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<div class="section" id="library-and-extension-faq">
<h1><a class="toc-backref" href="#id2">Library and Extension FAQ</a><a class="headerlink" href="#library-and-extension-faq" title="Permalink to this headline">¶</a></h1>
<div class="contents topic" id="contents">
<p class="topic-title first">Contents</p>
<ul class="simple">
<li><a class="reference internal" href="#library-and-extension-faq" id="id2">Library and Extension FAQ</a><ul>
<li><a class="reference internal" href="#general-library-questions" id="id3">General Library Questions</a><ul>
<li><a class="reference internal" href="#how-do-i-find-a-module-or-application-to-perform-task-x" id="id4">How do I find a module or application to perform task X?</a></li>
<li><a class="reference internal" href="#where-is-the-math-py-socket-py-regex-py-etc-source-file" id="id5">Where is the math.py (socket.py, regex.py, etc.) source file?</a></li>
<li><a class="reference internal" href="#how-do-i-make-a-python-script-executable-on-unix" id="id6">How do I make a Python script executable on Unix?</a></li>
<li><a class="reference internal" href="#is-there-a-curses-termcap-package-for-python" id="id7">Is there a curses/termcap package for Python?</a></li>
<li><a class="reference internal" href="#is-there-an-equivalent-to-c-s-onexit-in-python" id="id8">Is there an equivalent to C’s onexit() in Python?</a></li>
<li><a class="reference internal" href="#why-don-t-my-signal-handlers-work" id="id9">Why don’t my signal handlers work?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#common-tasks" id="id10">Common tasks</a><ul>
<li><a class="reference internal" href="#how-do-i-test-a-python-program-or-component" id="id11">How do I test a Python program or component?</a></li>
<li><a class="reference internal" href="#how-do-i-create-documentation-from-doc-strings" id="id12">How do I create documentation from doc strings?</a></li>
<li><a class="reference internal" href="#how-do-i-get-a-single-keypress-at-a-time" id="id13">How do I get a single keypress at a time?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#threads" id="id14">Threads</a><ul>
<li><a class="reference internal" href="#how-do-i-program-using-threads" id="id15">How do I program using threads?</a></li>
<li><a class="reference internal" href="#none-of-my-threads-seem-to-run-why" id="id16">None of my threads seem to run: why?</a></li>
<li><a class="reference internal" href="#how-do-i-parcel-out-work-among-a-bunch-of-worker-threads" id="id17">How do I parcel out work among a bunch of worker threads?</a></li>
<li><a class="reference internal" href="#what-kinds-of-global-value-mutation-are-thread-safe" id="id18">What kinds of global value mutation are thread-safe?</a></li>
<li><a class="reference internal" href="#can-t-we-get-rid-of-the-global-interpreter-lock" id="id19">Can’t we get rid of the Global Interpreter Lock?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#input-and-output" id="id20">Input and Output</a><ul>
<li><a class="reference internal" href="#how-do-i-delete-a-file-and-other-file-questions" id="id21">How do I delete a file? (And other file questions...)</a></li>
<li><a class="reference internal" href="#how-do-i-copy-a-file" id="id22">How do I copy a file?</a></li>
<li><a class="reference internal" href="#how-do-i-read-or-write-binary-data" id="id23">How do I read (or write) binary data?</a></li>
<li><a class="reference internal" href="#i-can-t-seem-to-use-os-read-on-a-pipe-created-with-os-popen-why" id="id24">I can’t seem to use os.read() on a pipe created with os.popen(); why?</a></li>
<li><a class="reference internal" href="#how-do-i-access-the-serial-rs232-port" id="id25">How do I access the serial (RS232) port?</a></li>
