource.getrusage"><code class="xref py py-func docutils literal notranslate"><span class="pre">resource.getrusage()</span></code></a> function. <code class="docutils literal notranslate"><span class="pre">wait4(pid)</span></code> does take a process ID.
(Contributed by Chad J. Schroeder.)</p>
<p>On FreeBSD, the <a class="reference internal" href="../library/os.html#os.stat" title="os.stat"><code class="xref py py-func docutils literal notranslate"><span class="pre">os.stat()</span></code></a> function now returns times with nanosecond
resolution, and the returned object now has <code class="xref py py-attr docutils literal notranslate"><span class="pre">st_gen</span></code> and
<code class="xref py py-attr docutils literal notranslate"><span class="pre">st_birthtime</span></code>. The <code class="xref py py-attr docutils literal notranslate"><span class="pre">st_flags</span></code> attribute is also available, if the
platform supports it. (Contributed by Antti Louko and Diego Pettenò.)</p>
</li>
<li><p>The Python debugger provided by the <a class="reference internal" href="../library/pdb.html#module-pdb" title="pdb: The Python debugger for interactive interpreters."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pdb</span></code></a> module can now store lists of
commands to execute when a breakpoint is reached and execution stops. Once
breakpoint #1 has been created, enter <code class="docutils literal notranslate"><span class="pre">commands</span> <span class="pre">1</span></code> and enter a series of
commands to be executed, finishing the list with <code class="docutils literal notranslate"><span class="pre">end</span></code>. The command list can
include commands that resume execution, such as <code class="docutils literal notranslate"><span class="pre">continue</span></code> or <code class="docutils literal notranslate"><span class="pre">next</span></code>.
(Contributed by Grégoire Dooms.)</p>
</li>
<li><p>The <a class="reference internal" href="../library/pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pickle</span></code></a> and <code class="xref py py-mod docutils literal notranslate"><span class="pre">cPickle</span></code> modules no longer accept a return value
of <code class="docutils literal notranslate"><span class="pre">None</span></code> from the <a class="reference internal" href="../library/pickle.html#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__reduce__()</span></code></a> method; the method must return a tuple
of arguments instead. The ability to return <code class="docutils literal notranslate"><span class="pre">None</span></code> was deprecated in Python
2.4, so this completes the removal of the feature.</p></li>
<li><p>The <a class="reference internal" href="../library/pkgutil.html#module-pkgutil" title="pkgutil: Utilities for the import system."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pkgutil</span></code></a> module, containing various utility functions for finding
packages, was enhanced to support PEP 302’s import hooks and now also works for
packages stored in ZIP-format archives. (Contributed by Phillip J. Eby.)</p></li>
<li><p>The pybench benchmark suite by Marc-André Lemburg is now included in the
<code class="file docutils literal notranslate"><span class="pre">Tools/pybench</span></code> directory. The pybench suite is an improvement on the
commonly used <code class="file docutils literal notranslate"><span class="pre">pystone.py</span></code> program because pybench provides a more
detailed measurement of the interpreter’s speed. It times particular operations
such as function calls, tuple slicing, method lookups, and numeric operations,
instead of performing many different operations and reducing the result to a
single number as <code class="file docutils literal notranslate"><span class="pre">pystone.py</span></code> does.</p></li>
<li><p>The <code class="xref py py-mod docutils literal notranslate"><span class="pre">pyexpat</span></code> module now uses version 2.0 of the Expat parser.
(Contributed by Trent Mick.)</p></li>
<li><p>The <a class="reference internal" href="../library/queue.html#queue.Queue" title="queue.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">Queue</span></code></a> class provided by the <code class="xref py py-mod docutils literal notranslate"><span class="pre">Queue</span></code> module gained two new
methods. <code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code> blocks until all items in the queue have been retrieved
and all processing work on the items have been completed. Worker threads call
the other new method, <code class="xref py py-meth docutils literal notranslate"><span class="pre">task_done()</span></code>, to signal that processing for an item
has been completed. (Contributed by Raymond Hettinger.)</p></li>
<li><p>The old <code class="xref py py-mod docutils literal notranslate"><span class="pre">regex</span></code> and <code class="xref py py-mod docutils literal notranslate"><span class="pre">regsub</span></code> modules, which have been deprecated
ever since Python 2.0, have finally been deleted. Other deleted modules:
<code class="xref py py-mod docutils literal notranslate"><span class="pre">statcache</span></code>, <code class="xref py py-mod docutils literal notranslate"><span class="pre">tzparse</span></code>, <code class="xref py py-mod docutils literal notranslate"><span class="pre">whrandom</span></code>.</p></li>
<li><p>Also deleted: the <code class="file docutils literal notranslate"><span class="pre">lib-old</span></code> directory, which includes ancient modules
such as <code class="xref py py-mod docutils literal notranslate"><span class="pre">dircmp</span></code> and <code class="xref py py-mod docutils literal notranslate"><span class="pre">ni</span></code>, was removed. <code class="file docutils literal notranslate"><span class="pre">lib-old</span></code> wasn’t on the
default <code class="docutils literal notranslate"><span class="pre">sys.path</span></code>, so unless your programs explicitly added the directory to
<code class="docutils literal notranslate"><span class="pre">sys.path</span></code>, this removal shouldn’t affect your code.</p></li>
<li><p>The <a class="reference internal" href="../library/rlcompleter.html#module-rlcompleter" title="rlcompleter: Python identifier completion, suitable for the GNU readline library."><code class="xref py py-mod docutils literal notranslate"><span class="pre">rlcompleter</span></code></a> module is no longer dependent on importing the
<a class="reference internal" href="../library/readline.html#module-readline" title="readline: GNU readline support for Python. (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">readline</span></code></a> module and therefore now works on non-Unix platforms. (Patch
from Robert Kiendl.)</p>
</li>
<li><p>The <code class="xref py py-mod docutils literal notranslate"><span class="pre">SimpleXMLRPCServer</span></code> and <code class="xref py py-mod docutils literal notranslate"><span class="pre">DocXMLRPCServer</span></code> classes now have a
<code class="xref py py-attr docutils literal notranslate"><span class="pre">rpc_paths</span></code> attribute that constrains XML-RPC operations to a limited set
of URL paths; the default is to allow only <code class="docutils literal notranslate"><span class="pre">'/'</span></code> and <code class="docutils literal notranslate"><span class="pre">'/RPC2'</span></code>. Setting
<code class="xref py py-attr docutils literal notranslate"><span class="pre">rpc_paths</span></code> to <code class="docutils literal notranslate"><span class="pre">None</span></code> or an empty tuple disables this path checking.</p>
</li>
<li><p>The <a class="reference internal" href="../library/socket.html#module-socket" title="socket: Low-level networking interface."><code class="xref py py-mod docutils literal notranslate"><span class="pre">socket</span></code></a> module now supports <code class="xref py py-const docutils literal notranslate"><span class="pre">AF_NETLINK</span></code> sockets on Linux,
thanks to a patch from Philippe Biondi. Netlink sockets are a Linux-specific
mechanism for communications between a user-space process and kernel code; an
introductory article about them is at <a class="reference external" href="https://www.linuxjournal.com/article/7356">https://www.linuxjournal.com/article/7356</a>.
In Python code, netlink addresses are represented as a tuple of 2 integers,
<code class="docutils literal notranslate"><span class="pre">(pid,</span> <span class="pre">group_mask)</span></code>.</p>
<p>Two new methods on socket objects, <code class="docutils literal notranslate"><span class="pre">recv_into(buffer)</span></code> and
<code class="docutils literal notranslate"><span class="pre">recvfrom_into(buffer)</span></code>, store the received data in an object that
supports the buffer protocol instead of returning the data as a string. This
means you can put the data directly into an array or a memory-mapped file.</p>
<p>Socket objects also gained <code class="xref py py-meth docutils literal notranslate"><span class="pre">getfamily()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">gettype()</span></code>, and
<code class="xref py py-meth docutils literal notranslate"><span class="pre">getproto()</span></code> accessor methods to retrieve the family, type, and protocol
values for the socket.</p>
</li>
<li><p>New module: the <a class="reference internal" href="../library/spwd.html#module-spwd" title="spwd: The shadow password database (getspnam() and friends). (Unix)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">spwd</span></code></a> module provides functions for accessing the shadow
password database on systems that support shadow passwords.</p></li>
<li><p>The <a class="reference internal" href="../library/struct.html#module-struct" title="struct: Interpret bytes as packed binary data."><code class="xref py py-mod docutils literal notranslate"><span class="pre">struct</span></code></a> is now faster because it compiles format strings into
<code class="xref py py-class docutils literal notranslate"><span class="pre">Struct</span></code> objects with <code class="xref py py-meth docutils literal notranslate"><span class="pre">pack()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">unpack()</span></code> methods. This is
similar to how the <a class="reference internal" href="../library/re.html#module-re" title="re: Regular expression operations."><code class="xref py py-mod docutils literal notranslate"><span class="pre">re</span></code></a> module lets you create compiled regular expression
objects. You can still use the module-level <code class="xref py py-func docutils literal notranslate"><span class="pre">pack()</span></code> and <code class="xref py py-func docutils literal notranslate"><span class="pre">unpack()</span></code>
functions; they’ll create <code class="xref py py-class docutils literal notranslate"><span class="pre">Struct</span></code> objects and cache them. Or you can
use <code class="xref py py-class docutils literal notranslate"><span class="pre">Struct</span></code> instances directly:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">s</span> <span class="o">=</span> <span class="n">struct</span><span class="o">.</span><span class="n">Struct</span><span class="p">(</span><span class="s1">'ih3s'</span><span class="p">)</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">s</span><span class="o">.</span><span class="n">pack</span><span class="p">(</span><span class="mi">1972</span><span class="p">,</span> <span class="mi">187</span><span class="p">,</span> <span class="s1">'abc'</span><span class="p">)</span>
<span class="n">year</span><span class="p">,</span> <span class="n">number</span><span class="p">,</span> <span class="n">name</span> <span class="o">=</span> <span class="n">s</span><span class="o">.</span><span class="n">unpack</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
</pre></div>
</div>
<p>You can also pack and unpack data to and from buffer objects directly using the
<code class="docutils literal notranslate"><span class="pre">pack_into(buffer,</span> <span class="pre">offset,</span> <span class="pre">v1,</span> <span class="pre">v2,</span> <span class="pre">...)</span></code> and <code class="docutils literal notranslate"><span class="pre">unpack_from(buffer,</span>
<span class="pre">offset)</span></code> methods. This lets you store data directly into an array or a
memory-mapped file.</p>
<p>(<code class="xref py py-class docutils literal notranslate"><span class="pre">Struct</span></code> objects were implemented by Bob Ippolito at the NeedForSpeed
sprint. Support for buffer objects was added by Martin Blais, also at the
NeedForSpeed sprint.)</p>
</li>
<li><p>The Python developers switched from CVS to Subversion during the 2.5
development process. Information about the exact build version is available as
the <code class="docutils literal notranslate"><span class="pre">sys.subversion</span></code> variable, a 3-tuple of <code class="docutils literal notranslate"><span class="pre">(interpreter-name,</span> <span class="pre">branch-name,</span>
<span class="pre">revision-range)</span></code>. For example, at the time of writing my copy of 2.5 was
reporting <code class="docutils literal notranslate"><span class="pre">('CPython',</span> <span class="pre">'trunk',</span> <span class="pre">'45313:45315')</span></code>.</p>
<p>This information is also available to C extensions via the
<a class="reference internal" href="../c-api/init.html#c.Py_GetBuildInfo" title="Py_GetBuildInfo"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_GetBuildInfo()</span></code></a> function that returns a string of build information
like this: <code class="docutils literal notranslate"><span class="pre">"trunk:45355:45356M,</span> <span class="pre">Apr</span> <span class="pre">13</span> <span class="pre">2006,</span> <span class="pre">07:42:19"</span></code>. (Contributed by
Barry Warsaw.)</p>
</li>
<li><p>Another new function, <a class="reference internal" href="../library/sys.html#sys._current_frames" title="sys._current_frames"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys._current_frames()</span></code></a>, returns the current stack
frames for all running threads as a dictionary mapping thread identifiers to the
topmost stack frame currently active in that thread at the time the function is
called. (Contributed by Tim Peters.)</p></li>
<li><p>The <code class="xref py py-class docutils literal notranslate"><span class="pre">TarFile</span></code> class in the <a class="reference internal" href="../library/tarfile.html#module-tarfile" title="tarfile: Read and write tar-format archive files."><code class="xref py py-mod docutils literal notranslate"><span class="pre">tarfile</span></code></a> module now has an
<code class="xref py py-meth docutils literal notranslate"><span class="pre">extractall()</span></code> method that extracts all members from the archive into the
current working directory. It’s also possible to set a different directory as
the extraction target, and to unpack only a subset of the archive’s members.</p>
<p>The compression used for a tarfile opened in stream mode can now be autodetected
using the mode <code class="docutils literal notranslate"><span class="pre">'r|*'</span></code>. (Contributed by Lars Gustäbel.)</p>
</li>
<li><p>The <a class="reference internal" href="../library/threading.html#module-threading" title="threading: Thread-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">threading</span></code></a> module now lets you set the stack size used when new
threads are created. The <code class="docutils literal notranslate"><span class="pre">stack_size([*size*])</span></code> function returns the
currently configured stack size, and supplying the optional <em>size</em> parameter
sets a new value. Not all platforms support changing the stack size, but
Windows, POSIX threading, and OS/2 all do. (Contributed by Andrew MacIntyre.)</p>
</li>
<li><p>The <a class="reference internal" href="../library/unicodedata.html#module-unicodedata" title="unicodedata: Access the Unicode Database."><code class="xref py py-mod docutils literal notranslate"><span class="pre">unicodedata</span></code></a> module has been updated to use version 4.1.0 of the
Unicode character database. Version 3.2.0 is required by some specifications,
so it’s still available as <a class="reference internal" href="../library/unicodedata.html#unicodedata.ucd_3_2_0" title="unicodedata.ucd_3_2_0"><code class="xref py py-attr docutils literal notranslate"><span class="pre">unicodedata.ucd_3_2_0</span></code></a>.</p></li>
<li><p>New module: the <a class="reference internal" href="../library/uuid.html#module-uuid" title="uuid: UUID objects (universally unique identifiers) according to RFC 4122"><code class="xref py py-mod docutils literal notranslate"><span class="pre">uuid</span></code></a> module generates universally unique identifiers
(UUIDs) according to <span class="target" id="index-19"></span><a class="rfc reference external" href="https://tools.ietf.org/html/rfc4122.html"><strong>RFC 4122</strong></a>. The RFC defines several different UUID
versions that are generated from a starting string, from system properties, or
purely randomly. This module contains a <code class="xref py py-class docutils literal notranslate"><span class="pre">UUID</span></code> class and functions
named <code class="xref py py-func docutils literal notranslate"><span class="pre">uuid1()</span></code>, <code class="xref py py-func docutils literal notranslate"><span class="pre">uuid3()</span></code>, <code class="xref py py-func docutils literal notranslate"><span class="pre">uuid4()</span></code>, and <code class="xref py py-func docutils literal notranslate"><span class="pre">uuid5()</span></code> to
generate different versions of UUID. (Version 2 UUIDs are not specified in
<span class="target" id="index-20"></span><a class="rfc reference external" href="https://tools.ietf.org/html/rfc4122.html"><strong>RFC 4122</strong></a> and are not supported by this module.)</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">uuid</span>
<span class="gp">>>> </span><span class="c1"># make a UUID based on the host ID and current time</span>
<span class="gp">>>> </span><span class="n">uuid</span><span class="o">.</span><span class="n">uuid1</span><span class="p">()</span>
<span class="go">UUID('a8098c1a-f86e-11da-bd1a-00112444be1e')</span>
<span class="gp">>>> </span><span class="c1"># make a UUID using an MD5 hash of a namespace UUID and a name</span>
<span class="gp">>>> </span><span class="n">uuid</span><span class="o">.</span><span class="n">uuid3</span><span class="p">(</span><span class="n">uuid</span><span class="o">.</span><span class="n">NAMESPACE_DNS</span><span class="p">,</span> <span class="s1">'python.org'</span><span class="p">)</span>
<span class="go">UUID('6fa459ea-ee8a-3ca4-894e-db77e160355e')</span>
<span class="gp">>>> </span><span class="c1"># make a random UUID</span>
<span class="gp">>>> </span><span class="n">uuid</span><span class="o">.</span><span class="n">uuid4</span><span class="p">()</span>
<span class="go">UUID('16fd2706-8baf-433b-82eb-8c7fada847da')</span>