<li><a class="reference internal" href="#why-doesn-t-closing-sys-stdout-stdin-stderr-really-close-it" id="id26">Why doesn’t closing sys.stdout (stdin, stderr) really close it?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#network-internet-programming" id="id27">Network/Internet Programming</a><ul>
<li><a class="reference internal" href="#what-www-tools-are-there-for-python" id="id28">What WWW tools are there for Python?</a></li>
<li><a class="reference internal" href="#how-can-i-mimic-cgi-form-submission-method-post" id="id29">How can I mimic CGI form submission (METHOD=POST)?</a></li>
<li><a class="reference internal" href="#what-module-should-i-use-to-help-with-generating-html" id="id30">What module should I use to help with generating HTML?</a></li>
<li><a class="reference internal" href="#how-do-i-send-mail-from-a-python-script" id="id31">How do I send mail from a Python script?</a></li>
<li><a class="reference internal" href="#how-do-i-avoid-blocking-in-the-connect-method-of-a-socket" id="id32">How do I avoid blocking in the connect() method of a socket?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#databases" id="id33">Databases</a><ul>
<li><a class="reference internal" href="#are-there-any-interfaces-to-database-packages-in-python" id="id34">Are there any interfaces to database packages in Python?</a></li>
<li><a class="reference internal" href="#how-do-you-implement-persistent-objects-in-python" id="id35">How do you implement persistent objects in Python?</a></li>
<li><a class="reference internal" href="#if-my-program-crashes-with-a-bsddb-or-anydbm-database-open-it-gets-corrupted-how-come" id="id36">If my program crashes with a bsddb (or anydbm) database open, it gets corrupted. How come?</a></li>
<li><a class="reference internal" href="#i-tried-to-open-berkeley-db-file-but-bsddb-produces-bsddb-error-22-invalid-argument-help-how-can-i-restore-my-data" id="id37">I tried to open Berkeley DB file, but bsddb produces bsddb.error: (22, ‘Invalid argument’). Help! How can I restore my data?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#mathematics-and-numerics" id="id38">Mathematics and Numerics</a><ul>
<li><a class="reference internal" href="#how-do-i-generate-random-numbers-in-python" id="id39">How do I generate random numbers in Python?</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</div>
<div class="section" id="general-library-questions">
<h2><a class="toc-backref" href="#id3">General Library Questions</a><a class="headerlink" href="#general-library-questions" title="Permalink to this headline">¶</a></h2>
<div class="section" id="how-do-i-find-a-module-or-application-to-perform-task-x">
<h3><a class="toc-backref" href="#id4">How do I find a module or application to perform task X?</a><a class="headerlink" href="#how-do-i-find-a-module-or-application-to-perform-task-x" title="Permalink to this headline">¶</a></h3>
<p>Check <a class="reference internal" href="../library/index.html#library-index"><em>the Library Reference</em></a> to see if there’s a relevant
standard library module. (Eventually you’ll learn what’s in the standard
library and will able to skip this step.)</p>
<p>For third-party packages, search the <a class="reference external" href="http://pypi.python.org/pypi">Python Package Index</a> or try <a class="reference external" href="http://www.google.com">Google</a> or
another Web search engine. Searching for “Python” plus a keyword or two for
your topic of interest will usually find something helpful.</p>
</div>
<div class="section" id="where-is-the-math-py-socket-py-regex-py-etc-source-file">
<h3><a class="toc-backref" href="#id5">Where is the math.py (socket.py, regex.py, etc.) source file?</a><a class="headerlink" href="#where-is-the-math-py-socket-py-regex-py-etc-source-file" title="Permalink to this headline">¶</a></h3>
<p>If you can’t find a source file for a module it may be a built-in or
dynamically loaded module implemented in C, C++ or other compiled language.