<span class="gp">>>> </span><span class="c1"># make a UUID using a SHA-1 hash of a namespace UUID and a name</span>
<span class="gp">>>> </span><span class="n">uuid</span><span class="o">.</span><span class="n">uuid5</span><span class="p">(</span><span class="n">uuid</span><span class="o">.</span><span class="n">NAMESPACE_DNS</span><span class="p">,</span> <span class="s1">'python.org'</span><span class="p">)</span>
<span class="go">UUID('886313e1-3b8a-5372-9b90-0c9aee199e5d')</span>
</pre></div>
</div>
<p>(Contributed by Ka-Ping Yee.)</p>
</li>
<li><p>The <a class="reference internal" href="../library/weakref.html#module-weakref" title="weakref: Support for weak references and weak dictionaries."><code class="xref py py-mod docutils literal notranslate"><span class="pre">weakref</span></code></a> module’s <code class="xref py py-class docutils literal notranslate"><span class="pre">WeakKeyDictionary</span></code> and
<code class="xref py py-class docutils literal notranslate"><span class="pre">WeakValueDictionary</span></code> types gained new methods for iterating over the
weak references contained in the dictionary. <code class="xref py py-meth docutils literal notranslate"><span class="pre">iterkeyrefs()</span></code> and
<code class="xref py py-meth docutils literal notranslate"><span class="pre">keyrefs()</span></code> methods were added to <code class="xref py py-class docutils literal notranslate"><span class="pre">WeakKeyDictionary</span></code>, and
<code class="xref py py-meth docutils literal notranslate"><span class="pre">itervaluerefs()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">valuerefs()</span></code> were added to
<code class="xref py py-class docutils literal notranslate"><span class="pre">WeakValueDictionary</span></code>. (Contributed by Fred L. Drake, Jr.)</p></li>
<li><p>The <a class="reference internal" href="../library/webbrowser.html#module-webbrowser" title="webbrowser: Easy-to-use controller for Web browsers."><code class="xref py py-mod docutils literal notranslate"><span class="pre">webbrowser</span></code></a> module received a number of enhancements. It’s now
usable as a script with <code class="docutils literal notranslate"><span class="pre">python</span> <span class="pre">-m</span> <span class="pre">webbrowser</span></code>, taking a URL as the argument;
there are a number of switches to control the behaviour (<code class="xref std std-option docutils literal notranslate"><span class="pre">-n</span></code> for a new
browser window, <code class="xref std std-option docutils literal notranslate"><span class="pre">-t</span></code> for a new tab). New module-level functions,
<code class="xref py py-func docutils literal notranslate"><span class="pre">open_new()</span></code> and <code class="xref py py-func docutils literal notranslate"><span class="pre">open_new_tab()</span></code>, were added to support this. The
module’s <a class="reference internal" href="../library/functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate"><span class="pre">open()</span></code></a> function supports an additional feature, an <em>autoraise</em>
parameter that signals whether to raise the open window when possible. A number
of additional browsers were added to the supported list such as Firefox, Opera,
Konqueror, and elinks. (Contributed by Oleg Broytmann and Georg Brandl.)</p>
</li>
<li><p>The <code class="xref py py-mod docutils literal notranslate"><span class="pre">xmlrpclib</span></code> module now supports returning <a class="reference internal" href="../library/datetime.html#datetime.datetime" title="datetime.datetime"><code class="xref py py-class docutils literal notranslate"><span class="pre">datetime</span></code></a> objects
for the XML-RPC date type. Supply <code class="docutils literal notranslate"><span class="pre">use_datetime=True</span></code> to the <code class="xref py py-func docutils literal notranslate"><span class="pre">loads()</span></code>
function or the <code class="xref py py-class docutils literal notranslate"><span class="pre">Unmarshaller</span></code> class to enable this feature. (Contributed
by Skip Montanaro.)</p>
</li>
<li><p>The <a class="reference internal" href="../library/zipfile.html#module-zipfile" title="zipfile: Read and write ZIP-format archive files."><code class="xref py py-mod docutils literal notranslate"><span class="pre">zipfile</span></code></a> module now supports the ZIP64 version of the format,
meaning that a .zip archive can now be larger than 4 GiB and can contain
individual files larger than 4 GiB. (Contributed by Ronald Oussoren.)</p>
</li>
<li><p>The <a class="reference internal" href="../library/zlib.html#module-zlib" title="zlib: Low-level interface to compression and decompression routines compatible with gzip."><code class="xref py py-mod docutils literal notranslate"><span class="pre">zlib</span></code></a> module’s <code class="xref py py-class docutils literal notranslate"><span class="pre">Compress</span></code> and <code class="xref py py-class docutils literal notranslate"><span class="pre">Decompress</span></code> objects now
support a <a class="reference internal" href="../library/copy.html#module-copy" title="copy: Shallow and deep copy operations."><code class="xref py py-meth docutils literal notranslate"><span class="pre">copy()</span></code></a> method that makes a copy of the object’s internal state
and returns a new <code class="xref py py-class docutils literal notranslate"><span class="pre">Compress</span></code> or <code class="xref py py-class docutils literal notranslate"><span class="pre">Decompress</span></code> object.
(Contributed by Chris AtLee.)</p>
</li>
</ul>
<div class="section" id="the-ctypes-package">
<span id="module-ctypes"></span><h3>The ctypes package<a class="headerlink" href="#the-ctypes-package" title="Permalink to this headline">¶</a></h3>
<p>The <a class="reference internal" href="../library/ctypes.html#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate"><span class="pre">ctypes</span></code></a> package, written by Thomas Heller, has been added to the
standard library. <a class="reference internal" href="../library/ctypes.html#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate"><span class="pre">ctypes</span></code></a> lets you call arbitrary functions in shared
libraries or DLLs. Long-time users may remember the <code class="xref py py-mod docutils literal notranslate"><span class="pre">dl</span></code> module, which
provides functions for loading shared libraries and calling functions in them.
The <a class="reference internal" href="../library/ctypes.html#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate"><span class="pre">ctypes</span></code></a> package is much fancier.</p>
<p>To load a shared library or DLL, you must create an instance of the
<code class="xref py py-class docutils literal notranslate"><span class="pre">CDLL</span></code> class and provide the name or path of the shared library or DLL.
Once that’s done, you can call arbitrary functions by accessing them as
attributes of the <code class="xref py py-class docutils literal notranslate"><span class="pre">CDLL</span></code> object.</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">ctypes</span>
<span class="n">libc</span> <span class="o">=</span> <span class="n">ctypes</span><span class="o">.</span><span class="n">CDLL</span><span class="p">(</span><span class="s1">'libc.so.6'</span><span class="p">)</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">libc</span><span class="o">.</span><span class="n">printf</span><span class="p">(</span><span class="s2">"Line of output</span><span class="se">\n</span><span class="s2">"</span><span class="p">)</span>
</pre></div>
</div>
<p>Type constructors for the various C types are provided: <code class="xref py py-func docutils literal notranslate"><span class="pre">c_int()</span></code>,
<code class="xref py py-func docutils literal notranslate"><span class="pre">c_float()</span></code>, <code class="xref py py-func docutils literal notranslate"><span class="pre">c_double()</span></code>, <code class="xref py py-func docutils literal notranslate"><span class="pre">c_char_p()</span></code> (equivalent to <code class="xref c c-type docutils literal notranslate"><span class="pre">char</span>
<span class="pre">*</span></code>), and so forth. Unlike Python’s types, the C versions are all mutable; you
can assign to their <code class="xref py py-attr docutils literal notranslate"><span class="pre">value</span></code> attribute to change the wrapped value. Python
integers and strings will be automatically converted to the corresponding C
types, but for other types you must call the correct type constructor. (And I
mean <em>must</em>; getting it wrong will often result in the interpreter crashing
with a segmentation fault.)</p>
<p>You shouldn’t use <code class="xref py py-func docutils literal notranslate"><span class="pre">c_char_p()</span></code> with a Python string when the C function will
be modifying the memory area, because Python strings are supposed to be
immutable; breaking this rule will cause puzzling bugs. When you need a
modifiable memory area, use <code class="xref py py-func docutils literal notranslate"><span class="pre">create_string_buffer()</span></code>:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">s</span> <span class="o">=</span> <span class="s2">"this is a string"</span>
<span class="n">buf</span> <span class="o">=</span> <span class="n">ctypes</span><span class="o">.</span><span class="n">create_string_buffer</span><span class="p">(</span><span class="n">s</span><span class="p">)</span>
<span class="n">libc</span><span class="o">.</span><span class="n">strfry</span><span class="p">(</span><span class="n">buf</span><span class="p">)</span>
</pre></div>
</div>
<p>C functions are assumed to return integers, but you can set the <code class="xref py py-attr docutils literal notranslate"><span class="pre">restype</span></code>
attribute of the function object to change this:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">libc</span><span class="o">.</span><span class="n">atof</span><span class="p">(</span><span class="s1">'2.71828'</span><span class="p">)</span>
<span class="go">-1783957616</span>
<span class="gp">>>> </span><span class="n">libc</span><span class="o">.</span><span class="n">atof</span><span class="o">.</span><span class="n">restype</span> <span class="o">=</span> <span class="n">ctypes</span><span class="o">.</span><span class="n">c_double</span>
<span class="gp">>>> </span><span class="n">libc</span><span class="o">.</span><span class="n">atof</span><span class="p">(</span><span class="s1">'2.71828'</span><span class="p">)</span>
<span class="go">2.71828</span>
</pre></div>
</div>
<p><a class="reference internal" href="../library/ctypes.html#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate"><span class="pre">ctypes</span></code></a> also provides a wrapper for Python’s C API as the