In this case you may not have the source file or it may be something like
mathmodule.c, somewhere in a C source directory (not on the Python Path).</p>
<p>There are (at least) three kinds of modules in Python:</p>
<ol class="arabic">
<li><p class="first">modules written in Python (.py);</p>
</li>
<li><p class="first">modules written in C and dynamically loaded (.dll, .pyd, .so, .sl, etc);</p>
</li>
<li><p class="first">modules written in C and linked with the interpreter; to get a list of these,
type:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">sys</span>
<span class="nb">print</span><span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">builtin_module_names</span><span class="p">)</span>
</pre></div>
</div>
</li>
</ol>
</div>
<div class="section" id="how-do-i-make-a-python-script-executable-on-unix">
<h3><a class="toc-backref" href="#id6">How do I make a Python script executable on Unix?</a><a class="headerlink" href="#how-do-i-make-a-python-script-executable-on-unix" title="Permalink to this headline">¶</a></h3>
<p>You need to do two things: the script file’s mode must be executable and the
first line must begin with <tt class="docutils literal"><span class="pre">#!</span></tt> followed by the path of the Python
interpreter.</p>
<p>The first is done by executing <tt class="docutils literal"><span class="pre">chmod</span> <span class="pre">+x</span> <span class="pre">scriptfile</span></tt> or perhaps <tt class="docutils literal"><span class="pre">chmod</span> <span class="pre">755</span>
<span class="pre">scriptfile</span></tt>.</p>
<p>The second can be done in a number of ways. The most straightforward way is to
write</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="c">#!/usr/bin/python3</span>
</pre></div>
</div>
<p>as the very first line of your file, using the pathname for where the Python
interpreter is installed on your platform.</p>
<p>If you would like the script to be independent of where the Python interpreter
lives, you can use the “env” program. Almost all Unix variants support the
following, assuming the Python interpreter is in a directory on the user’s
$PATH:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="c">#!/usr/bin/env python</span>
</pre></div>
</div>
<p><em>Don’t</em> do this for CGI scripts. The $PATH variable for CGI scripts is often
very minimal, so you need to use the actual absolute pathname of the
interpreter.</p>
<p>Occasionally, a user’s environment is so full that the /usr/bin/env program
fails; or there’s no env program at all. In that case, you can try the
following hack (due to Alex Rezinsky):</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="c">#! /bin/sh</span>
<span class="sd">""":"</span>
<span class="sd">exec python $0 ${1+"$@"}</span>
<span class="sd">"""</span>
</pre></div>
</div>
<p>The minor disadvantage is that this defines the script’s __doc__ string.
However, you can fix that by adding</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="n">__doc__</span> <span class="o">=</span> <span class="s">"""...Whatever..."""</span>
</pre></div>
</div>
</div>
<div class="section" id="is-there-a-curses-termcap-package-for-python">
<h3><a class="toc-backref" href="#id7">Is there a curses/termcap package for Python?</a><a class="headerlink" href="#is-there-a-curses-termcap-package-for-python" title="Permalink to this headline">¶</a></h3>
<p>For Unix variants: The standard Python source distribution comes with a curses
module in the <tt class="docutils literal"><span class="pre">Modules/</span></tt> subdirectory, though it’s not compiled by default
(note that this is not available in the Windows distribution – there is no
curses module for Windows).</p>
<p>The curses module supports basic curses features as well as many additional
functions from ncurses and SYSV curses such as colour, alternative character set
support, pads, and mouse support. This means the module isn’t compatible with
operating systems that only have BSD curses, but there don’t seem to be any
currently maintained OSes that fall into this category.</p>
<p>For Windows: use <a class="reference external" href="http://effbot.org/zone/console-index.htm">the consolelib module</a>.</p>
</div>
<div class="section" id="is-there-an-equivalent-to-c-s-onexit-in-python">
<h3><a class="toc-backref" href="#id8">Is there an equivalent to C’s onexit() in Python?</a><a class="headerlink" href="#is-there-an-equivalent-to-c-s-onexit-in-python" title="Permalink to this headline">¶</a></h3>
<p>The <a class="reference internal" href="../library/atexit.html#module-atexit" title="atexit: Register and execute cleanup functions."><tt class="xref py py-mod docutils literal"><span class="pre">atexit</span></tt></a> module provides a register function that is similar to C’s
onexit.</p>
</div>
<div class="section" id="why-don-t-my-signal-handlers-work">
<h3><a class="toc-backref" href="#id9">Why don’t my signal handlers work?</a><a class="headerlink" href="#why-don-t-my-signal-handlers-work" title="Permalink to this headline">¶</a></h3>
<p>The most common problem is that the signal handler is declared with the wrong
argument list. It is called as</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="n">handler</span><span class="p">(</span><span class="n">signum</span><span class="p">,</span> <span class="n">frame</span><span class="p">)</span>
</pre></div>
</div>
<p>so it should be declared with two arguments:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="k">def</span> <span class="nf">handler</span><span class="p">(</span><span class="n">signum</span><span class="p">,</span> <span class="n">frame</span><span class="p">):</span>
<span class="o">...</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="common-tasks">
<h2><a class="toc-backref" href="#id10">Common tasks</a><a class="headerlink" href="#common-tasks" title="Permalink to this headline">¶</a></h2>
<div class="section" id="how-do-i-test-a-python-program-or-component">
<h3><a class="toc-backref" href="#id11">How do I test a Python program or component?</a><a class="headerlink" href="#how-do-i-test-a-python-program-or-component" title="Permalink to this headline">¶</a></h3>
<p>Python comes with two testing frameworks. The <a class="reference internal" href="../library/doctest.html#module-doctest" title="doctest: Test pieces of code within docstrings."><tt class="xref py py-mod docutils literal"><span class="pre">doctest</span></tt></a> module finds
examples in the docstrings for a module and runs them, comparing the output with
the expected output given in the docstring.</p>
<p>The <a class="reference internal" href="../library/unittest.html#module-unittest" title="unittest: Unit testing framework for Python."><tt class="xref py py-mod docutils literal"><span class="pre">unittest</span></tt></a> module is a fancier testing framework modelled on Java and
Smalltalk testing frameworks.</p>
<p>For testing, it helps to write the program so that it may be easily tested by
using good modular design. Your program should have almost all functionality
encapsulated in either functions or class methods – and this sometimes has the
surprising and delightful effect of making the program run faster (because local
variable accesses are faster than global accesses). Furthermore the program
should avoid depending on mutating global variables, since this makes testing
much more difficult to do.</p>
<p>The “global main logic” of your program may be as simple as</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s">"__main__"</span><span class="p">:</span>
<span class="n">main_logic</span><span class="p">()</span>
</pre></div>
</div>
<p>at the bottom of the main module of your program.</p>
<p>Once your program is organized as a tractable collection of functions and class
behaviours you should write test functions that exercise the behaviours. A test
suite can be associated with each module which automates a sequence of tests.
This sounds like a lot of work, but since Python is so terse and flexible it’s
surprisingly easy. You can make coding much more pleasant and fun by writing
your test functions in parallel with the “production code”, since this makes it
easy to find bugs and even design flaws earlier.</p>
<p>“Support modules” that are not intended to be the main module of a program may
include a self-test of the module.</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="k">if</span> <span class="n">__name__</span> <span class="o">==</span> <span class="s">"__main__"</span><span class="p">:</span>
<span class="n">self_test</span><span class="p">()</span>
</pre></div>
</div>
<p>Even programs that interact with complex external interfaces may be tested when
the external interfaces are unavailable by using “fake” interfaces implemented
in Python.</p>
</div>
<div class="section" id="how-do-i-create-documentation-from-doc-strings">
<h3><a class="toc-backref" href="#id12">How do I create documentation from doc strings?</a><a class="headerlink" href="#how-do-i-create-documentation-from-doc-strings" title="Permalink to this headline">¶</a></h3>
<p>The <a class="reference internal" href="../library/pydoc.html#module-pydoc" title="pydoc: Documentation generator and online help system."><tt class="xref py py-mod docutils literal"><span class="pre">pydoc</span></tt></a> module can create HTML from the doc strings in your Python
source code. An alternative for creating API documentation purely from
docstrings is <a class="reference external" href="http://epydoc.sf.net/">epydoc</a>. <a class="reference external" href="http://sphinx.pocoo.org">Sphinx</a> can also include docstring content.</p>
</div>
<div class="section" id="how-do-i-get-a-single-keypress-at-a-time">
<h3><a class="toc-backref" href="#id13">How do I get a single keypress at a time?</a><a class="headerlink" href="#how-do-i-get-a-single-keypress-at-a-time" title="Permalink to this headline">¶</a></h3>
<p>For Unix variants: There are several solutions. It’s straightforward to do this
using curses, but curses is a fairly large module to learn.</p>
</div>
</div>
<div class="section" id="threads">
<h2><a class="toc-backref" href="#id14">Threads</a><a class="headerlink" href="#threads" title="Permalink to this headline">¶</a></h2>
<div class="section" id="how-do-i-program-using-threads">
<h3><a class="toc-backref" href="#id15">How do I program using threads?</a><a class="headerlink" href="#how-do-i-program-using-threads" title="Permalink to this headline">¶</a></h3>
<p>Be sure to use the <a class="reference internal" href="../library/threading.html#module-threading" title="threading: Thread-based parallelism."><tt class="xref py py-mod docutils literal"><span class="pre">threading</span></tt></a> module and not the <a class="reference internal" href="../library/_thread.html#module-_thread" title="_thread: Low-level threading API."><tt class="xref py py-mod docutils literal"><span class="pre">_thread</span></tt></a> module.