<code class="docutils literal notranslate"><span class="pre">ctypes.pythonapi</span></code> object. This object does <em>not</em> release the global
interpreter lock before calling a function, because the lock must be held when
calling into the interpreter’s code. There’s a <code class="xref py py-class docutils literal notranslate"><span class="pre">py_object()</span></code> type
constructor that will create a <a class="reference internal" href="../c-api/structures.html#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal notranslate"><span class="pre">PyObject</span> <span class="pre">*</span></code></a> pointer. A simple usage:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">ctypes</span>
<span class="n">d</span> <span class="o">=</span> <span class="p">{}</span>
<span class="n">ctypes</span><span class="o">.</span><span class="n">pythonapi</span><span class="o">.</span><span class="n">PyObject_SetItem</span><span class="p">(</span><span class="n">ctypes</span><span class="o">.</span><span class="n">py_object</span><span class="p">(</span><span class="n">d</span><span class="p">),</span>
<span class="n">ctypes</span><span class="o">.</span><span class="n">py_object</span><span class="p">(</span><span class="s2">"abc"</span><span class="p">),</span> <span class="n">ctypes</span><span class="o">.</span><span class="n">py_object</span><span class="p">(</span><span class="mi">1</span><span class="p">))</span>
<span class="c1"># d is now {'abc', 1}.</span>
</pre></div>
</div>
<p>Don’t forget to use <code class="xref py py-class docutils literal notranslate"><span class="pre">py_object()</span></code>; if it’s omitted you end up with a
segmentation fault.</p>
<p><a class="reference internal" href="../library/ctypes.html#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate"><span class="pre">ctypes</span></code></a> has been around for a while, but people still write and
distribution hand-coded extension modules because you can’t rely on
<a class="reference internal" href="../library/ctypes.html#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate"><span class="pre">ctypes</span></code></a> being present. Perhaps developers will begin to write Python
wrappers atop a library accessed through <a class="reference internal" href="../library/ctypes.html#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate"><span class="pre">ctypes</span></code></a> instead of extension
modules, now that <a class="reference internal" href="../library/ctypes.html#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate"><span class="pre">ctypes</span></code></a> is included with core Python.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><a class="reference external" href="http://starship.python.net/crew/theller/ctypes/">http://starship.python.net/crew/theller/ctypes/</a></dt><dd><p>The ctypes web page, with a tutorial, reference, and FAQ.</p>
</dd>
</dl>
<p>The documentation for the <a class="reference internal" href="../library/ctypes.html#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate"><span class="pre">ctypes</span></code></a> module.</p>
</div>
</div>
<div class="section" id="the-elementtree-package">
<span id="module-etree"></span><h3>The ElementTree package<a class="headerlink" href="#the-elementtree-package" title="Permalink to this headline">¶</a></h3>
<p>A subset of Fredrik Lundh’s ElementTree library for processing XML has been
added to the standard library as <code class="xref py py-mod docutils literal notranslate"><span class="pre">xml.etree</span></code>. The available modules are
<code class="xref py py-mod docutils literal notranslate"><span class="pre">ElementTree</span></code>, <code class="xref py py-mod docutils literal notranslate"><span class="pre">ElementPath</span></code>, and <code class="xref py py-mod docutils literal notranslate"><span class="pre">ElementInclude</span></code> from
ElementTree 1.2.6. The <code class="xref py py-mod docutils literal notranslate"><span class="pre">cElementTree</span></code> accelerator module is also
included.</p>
<p>The rest of this section will provide a brief overview of using ElementTree.
Full documentation for ElementTree is available at
<a class="reference external" href="http://effbot.org/zone/element-index.htm">http://effbot.org/zone/element-index.htm</a>.</p>
<p>ElementTree represents an XML document as a tree of element nodes. The text
content of the document is stored as the <code class="xref py py-attr docutils literal notranslate"><span class="pre">text</span></code> and <code class="xref py py-attr docutils literal notranslate"><span class="pre">tail</span></code>
attributes of (This is one of the major differences between ElementTree and
the Document Object Model; in the DOM there are many different types of node,
including <code class="xref py py-class docutils literal notranslate"><span class="pre">TextNode</span></code>.)</p>
<p>The most commonly used parsing function is <code class="xref py py-func docutils literal notranslate"><span class="pre">parse()</span></code>, that takes either a
string (assumed to contain a filename) or a file-like object and returns an
<code class="xref py py-class docutils literal notranslate"><span class="pre">ElementTree</span></code> instance:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">xml.etree</span> <span class="kn">import</span> <span class="n">ElementTree</span> <span class="k">as</span> <span class="n">ET</span>
<span class="n">tree</span> <span class="o">=</span> <span class="n">ET</span><span class="o">.</span><span class="n">parse</span><span class="p">(</span><span class="s1">'ex-1.xml'</span><span class="p">)</span>
<span class="n">feed</span> <span class="o">=</span> <span class="n">urllib</span><span class="o">.</span><span class="n">urlopen</span><span class="p">(</span>
<span class="s1">'http://planet.python.org/rss10.xml'</span><span class="p">)</span>
<span class="n">tree</span> <span class="o">=</span> <span class="n">ET</span><span class="o">.</span><span class="n">parse</span><span class="p">(</span><span class="n">feed</span><span class="p">)</span>
</pre></div>
</div>
<p>Once you have an <code class="xref py py-class docutils literal notranslate"><span class="pre">ElementTree</span></code> instance, you can call its <code class="xref py py-meth docutils literal notranslate"><span class="pre">getroot()</span></code>
method to get the root <code class="xref py py-class docutils literal notranslate"><span class="pre">Element</span></code> node.</p>
<p>There’s also an <code class="xref py py-func docutils literal notranslate"><span class="pre">XML()</span></code> function that takes a string literal and returns an
<code class="xref py py-class docutils literal notranslate"><span class="pre">Element</span></code> node (not an <code class="xref py py-class docutils literal notranslate"><span class="pre">ElementTree</span></code>). This function provides a
tidy way to incorporate XML fragments, approaching the convenience of an XML
literal:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">svg</span> <span class="o">=</span> <span class="n">ET</span><span class="o">.</span><span class="n">XML</span><span class="p">(</span><span class="s2">"""<svg width="10px" version="1.0"></span>
<span class="s2"> </svg>"""</span><span class="p">)</span>
<span class="n">svg</span><span class="o">.</span><span class="n">set</span><span class="p">(</span><span class="s1">'height'</span><span class="p">,</span> <span class="s1">'320px'</span><span class="p">)</span>
<span class="n">svg</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">elem1</span><span class="p">)</span>
</pre></div>
</div>
<p>Each XML element supports some dictionary-like and some list-like access
methods. Dictionary-like operations are used to access attribute values, and
list-like operations are used to access child nodes.</p>
<table class="docutils align-default">
<colgroup>
<col style="width: 41%" />
<col style="width: 59%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Operation</p></th>
<th class="head"><p>Result</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">elem[n]</span></code></p></td>
<td><p>Returns n’th child element.</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">elem[m:n]</span></code></p></td>
<td><p>Returns list of m’th through n’th child
elements.</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">len(elem)</span></code></p></td>
<td><p>Returns number of child elements.</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">list(elem)</span></code></p></td>
<td><p>Returns list of child elements.</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">elem.append(elem2)</span></code></p></td>
<td><p>Adds <em>elem2</em> as a child.</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">elem.insert(index,</span> <span class="pre">elem2)</span></code></p></td>
<td><p>Inserts <em>elem2</em> at the specified location.</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">del</span> <span class="pre">elem[n]</span></code></p></td>
<td><p>Deletes n’th child element.</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">elem.keys()</span></code></p></td>
<td><p>Returns list of attribute names.</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">elem.get(name)</span></code></p></td>
<td><p>Returns value of attribute <em>name</em>.</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">elem.set(name,</span> <span class="pre">value)</span></code></p></td>
<td><p>Sets new value for attribute <em>name</em>.</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">elem.attrib</span></code></p></td>
<td><p>Retrieves the dictionary containing
attributes.</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">del</span> <span class="pre">elem.attrib[name]</span></code></p></td>
<td><p>Deletes attribute <em>name</em>.</p></td>
</tr>
</tbody>
</table>
<p>Comments and processing instructions are also represented as <code class="xref py py-class docutils literal notranslate"><span class="pre">Element</span></code>
nodes. To check if a node is a comment or processing instructions:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">elem</span><span class="o">.</span><span class="n">tag</span> <span class="ow">is</span> <span class="n">ET</span><span class="o">.</span><span class="n">Comment</span><span class="p">:</span>
<span class="o">...</span>
<span class="k">elif</span> <span class="n">elem</span><span class="o">.</span><span class="n">tag</span> <span class="ow">is</span> <span class="n">ET</span><span class="o">.</span><span class="n">ProcessingInstruction</span><span class="p">:</span>
<span class="o">...</span>
</pre></div>
</div>
<p>To generate XML output, you should call the <code class="xref py py-meth docutils literal notranslate"><span class="pre">ElementTree.write()</span></code> method.