The <a class="reference internal" href="../library/threading.html#module-threading" title="threading: Thread-based parallelism."><tt class="xref py py-mod docutils literal"><span class="pre">threading</span></tt></a> module builds convenient abstractions on top of the
low-level primitives provided by the <a class="reference internal" href="../library/_thread.html#module-_thread" title="_thread: Low-level threading API."><tt class="xref py py-mod docutils literal"><span class="pre">_thread</span></tt></a> module.</p>
<p>Aahz has a set of slides from his threading tutorial that are helpful; see
<a class="reference external" href="http://www.pythoncraft.com/OSCON2001/">http://www.pythoncraft.com/OSCON2001/</a>.</p>
</div>
<div class="section" id="none-of-my-threads-seem-to-run-why">
<h3><a class="toc-backref" href="#id16">None of my threads seem to run: why?</a><a class="headerlink" href="#none-of-my-threads-seem-to-run-why" title="Permalink to this headline">¶</a></h3>
<p>As soon as the main thread exits, all threads are killed. Your main thread is
running too quickly, giving the threads no time to do any work.</p>
<p>A simple fix is to add a sleep to the end of the program that’s long enough for
all the threads to finish:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">threading</span><span class="o">,</span> <span class="nn">time</span>
<span class="k">def</span> <span class="nf">thread_task</span><span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="n">n</span><span class="p">):</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">n</span><span class="p">):</span> <span class="nb">print</span><span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="n">i</span><span class="p">)</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>
<span class="n">T</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">thread_task</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="nb">str</span><span class="p">(</span><span class="n">i</span><span class="p">),</span> <span class="n">i</span><span class="p">))</span>
<span class="n">T</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span> <span class="c"># <---------------------------!</span>
</pre></div>
</div>
<p>But now (on many platforms) the threads don’t run in parallel, but appear to run
sequentially, one at a time! The reason is that the OS thread scheduler doesn’t
start a new thread until the previous thread is blocked.</p>
<p>A simple fix is to add a tiny sleep to the start of the run function:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="k">def</span> <span class="nf">thread_task</span><span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="n">n</span><span class="p">):</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.001</span><span class="p">)</span> <span class="c"># <--------------------!</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">n</span><span class="p">):</span> <span class="nb">print</span><span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="n">i</span><span class="p">)</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">):</span>
<span class="n">T</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">thread_task</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">(</span><span class="nb">str</span><span class="p">(</span><span class="n">i</span><span class="p">),</span> <span class="n">i</span><span class="p">))</span>
<span class="n">T</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span>
</pre></div>
</div>
<p>Instead of trying to guess how long a <a class="reference internal" href="../library/time.html#time.sleep" title="time.sleep"><tt class="xref py py-func docutils literal"><span class="pre">time.sleep()</span></tt></a> delay will be enough,
it’s better to use some kind of semaphore mechanism. One idea is to use the
<a class="reference internal" href="../library/queue.html#module-queue" title="queue: A synchronized queue class."><tt class="xref py py-mod docutils literal"><span class="pre">queue</span></tt></a> module to create a queue object, let each thread append a token to
the queue when it finishes, and let the main thread read as many tokens from the
queue as there are threads.</p>
</div>
<div class="section" id="how-do-i-parcel-out-work-among-a-bunch-of-worker-threads">
<h3><a class="toc-backref" href="#id17">How do I parcel out work among a bunch of worker threads?</a><a class="headerlink" href="#how-do-i-parcel-out-work-among-a-bunch-of-worker-threads" title="Permalink to this headline">¶</a></h3>
<p>The easiest way is to use the new <a class="reference internal" href="../library/concurrent.futures.html#module-concurrent.futures" title="concurrent.futures: Execute computations concurrently using threads or processes."><tt class="xref py py-mod docutils literal"><span class="pre">concurrent.futures</span></tt></a> module,