Like <code class="xref py py-func docutils literal notranslate"><span class="pre">parse()</span></code>, it can take either a string or a file-like object:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># Encoding is US-ASCII</span>
<span class="n">tree</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s1">'output.xml'</span><span class="p">)</span>
<span class="c1"># Encoding is UTF-8</span>
<span class="n">f</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="s1">'output.xml'</span><span class="p">,</span> <span class="s1">'w'</span><span class="p">)</span>
<span class="n">tree</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="n">encoding</span><span class="o">=</span><span class="s1">'utf-8'</span><span class="p">)</span>
</pre></div>
</div>
<p>(Caution: the default encoding used for output is ASCII. For general XML work,
where an element’s name may contain arbitrary Unicode characters, ASCII isn’t a
very useful encoding because it will raise an exception if an element’s name
contains any characters with values greater than 127. Therefore, it’s best to
specify a different encoding such as UTF-8 that can handle any Unicode
character.)</p>
<p>This section is only a partial description of the ElementTree interfaces. Please
read the package’s official documentation for more details.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><a class="reference external" href="http://effbot.org/zone/element-index.htm">http://effbot.org/zone/element-index.htm</a></dt><dd><p>Official documentation for ElementTree.</p>
</dd>
</dl>
</div>
</div>
<div class="section" id="the-hashlib-package">
<span id="module-hashlib"></span><h3>The hashlib package<a class="headerlink" href="#the-hashlib-package" title="Permalink to this headline">¶</a></h3>
<p>A new <a class="reference internal" href="../library/hashlib.html#module-hashlib" title="hashlib: Secure hash and message digest algorithms."><code class="xref py py-mod docutils literal notranslate"><span class="pre">hashlib</span></code></a> module, written by Gregory P. Smith, has been added to
replace the <code class="xref py py-mod docutils literal notranslate"><span class="pre">md5</span></code> and <code class="xref py py-mod docutils literal notranslate"><span class="pre">sha</span></code> modules. <a class="reference internal" href="../library/hashlib.html#module-hashlib" title="hashlib: Secure hash and message digest algorithms."><code class="xref py py-mod docutils literal notranslate"><span class="pre">hashlib</span></code></a> adds support for
additional secure hashes (SHA-224, SHA-256, SHA-384, and SHA-512). When
available, the module uses OpenSSL for fast platform optimized implementations
of algorithms.</p>
<p>The old <code class="xref py py-mod docutils literal notranslate"><span class="pre">md5</span></code> and <code class="xref py py-mod docutils literal notranslate"><span class="pre">sha</span></code> modules still exist as wrappers around hashlib
to preserve backwards compatibility. The new module’s interface is very close
to that of the old modules, but not identical. The most significant difference
is that the constructor functions for creating new hashing objects are named
differently.</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># Old versions</span>
<span class="n">h</span> <span class="o">=</span> <span class="n">md5</span><span class="o">.</span><span class="n">md5</span><span class="p">()</span>
<span class="n">h</span> <span class="o">=</span> <span class="n">md5</span><span class="o">.</span><span class="n">new</span><span class="p">()</span>
<span class="c1"># New version</span>
<span class="n">h</span> <span class="o">=</span> <span class="n">hashlib</span><span class="o">.</span><span class="n">md5</span><span class="p">()</span>
<span class="c1"># Old versions</span>
<span class="n">h</span> <span class="o">=</span> <span class="n">sha</span><span class="o">.</span><span class="n">sha</span><span class="p">()</span>
<span class="n">h</span> <span class="o">=</span> <span class="n">sha</span><span class="o">.</span><span class="n">new</span><span class="p">()</span>
<span class="c1"># New version</span>
<span class="n">h</span> <span class="o">=</span> <span class="n">hashlib</span><span class="o">.</span><span class="n">sha1</span><span class="p">()</span>
<span class="c1"># Hash that weren't previously available</span>
<span class="n">h</span> <span class="o">=</span> <span class="n">hashlib</span><span class="o">.</span><span class="n">sha224</span><span class="p">()</span>
<span class="n">h</span> <span class="o">=</span> <span class="n">hashlib</span><span class="o">.</span><span class="n">sha256</span><span class="p">()</span>
<span class="n">h</span> <span class="o">=</span> <span class="n">hashlib</span><span class="o">.</span><span class="n">sha384</span><span class="p">()</span>
<span class="n">h</span> <span class="o">=</span> <span class="n">hashlib</span><span class="o">.</span><span class="n">sha512</span><span class="p">()</span>
<span class="c1"># Alternative form</span>
<span class="n">h</span> <span class="o">=</span> <span class="n">hashlib</span><span class="o">.</span><span class="n">new</span><span class="p">(</span><span class="s1">'md5'</span><span class="p">)</span> <span class="c1"># Provide algorithm as a string</span>
</pre></div>
</div>
<p>Once a hash object has been created, its methods are the same as before:
<code class="docutils literal notranslate"><span class="pre">update(string)</span></code> hashes the specified string into the current digest
state, <code class="xref py py-meth docutils literal notranslate"><span class="pre">digest()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">hexdigest()</span></code> return the digest value as a binary
string or a string of hex digits, and <a class="reference internal" href="../library/copy.html#module-copy" title="copy: Shallow and deep copy operations."><code class="xref py py-meth docutils literal notranslate"><span class="pre">copy()</span></code></a> returns a new hashing object
with the same digest state.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p>The documentation for the <a class="reference internal" href="../library/hashlib.html#module-hashlib" title="hashlib: Secure hash and message digest algorithms."><code class="xref py py-mod docutils literal notranslate"><span class="pre">hashlib</span></code></a> module.</p>
</div>
</div>
<div class="section" id="the-sqlite3-package">
<span id="module-sqlite"></span><h3>The sqlite3 package<a class="headerlink" href="#the-sqlite3-package" title="Permalink to this headline">¶</a></h3>
<p>The pysqlite module (<a class="reference external" href="http://www.pysqlite.org">http://www.pysqlite.org</a>), a wrapper for the SQLite embedded
database, has been added to the standard library under the package name
<a class="reference internal" href="../library/sqlite3.html#module-sqlite3" title="sqlite3: A DB-API 2.0 implementation using SQLite 3.x."><code class="xref py py-mod docutils literal notranslate"><span class="pre">sqlite3</span></code></a>.</p>
<p>SQLite is a C library that provides a lightweight disk-based database that
doesn’t require a separate server process and allows accessing the database
using a nonstandard variant of the SQL query language. Some applications can use
SQLite for internal data storage. It’s also possible to prototype an
application using SQLite and then port the code to a larger database such as
PostgreSQL or Oracle.</p>
<p>pysqlite was written by Gerhard Häring and provides a SQL interface compliant
with the DB-API 2.0 specification described by <span class="target" id="index-21"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0249"><strong>PEP 249</strong></a>.</p>
<p>If you’re compiling the Python source yourself, note that the source tree
doesn’t include the SQLite code, only the wrapper module. You’ll need to have
the SQLite libraries and headers installed before compiling Python, and the
build process will compile the module when the necessary headers are available.</p>
<p>To use the module, you must first create a <code class="xref py py-class docutils literal notranslate"><span class="pre">Connection</span></code> object that
represents the database. Here the data will be stored in the
<code class="file docutils literal notranslate"><span class="pre">/tmp/example</span></code> file:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">conn</span> <span class="o">=</span> <span class="n">sqlite3</span><span class="o">.</span><span class="n">connect</span><span class="p">(</span><span class="s1">'/tmp/example'</span><span class="p">)</span>
</pre></div>
</div>
<p>You can also supply the special name <code class="docutils literal notranslate"><span class="pre">:memory:</span></code> to create a database in RAM.</p>
<p>Once you have a <code class="xref py py-class docutils literal notranslate"><span class="pre">Connection</span></code>, you can create a <code class="xref py py-class docutils literal notranslate"><span class="pre">Cursor</span></code> object
and call its <code class="xref py py-meth docutils literal notranslate"><span class="pre">execute()</span></code> method to perform SQL commands:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="n">c</span> <span class="o">=</span> <span class="n">conn</span><span class="o">.</span><span class="n">cursor</span><span class="p">()</span>
<span class="c1"># Create table</span>
<span class="n">c</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s1">'''create table stocks</span>
<span class="s1">(date text, trans text, symbol text,</span>
<span class="s1"> qty real, price real)'''</span><span class="p">)</span>
<span class="c1"># Insert a row of data</span>
<span class="n">c</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s2">"""insert into stocks</span>
<span class="s2"> values ('2006-01-05','BUY','RHAT',100,35.14)"""</span><span class="p">)</span>
</pre></div>
</div>
<p>Usually your SQL operations will need to use values from Python variables. You
shouldn’t assemble your query using Python’s string operations because doing so
is insecure; it makes your program vulnerable to an SQL injection attack.</p>
<p>Instead, use the DB-API’s parameter substitution. Put <code class="docutils literal notranslate"><span class="pre">?</span></code> as a placeholder
wherever you want to use a value, and then provide a tuple of values as the
second argument to the cursor’s <code class="xref py py-meth docutils literal notranslate"><span class="pre">execute()</span></code> method. (Other database modules
may use a different placeholder, such as <code class="docutils literal notranslate"><span class="pre">%s</span></code> or <code class="docutils literal notranslate"><span class="pre">:1</span></code>.) For example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># Never do this -- insecure!</span>
<span class="n">symbol</span> <span class="o">=</span> <span class="s1">'IBM'</span>