especially the <a class="reference internal" href="../library/concurrent.futures.html#concurrent.futures.ThreadPoolExecutor" title="concurrent.futures.ThreadPoolExecutor"><tt class="xref py py-mod docutils literal"><span class="pre">ThreadPoolExecutor</span></tt></a> class.</p>
<p>Or, if you want fine control over the dispatching algorithm, you can write
your own logic manually. Use the <a class="reference internal" href="../library/queue.html#module-queue" title="queue: A synchronized queue class."><tt class="xref py py-mod docutils literal"><span class="pre">queue</span></tt></a> module to create a queue
containing a list of jobs. The <a class="reference internal" href="../library/queue.html#queue.Queue" title="queue.Queue"><tt class="xref py py-class docutils literal"><span class="pre">Queue</span></tt></a> class maintains a
list of objects with <tt class="docutils literal"><span class="pre">.put(obj)</span></tt> to add an item to the queue and <tt class="docutils literal"><span class="pre">.get()</span></tt>
to return an item. The class will take care of the locking necessary to
ensure that each job is handed out exactly once.</p>
<p>Here’s a trivial example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">threading</span><span class="o">,</span> <span class="nn">queue</span><span class="o">,</span> <span class="nn">time</span>
<span class="c"># The worker thread gets jobs off the queue. When the queue is empty, it</span>
<span class="c"># assumes there will be no more work and exits.</span>
<span class="c"># (Realistically workers will run until terminated.)</span>
<span class="k">def</span> <span class="nf">worker</span> <span class="p">():</span>
<span class="nb">print</span><span class="p">(</span><span class="s">'Running worker'</span><span class="p">)</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.1</span><span class="p">)</span>
<span class="k">while</span> <span class="k">True</span><span class="p">:</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">arg</span> <span class="o">=</span> <span class="n">q</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">block</span><span class="o">=</span><span class="k">False</span><span class="p">)</span>
<span class="k">except</span> <span class="n">queue</span><span class="o">.</span><span class="n">Empty</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s">'Worker'</span><span class="p">,</span> <span class="n">threading</span><span class="o">.</span><span class="n">currentThread</span><span class="p">(),</span> <span class="n">end</span><span class="o">=</span><span class="s">' '</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s">'queue empty'</span><span class="p">)</span>
<span class="k">break</span>
<span class="k">else</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s">'Worker'</span><span class="p">,</span> <span class="n">threading</span><span class="o">.</span><span class="n">currentThread</span><span class="p">(),</span> <span class="n">end</span><span class="o">=</span><span class="s">' '</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s">'running with argument'</span><span class="p">,</span> <span class="n">arg</span><span class="p">)</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mf">0.5</span><span class="p">)</span>
<span class="c"># Create queue</span>
<span class="n">q</span> <span class="o">=</span> <span class="n">queue</span><span class="o">.</span><span class="n">Queue</span><span class="p">()</span>
<span class="c"># Start a pool of 5 workers</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">5</span><span class="p">):</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">threading</span><span class="o">.</span><span class="n">Thread</span><span class="p">(</span><span class="n">target</span><span class="o">=</span><span class="n">worker</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="s">'worker %i'</span> <span class="o">%</span> <span class="p">(</span><span class="n">i</span><span class="o">+</span><span class="mi">1</span><span class="p">))</span>
<span class="n">t</span><span class="o">.</span><span class="n">start</span><span class="p">()</span>
<span class="c"># Begin adding work to the queue</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">50</span><span class="p">):</span>
<span class="n">q</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="n">i</span><span class="p">)</span>
<span class="c"># Give threads time to run</span>
<span class="nb">print</span><span class="p">(</span><span class="s">'Main thread sleeping'</span><span class="p">)</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
</pre></div>
</div>
<p>When run, this will produce the following output:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="n">Running</span> <span class="n">worker</span>
<span class="n">Running</span> <span class="n">worker</span>
<span class="n">Running</span> <span class="n">worker</span>
<span class="n">Running</span> <span class="n">worker</span>
<span class="n">Running</span> <span class="n">worker</span>
<span class="n">Main</span> <span class="n">thread</span> <span class="n">sleeping</span>
<span class="n">Worker</span> <span class="o"><</span><span class="n">Thread</span><span class="p">(</span><span class="n">worker</span> <span class="mi">1</span><span class="p">,</span> <span class="n">started</span> <span class="mi">130283832797456</span><span class="p">)</span><span class="o">></span> <span class="n">running</span> <span class="k">with</span> <span class="n">argument</span> <span class="mi">0</span>
<span class="n">Worker</span> <span class="o"><</span><span class="n">Thread</span><span class="p">(</span><span class="n">worker</span> <span class="mi">2</span><span class="p">,</span> <span class="n">started</span> <span class="mi">130283824404752</span><span class="p">)</span><span class="o">></span> <span class="n">running</span> <span class="k">with</span> <span class="n">argument</span> <span class="mi">1</span>
<span class="n">Worker</span> <span class="o"><</span><span class="n">Thread</span><span class="p">(</span><span class="n">worker</span> <span class="mi">3</span><span class="p">,</span> <span class="n">started</span> <span class="mi">130283816012048</span><span class="p">)</span><span class="o">></span> <span class="n">running</span> <span class="k">with</span> <span class="n">argument</span> <span class="mi">2</span>
<span class="n">Worker</span> <span class="o"><</span><span class="n">Thread</span><span class="p">(</span><span class="n">worker</span> <span class="mi">4</span><span class="p">,</span> <span class="n">started</span> <span class="mi">130283807619344</span><span class="p">)</span><span class="o">></span> <span class="n">running</span> <span class="k">with</span> <span class="n">argument</span> <span class="mi">3</span>
<span class="n">Worker</span> <span class="o"><</span><span class="n">Thread</span><span class="p">(</span><span class="n">worker</span> <span class="mi">5</span><span class="p">,</span> <span class="n">started</span> <span class="mi">130283799226640</span><span class="p">)</span><span class="o">></span> <span class="n">running</span> <span class="k">with</span> <span class="n">argument</span> <span class="mi">4</span>
<span class="n">Worker</span> <span class="o"><</span><span class="n">Thread</span><span class="p">(</span><span class="n">worker</span> <span class="mi">1</span><span class="p">,</span> <span class="n">started</span> <span class="mi">130283832797456</span><span class="p">)</span><span class="o">></span> <span class="n">running</span> <span class="k">with</span> <span class="n">argument</span> <span class="mi">5</span>
<span class="o">...</span>
</pre></div>
</div>
<p>Consult the module’s documentation for more details; the <tt class="docutils literal"><span class="pre">Queue</span></tt> class
provides a featureful interface.</p>
</div>
<div class="section" id="what-kinds-of-global-value-mutation-are-thread-safe">
<h3><a class="toc-backref" href="#id18">What kinds of global value mutation are thread-safe?</a><a class="headerlink" href="#what-kinds-of-global-value-mutation-are-thread-safe" title="Permalink to this headline">¶</a></h3>
<p>A <a class="reference internal" href="../glossary.html#term-global-interpreter-lock"><em class="xref std std-term">global interpreter lock</em></a> (GIL) is used internally to ensure that only one
thread runs in the Python VM at a time. In general, Python offers to switch
among threads only between bytecode instructions; how frequently it switches can
be set via <a class="reference internal" href="../library/sys.html#sys.setswitchinterval" title="sys.setswitchinterval"><tt class="xref py py-func docutils literal"><span class="pre">sys.setswitchinterval()</span></tt></a>. Each bytecode instruction and
therefore all the C implementation code reached from each instruction is
therefore atomic from the point of view of a Python program.</p>
<p>In theory, this means an exact accounting requires an exact understanding of the
PVM bytecode implementation. In practice, it means that operations on shared
variables of built-in data types (ints, lists, dicts, etc) that “look atomic”
really are.</p>
<p>For example, the following operations are all atomic (L, L1, L2 are lists, D,
D1, D2 are dicts, x, y are objects, i, j are ints):</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="n">L</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="n">L1</span><span class="o">.</span><span class="n">extend</span><span class="p">(</span><span class="n">L2</span><span class="p">)</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">L</span><span class="p">[</span><span class="n">i</span><span class="p">]</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">L</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span>
<span class="n">L1</span><span class="p">[</span><span class="n">i</span><span class="p">:</span><span class="n">j</span><span class="p">]</span> <span class="o">=</span> <span class="n">L2</span>