<span class="n">c</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s2">"... where symbol = '</span><span class="si">%s</span><span class="s2">'"</span> <span class="o">%</span> <span class="n">symbol</span><span class="p">)</span>
<span class="c1"># Do this instead</span>
<span class="n">t</span> <span class="o">=</span> <span class="p">(</span><span class="n">symbol</span><span class="p">,)</span>
<span class="n">c</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s1">'select * from stocks where symbol=?'</span><span class="p">,</span> <span class="n">t</span><span class="p">)</span>
<span class="c1"># Larger example</span>
<span class="k">for</span> <span class="n">t</span> <span class="ow">in</span> <span class="p">((</span><span class="s1">'2006-03-28'</span><span class="p">,</span> <span class="s1">'BUY'</span><span class="p">,</span> <span class="s1">'IBM'</span><span class="p">,</span> <span class="mi">1000</span><span class="p">,</span> <span class="mf">45.00</span><span class="p">),</span>
<span class="p">(</span><span class="s1">'2006-04-05'</span><span class="p">,</span> <span class="s1">'BUY'</span><span class="p">,</span> <span class="s1">'MSOFT'</span><span class="p">,</span> <span class="mi">1000</span><span class="p">,</span> <span class="mf">72.00</span><span class="p">),</span>
<span class="p">(</span><span class="s1">'2006-04-06'</span><span class="p">,</span> <span class="s1">'SELL'</span><span class="p">,</span> <span class="s1">'IBM'</span><span class="p">,</span> <span class="mi">500</span><span class="p">,</span> <span class="mf">53.00</span><span class="p">),</span>
<span class="p">):</span>
<span class="n">c</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s1">'insert into stocks values (?,?,?,?,?)'</span><span class="p">,</span> <span class="n">t</span><span class="p">)</span>
</pre></div>
</div>
<p>To retrieve data after executing a SELECT statement, you can either treat the
cursor as an iterator, call the cursor’s <code class="xref py py-meth docutils literal notranslate"><span class="pre">fetchone()</span></code> method to retrieve a
single matching row, or call <code class="xref py py-meth docutils literal notranslate"><span class="pre">fetchall()</span></code> to get a list of the matching
rows.</p>
<p>This example uses the iterator form:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">c</span> <span class="o">=</span> <span class="n">conn</span><span class="o">.</span><span class="n">cursor</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">c</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s1">'select * from stocks order by price'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="k">for</span> <span class="n">row</span> <span class="ow">in</span> <span class="n">c</span><span class="p">:</span>
<span class="gp">... </span> <span class="nb">print</span> <span class="n">row</span>
<span class="gp">...</span>
<span class="go">(u'2006-01-05', u'BUY', u'RHAT', 100, 35.140000000000001)</span>
<span class="go">(u'2006-03-28', u'BUY', u'IBM', 1000, 45.0)</span>
<span class="go">(u'2006-04-06', u'SELL', u'IBM', 500, 53.0)</span>
<span class="go">(u'2006-04-05', u'BUY', u'MSOFT', 1000, 72.0)</span>
<span class="go">>>></span>
</pre></div>
</div>
<p>For more information about the SQL dialect supported by SQLite, see
<a class="reference external" href="https://www.sqlite.org">https://www.sqlite.org</a>.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><a class="reference external" href="http://www.pysqlite.org">http://www.pysqlite.org</a></dt><dd><p>The pysqlite web page.</p>
</dd>
<dt><a class="reference external" href="https://www.sqlite.org">https://www.sqlite.org</a></dt><dd><p>The SQLite web page; the documentation describes the syntax and the available
data types for the supported SQL dialect.</p>
</dd>
</dl>
<p>The documentation for the <a class="reference internal" href="../library/sqlite3.html#module-sqlite3" title="sqlite3: A DB-API 2.0 implementation using SQLite 3.x."><code class="xref py py-mod docutils literal notranslate"><span class="pre">sqlite3</span></code></a> module.</p>
<dl class="simple">
<dt><span class="target" id="index-22"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0249"><strong>PEP 249</strong></a> - Database API Specification 2.0</dt><dd><p>PEP written by Marc-André Lemburg.</p>
</dd>
</dl>
</div>
</div>
<div class="section" id="the-wsgiref-package">
<span id="module-wsgiref"></span><h3>The wsgiref package<a class="headerlink" href="#the-wsgiref-package" title="Permalink to this headline">¶</a></h3>
<p>The Web Server Gateway Interface (WSGI) v1.0 defines a standard interface
between web servers and Python web applications and is described in <span class="target" id="index-23"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0333"><strong>PEP 333</strong></a>.
The <a class="reference internal" href="../library/wsgiref.html#module-wsgiref" title="wsgiref: WSGI Utilities and Reference Implementation."><code class="xref py py-mod docutils literal notranslate"><span class="pre">wsgiref</span></code></a> package is a reference implementation of the WSGI
specification.</p>
<p>The package includes a basic HTTP server that will run a WSGI application; this
server is useful for debugging but isn’t intended for production use. Setting
up a server takes only a few lines of code:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">wsgiref</span> <span class="kn">import</span> <span class="n">simple_server</span>
<span class="n">wsgi_app</span> <span class="o">=</span> <span class="o">...</span>
<span class="n">host</span> <span class="o">=</span> <span class="s1">''</span>
<span class="n">port</span> <span class="o">=</span> <span class="mi">8000</span>
<span class="n">httpd</span> <span class="o">=</span> <span class="n">simple_server</span><span class="o">.</span><span class="n">make_server</span><span class="p">(</span><span class="n">host</span><span class="p">,</span> <span class="n">port</span><span class="p">,</span> <span class="n">wsgi_app</span><span class="p">)</span>
<span class="n">httpd</span><span class="o">.</span><span class="n">serve_forever</span><span class="p">()</span>
</pre></div>
</div>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><a class="reference external" href="http://www.wsgi.org">http://www.wsgi.org</a></dt><dd><p>A central web site for WSGI-related resources.</p>
</dd>
<dt><span class="target" id="index-24"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0333"><strong>PEP 333</strong></a> - Python Web Server Gateway Interface v1.0</dt><dd><p>PEP written by Phillip J. Eby.</p>
</dd>
</dl>
</div>
</div>
</div>
<div class="section" id="build-and-c-api-changes">
<span id="build-api"></span><h2>Build and C API Changes<a class="headerlink" href="#build-and-c-api-changes" title="Permalink to this headline">¶</a></h2>
<p>Changes to Python’s build process and to the C API include:</p>
<ul>
<li><p>The Python source tree was converted from CVS to Subversion, in a complex
migration procedure that was supervised and flawlessly carried out by Martin von
Löwis. The procedure was developed as <span class="target" id="index-25"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0347"><strong>PEP 347</strong></a>.</p></li>
<li><p>Coverity, a company that markets a source code analysis tool called Prevent,
provided the results of their examination of the Python source code. The
analysis found about 60 bugs that were quickly fixed. Many of the bugs were
refcounting problems, often occurring in error-handling code. See
<a class="reference external" href="https://scan.coverity.com">https://scan.coverity.com</a> for the statistics.</p></li>
<li><p>The largest change to the C API came from <span class="target" id="index-26"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0353"><strong>PEP 353</strong></a>, which modifies the
interpreter to use a <code class="xref c c-type docutils literal notranslate"><span class="pre">Py_ssize_t</span></code> type definition instead of
<code class="xref c c-type docutils literal notranslate"><span class="pre">int</span></code>. See the earlier section <a class="reference internal" href="#pep-353"><span class="std std-ref">PEP 353: Using ssize_t as the index type</span></a> for a discussion of this
change.</p></li>
<li><p>The design of the bytecode compiler has changed a great deal, no longer
generating bytecode by traversing the parse tree. Instead the parse tree is
converted to an abstract syntax tree (or AST), and it is the abstract syntax
tree that’s traversed to produce the bytecode.</p>
<p>It’s possible for Python code to obtain AST objects by using the
<a class="reference internal" href="../library/functions.html#compile" title="compile"><code class="xref py py-func docutils literal notranslate"><span class="pre">compile()</span></code></a> built-in and specifying <code class="docutils literal notranslate"><span class="pre">_ast.PyCF_ONLY_AST</span></code> as the value of
the <em>flags</em> parameter:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">_ast</span> <span class="kn">import</span> <span class="n">PyCF_ONLY_AST</span>
<span class="n">ast</span> <span class="o">=</span> <span class="nb">compile</span><span class="p">(</span><span class="s2">"""a=0</span>
<span class="s2">for i in range(10):</span>
<span class="s2"> a += i</span>
<span class="s2">"""</span><span class="p">,</span> <span class="s2">"<string>"</span><span class="p">,</span> <span class="s1">'exec'</span><span class="p">,</span> <span class="n">PyCF_ONLY_AST</span><span class="p">)</span>
<span class="n">assignment</span> <span class="o">=</span> <span class="n">ast</span><span class="o">.</span><span class="n">body</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
<span class="n">for_loop</span> <span class="o">=</span> <span class="n">ast</span><span class="o">.</span><span class="n">body</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span>
</pre></div>
</div>
<p>No official documentation has been written for the AST code yet, but <span class="target" id="index-27"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0339"><strong>PEP 339</strong></a>
discusses the design. To start learning about the code, read the definition of
the various AST nodes in <code class="file docutils literal notranslate"><span class="pre">Parser/Python.asdl</span></code>. A Python script reads this
file and generates a set of C structure definitions in
<code class="file docutils literal notranslate"><span class="pre">Include/Python-ast.h</span></code>. The <code class="xref c c-func docutils literal notranslate"><span class="pre">PyParser_ASTFromString()</span></code> and
<code class="xref c c-func docutils literal notranslate"><span class="pre">PyParser_ASTFromFile()</span></code>, defined in <code class="file docutils literal notranslate"><span class="pre">Include/pythonrun.h</span></code>, take
Python source as input and return the root of an AST representing the contents.
This AST can then be turned into a code object by <code class="xref c c-func docutils literal notranslate"><span class="pre">PyAST_Compile()</span></code>. For
more information, read the source code, and then ask questions on python-dev.</p>
<p>The AST code was developed under Jeremy Hylton’s management, and implemented by
(in alphabetical order) Brett Cannon, Nick Coghlan, Grant Edwards, John
Ehresman, Kurt Kaiser, Neal Norwitz, Tim Peters, Armin Rigo, and Neil
Schemenauer, plus the participants in a number of AST sprints at conferences
such as PyCon.</p>
</li>
<li><p>Evan Jones’s patch to obmalloc, first described in a talk at PyCon DC 2005,
was applied. Python 2.4 allocated small objects in 256K-sized arenas, but never
freed arenas. With this patch, Python will free arenas when they’re empty. The
net effect is that on some platforms, when you allocate many objects, Python’s
memory usage may actually drop when you delete them and the memory may be
returned to the operating system. (Implemented by Evan Jones, and reworked by
Tim Peters.)</p>
<p>Note that this change means extension modules must be more careful when
allocating memory. Python’s API has many different functions for allocating
memory that are grouped into families. For example, <a class="reference internal" href="../c-api/memory.html#c.PyMem_Malloc" title="PyMem_Malloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Malloc()</span></code></a>,
<a class="reference internal" href="../c-api/memory.html#c.PyMem_Realloc" title="PyMem_Realloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Realloc()</span></code></a>, and <a class="reference internal" href="../c-api/memory.html#c.PyMem_Free" title="PyMem_Free"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_Free()</span></code></a> are one family that allocates
raw memory, while <a class="reference internal" href="../c-api/memory.html#c.PyObject_Malloc" title="PyObject_Malloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Malloc()</span></code></a>, <a class="reference internal" href="../c-api/memory.html#c.PyObject_Realloc" title="PyObject_Realloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Realloc()</span></code></a>, and
<a class="reference internal" href="../c-api/memory.html#c.PyObject_Free" title="PyObject_Free"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Free()</span></code></a> are another family that’s supposed to be used for
creating Python objects.</p>
<p>Previously these different families all reduced to the platform’s
<code class="xref c c-func docutils literal notranslate"><span class="pre">malloc()</span></code> and <code class="xref c c-func docutils literal notranslate"><span class="pre">free()</span></code> functions. This meant it didn’t matter if
you got things wrong and allocated memory with the <code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem()</span></code> function but
freed it with the <a class="reference internal" href="../c-api/structures.html#c.PyObject" title="PyObject"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject()</span></code></a> function. With 2.5’s changes to obmalloc,
these families now do different things and mismatches will probably result in a
segfault. You should carefully test your C extension modules with Python 2.5.</p>
</li>
<li><p>The built-in set types now have an official C API. Call <a class="reference internal" href="../c-api/set.html#c.PySet_New" title="PySet_New"><code class="xref c c-func docutils literal notranslate"><span class="pre">PySet_New()</span></code></a>
and <a class="reference internal" href="../c-api/set.html#c.PyFrozenSet_New" title="PyFrozenSet_New"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyFrozenSet_New()</span></code></a> to create a new set, <a class="reference internal" href="../c-api/set.html#c.PySet_Add" title="PySet_Add"><code class="xref c c-func docutils literal notranslate"><span class="pre">PySet_Add()</span></code></a> and
<a class="reference internal" href="../c-api/set.html#c.PySet_Discard" title="PySet_Discard"><code class="xref c c-func docutils literal notranslate"><span class="pre">PySet_Discard()</span></code></a> to add and remove elements, and <a class="reference internal" href="../c-api/set.html#c.PySet_Contains" title="PySet_Contains"><code class="xref c c-func docutils literal notranslate"><span class="pre">PySet_Contains()</span></code></a>
and <a class="reference internal" href="../c-api/set.html#c.PySet_Size" title="PySet_Size"><code class="xref c c-func docutils literal notranslate"><span class="pre">PySet_Size()</span></code></a> to examine the set’s state. (Contributed by Raymond
Hettinger.)</p></li>
<li><p>C code can now obtain information about the exact revision of the Python
interpreter by calling the <a class="reference internal" href="../c-api/init.html#c.Py_GetBuildInfo" title="Py_GetBuildInfo"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_GetBuildInfo()</span></code></a> function that returns a
string of build information like this: <code class="docutils literal notranslate"><span class="pre">"trunk:45355:45356M,</span> <span class="pre">Apr</span> <span class="pre">13</span> <span class="pre">2006,</span>
<span class="pre">07:42:19"</span></code>. (Contributed by Barry Warsaw.)</p></li>
<li><p>Two new macros can be used to indicate C functions that are local to the
current file so that a faster calling convention can be used.
<code class="docutils literal notranslate"><span class="pre">Py_LOCAL(type)</span></code> declares the function as returning a value of the
specified <em>type</em> and uses a fast-calling qualifier.
<code class="docutils literal notranslate"><span class="pre">Py_LOCAL_INLINE(type)</span></code> does the same thing and also requests the
function be inlined. If <code class="xref c c-func docutils literal notranslate"><span class="pre">PY_LOCAL_AGGRESSIVE()</span></code> is defined before
<code class="file docutils literal notranslate"><span class="pre">python.h</span></code> is included, a set of more aggressive optimizations are enabled
for the module; you should benchmark the results to find out if these
optimizations actually make the code faster. (Contributed by Fredrik Lundh at
the NeedForSpeed sprint.)</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">PyErr_NewException(name,</span> <span class="pre">base,</span> <span class="pre">dict)</span></code> can now accept a tuple of base
classes as its <em>base</em> argument. (Contributed by Georg Brandl.)</p></li>
<li><p>The <code class="xref c c-func docutils literal notranslate"><span class="pre">PyErr_Warn()</span></code> function for issuing warnings is now deprecated in
favour of <code class="docutils literal notranslate"><span class="pre">PyErr_WarnEx(category,</span> <span class="pre">message,</span> <span class="pre">stacklevel)</span></code> which lets you
specify the number of stack frames separating this function and the caller. A
<em>stacklevel</em> of 1 is the function calling <a class="reference internal" href="../c-api/exceptions.html#c.PyErr_WarnEx" title="PyErr_WarnEx"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyErr_WarnEx()</span></code></a>, 2 is the
function above that, and so forth. (Added by Neal Norwitz.)</p></li>
<li><p>The CPython interpreter is still written in C, but the code can now be
compiled with a C++ compiler without errors. (Implemented by Anthony Baxter,
Martin von Löwis, Skip Montanaro.)</p></li>
<li><p>The <code class="xref c c-func docutils literal notranslate"><span class="pre">PyRange_New()</span></code> function was removed. It was never documented, never
used in the core code, and had dangerously lax error checking. In the unlikely
case that your extensions were using it, you can replace it by something like
the following:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="nb">range</span> <span class="o">=</span> <span class="n">PyObject_CallFunction</span><span class="p">((</span><span class="n">PyObject</span><span class="o">*</span><span class="p">)</span> <span class="o">&</span><span class="n">PyRange_Type</span><span class="p">,</span> <span class="s2">"lll"</span><span class="p">,</span>
<span class="n">start</span><span class="p">,</span> <span class="n">stop</span><span class="p">,</span> <span class="n">step</span><span class="p">);</span>
</pre></div>
</div>
</li>
</ul>
<div class="section" id="port-specific-changes">
<span id="ports"></span><h3>Port-Specific Changes<a class="headerlink" href="#port-specific-changes" title="Permalink to this headline">¶</a></h3>
<ul class="simple">
<li><p>MacOS X (10.3 and higher): dynamic loading of modules now uses the
<code class="xref c c-func docutils literal notranslate"><span class="pre">dlopen()</span></code> function instead of MacOS-specific functions.</p></li>
<li><p>MacOS X: an <code class="xref std std-option docutils literal notranslate"><span class="pre">--enable-universalsdk</span></code> switch was added to the
<strong class="program">configure</strong> script that compiles the interpreter as a universal binary
able to run on both PowerPC and Intel processors. (Contributed by Ronald
Oussoren; <a class="reference external" href="https://bugs.python.org/issue2573">bpo-2573</a>.)</p></li>
<li><p>Windows: <code class="file docutils literal notranslate"><span class="pre">.dll</span></code> is no longer supported as a filename extension for
extension modules. <code class="file docutils literal notranslate"><span class="pre">.pyd</span></code> is now the only filename extension that will be
searched for.</p></li>
</ul>
</div>
</div>
<div class="section" id="porting-to-python-2-5">
<span id="porting"></span><h2>Porting to Python 2.5<a class="headerlink" href="#porting-to-python-2-5" title="Permalink to this headline">¶</a></h2>
<p>This section lists previously described changes that may require changes to your
code:</p>
<ul class="simple">
<li><p>ASCII is now the default encoding for modules. It’s now a syntax error if a
module contains string literals with 8-bit characters but doesn’t have an
encoding declaration. In Python 2.4 this triggered a warning, not a syntax
error.</p></li>
<li><p>Previously, the <code class="xref py py-attr docutils literal notranslate"><span class="pre">gi_frame</span></code> attribute of a generator was always a frame
object. Because of the <span class="target" id="index-28"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0342"><strong>PEP 342</strong></a> changes described in section <a class="reference internal" href="#pep-342"><span class="std std-ref">PEP 342: New Generator Features</span></a>,
it’s now possible for <code class="xref py py-attr docutils literal notranslate"><span class="pre">gi_frame</span></code> to be <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p></li>
<li><p>A new warning, <a class="reference internal" href="../library/exceptions.html#UnicodeWarning" title="UnicodeWarning"><code class="xref py py-class docutils literal notranslate"><span class="pre">UnicodeWarning</span></code></a>, is triggered when you attempt to
compare a Unicode string and an 8-bit string that can’t be converted to Unicode
using the default ASCII encoding. Previously such comparisons would raise a
<a class="reference internal" href="../library/exceptions.html#UnicodeDecodeError" title="UnicodeDecodeError"><code class="xref py py-class docutils literal notranslate"><span class="pre">UnicodeDecodeError</span></code></a> exception.</p></li>
<li><p>Library: the <a class="reference internal" href="../library/csv.html#module-csv" title="csv: Write and read tabular data to and from delimited files."><code class="xref py py-mod docutils literal notranslate"><span class="pre">csv</span></code></a> module is now stricter about multi-line quoted fields.
If your files contain newlines embedded within fields, the input should be split
into lines in a manner which preserves the newline characters.</p></li>
<li><p>Library: the <a class="reference internal" href="../library/locale.html#module-locale" title="locale: Internationalization services."><code class="xref py py-mod docutils literal notranslate"><span class="pre">locale</span></code></a> module’s <a class="reference internal" href="../library/functions.html#format" title="format"><code class="xref py py-func docutils literal notranslate"><span class="pre">format()</span></code></a> function’s would
previously accept any string as long as no more than one %char specifier
appeared. In Python 2.5, the argument must be exactly one %char specifier with
no surrounding text.</p></li>
<li><p>Library: The <a class="reference internal" href="../library/pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pickle</span></code></a> and <code class="xref py py-mod docutils literal notranslate"><span class="pre">cPickle</span></code> modules no longer accept a
return value of <code class="docutils literal notranslate"><span class="pre">None</span></code> from the <a class="reference internal" href="../library/pickle.html#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__reduce__()</span></code></a> method; the method must
return a tuple of arguments instead. The modules also no longer accept the
deprecated <em>bin</em> keyword parameter.</p></li>
<li><p>Library: The <code class="xref py py-mod docutils literal notranslate"><span class="pre">SimpleXMLRPCServer</span></code> and <code class="xref py py-mod docutils literal notranslate"><span class="pre">DocXMLRPCServer</span></code> classes now
have a <code class="xref py py-attr docutils literal notranslate"><span class="pre">rpc_paths</span></code> attribute that constrains XML-RPC operations to a
limited set of URL paths; the default is to allow only <code class="docutils literal notranslate"><span class="pre">'/'</span></code> and <code class="docutils literal notranslate"><span class="pre">'/RPC2'</span></code>.
Setting <code class="xref py py-attr docutils literal notranslate"><span class="pre">rpc_paths</span></code> to <code class="docutils literal notranslate"><span class="pre">None</span></code> or an empty tuple disables this path
checking.</p></li>
<li><p>C API: Many functions now use <code class="xref c c-type docutils literal notranslate"><span class="pre">Py_ssize_t</span></code> instead of <code class="xref c c-type docutils literal notranslate"><span class="pre">int</span></code> to
allow processing more data on 64-bit machines. Extension code may need to make
the same change to avoid warnings and to support 64-bit machines. See the
earlier section <a class="reference internal" href="#pep-353"><span class="std std-ref">PEP 353: Using ssize_t as the index type</span></a> for a discussion of this change.</p></li>
<li><p>C API: The obmalloc changes mean that you must be careful to not mix usage
of the <code class="xref c c-func docutils literal notranslate"><span class="pre">PyMem_*()</span></code> and <code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_*()</span></code> families of functions. Memory
allocated with one family’s <code class="xref c c-func docutils literal notranslate"><span class="pre">*_Malloc()</span></code> must be freed with the
corresponding family’s <code class="xref c c-func docutils literal notranslate"><span class="pre">*_Free()</span></code> function.</p></li>
</ul>
</div>
<div class="section" id="acknowledgements">
<h2>Acknowledgements<a class="headerlink" href="#acknowledgements" title="Permalink to this headline">¶</a></h2>
<p>The author would like to thank the following people for offering suggestions,
corrections and assistance with various drafts of this article: Georg Brandl,
Nick Coghlan, Phillip J. Eby, Lars Gustäbel, Raymond Hettinger, Ralf W.
Grosse-Kunstleve, Kent Johnson, Iain Lowe, Martin von Löwis, Fredrik Lundh, Andrew
McNamara, Skip Montanaro, Gustavo Niemeyer, Paul Prescod, James Pryor, Mike
Rovner, Scott Weikart, Barry Warsaw, Thomas Wouters.</p>
</div>
</div>
</div>
</div>
</div>
<div class="sphinxsidebar" role="navigation" aria-label="main navigation">
<div class="sphinxsidebarwrapper">
<h3><a href="../contents.html">Table of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">What’s New in Python 2.5</a><ul>
<li><a class="reference internal" href="#pep-308-conditional-expressions">PEP 308: Conditional Expressions</a></li>
<li><a class="reference internal" href="#pep-309-partial-function-application">PEP 309: Partial Function Application</a></li>
<li><a class="reference internal" href="#pep-314-metadata-for-python-software-packages-v1-1">PEP 314: Metadata for Python Software Packages v1.1</a></li>
<li><a class="reference internal" href="#pep-328-absolute-and-relative-imports">PEP 328: Absolute and Relative Imports</a></li>
<li><a class="reference internal" href="#pep-338-executing-modules-as-scripts">PEP 338: Executing Modules as Scripts</a></li>
<li><a class="reference internal" href="#pep-341-unified-try-except-finally">PEP 341: Unified try/except/finally</a></li>
<li><a class="reference internal" href="#pep-342-new-generator-features">PEP 342: New Generator Features</a></li>
<li><a class="reference internal" href="#pep-343-the-with-statement">PEP 343: The ‘with’ statement</a><ul>
<li><a class="reference internal" href="#writing-context-managers">Writing Context Managers</a></li>
<li><a class="reference internal" href="#the-contextlib-module">The contextlib module</a></li>
</ul>
</li>
<li><a class="reference internal" href="#pep-352-exceptions-as-new-style-classes">PEP 352: Exceptions as New-Style Classes</a></li>
<li><a class="reference internal" href="#pep-353-using-ssize-t-as-the-index-type">PEP 353: Using ssize_t as the index type</a></li>
<li><a class="reference internal" href="#pep-357-the-index-method">PEP 357: The ‘__index__’ method</a></li>
<li><a class="reference internal" href="#other-language-changes">Other Language Changes</a><ul>
<li><a class="reference internal" href="#interactive-interpreter-changes">Interactive Interpreter Changes</a></li>
<li><a class="reference internal" href="#optimizations">Optimizations</a></li>
</ul>
</li>
<li><a class="reference internal" href="#new-improved-and-removed-modules">New, Improved, and Removed Modules</a><ul>
<li><a class="reference internal" href="#the-ctypes-package">The ctypes package</a></li>
<li><a class="reference internal" href="#the-elementtree-package">The ElementTree package</a></li>
<li><a class="reference internal" href="#the-hashlib-package">The hashlib package</a></li>
<li><a class="reference internal" href="#the-sqlite3-package">The sqlite3 package</a></li>
<li><a class="reference internal" href="#the-wsgiref-package">The wsgiref package</a></li>
</ul>
</li>
<li><a class="reference internal" href="#build-and-c-api-changes">Build and C API Changes</a><ul>
<li><a class="reference internal" href="#port-specific-changes">Port-Specific Changes</a></li>
</ul>
</li>
<li><a class="reference internal" href="#porting-to-python-2-5">Porting to Python 2.5</a></li>
<li><a class="reference internal" href="#acknowledgements">Acknowledgements</a></li>
</ul>
</li>
</ul>
<h4>Previous topic</h4>
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title="next chapter">What’s New in Python 2.4</a></p>
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