<span class="n">L</span><span class="o">.</span><span class="n">sort</span><span class="p">()</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">y</span>
<span class="n">x</span><span class="o">.</span><span class="n">field</span> <span class="o">=</span> <span class="n">y</span>
<span class="n">D</span><span class="p">[</span><span class="n">x</span><span class="p">]</span> <span class="o">=</span> <span class="n">y</span>
<span class="n">D1</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="n">D2</span><span class="p">)</span>
<span class="n">D</span><span class="o">.</span><span class="n">keys</span><span class="p">()</span>
</pre></div>
</div>
<p>These aren’t:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="n">i</span> <span class="o">=</span> <span class="n">i</span><span class="o">+</span><span class="mi">1</span>
<span class="n">L</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">L</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">])</span>
<span class="n">L</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">=</span> <span class="n">L</span><span class="p">[</span><span class="n">j</span><span class="p">]</span>
<span class="n">D</span><span class="p">[</span><span class="n">x</span><span class="p">]</span> <span class="o">=</span> <span class="n">D</span><span class="p">[</span><span class="n">x</span><span class="p">]</span> <span class="o">+</span> <span class="mi">1</span>
</pre></div>
</div>
<p>Operations that replace other objects may invoke those other objects’
<a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><tt class="xref py py-meth docutils literal"><span class="pre">__del__()</span></tt></a> method when their reference count reaches zero, and that can
affect things. This is especially true for the mass updates to dictionaries and
lists. When in doubt, use a mutex!</p>
</div>
<div class="section" id="can-t-we-get-rid-of-the-global-interpreter-lock">
<h3><a class="toc-backref" href="#id19">Can’t we get rid of the Global Interpreter Lock?</a><a class="headerlink" href="#can-t-we-get-rid-of-the-global-interpreter-lock" title="Permalink to this headline">¶</a></h3>
<p>The <a class="reference internal" href="../glossary.html#term-global-interpreter-lock"><em class="xref std std-term">global interpreter lock</em></a> (GIL) is often seen as a hindrance to Python’s
deployment on high-end multiprocessor server machines, because a multi-threaded
Python program effectively only uses one CPU, due to the insistence that
(almost) all Python code can only run while the GIL is held.</p>
<p>Back in the days of Python 1.5, Greg Stein actually implemented a comprehensive
patch set (the “free threading” patches) that removed the GIL and replaced it
with fine-grained locking. Adam Olsen recently did a similar experiment
in his <a class="reference external" href="http://code.google.com/p/python-safethread/">python-safethread</a>
project. Unfortunately, both experiments exhibited a sharp drop in single-thread
performance (at least 30% slower), due to the amount of fine-grained locking
necessary to compensate for the removal of the GIL.</p>
<p>This doesn’t mean that you can’t make good use of Python on multi-CPU machines!
You just have to be creative with dividing the work up between multiple
<em>processes</em> rather than multiple <em>threads</em>. The
<a class="reference internal" href="../library/concurrent.futures.html#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><tt class="xref py py-class docutils literal"><span class="pre">ProcessPoolExecutor</span></tt></a> class in the new
<a class="reference internal" href="../library/concurrent.futures.html#module-concurrent.futures" title="concurrent.futures: Execute computations concurrently using threads or processes."><tt class="xref py py-mod docutils literal"><span class="pre">concurrent.futures</span></tt></a> module provides an easy way of doing so; the
<a class="reference internal" href="../library/multiprocessing.html#module-multiprocessing" title="multiprocessing: Process-based parallelism."><tt class="xref py py-mod docutils literal"><span class="pre">multiprocessing</span></tt></a> module provides a lower-level API in case you want
more control over dispatching of tasks.</p>
<p>Judicious use of C extensions will also help; if you use a C extension to
perform a time-consuming task, the extension can release the GIL while the
thread of execution is in the C code and allow other threads to get some work
done. Some standard library modules such as <a class="reference internal" href="../library/zlib.html#module-zlib" title="zlib: Low-level interface to compression and decompression routines compatible with gzip."><tt class="xref py py-mod docutils literal"><span class="pre">zlib</span></tt></a> and <a class="reference internal" href="../library/hashlib.html#module-hashlib" title="hashlib: Secure hash and message digest algorithms."><tt class="xref py py-mod docutils literal"><span class="pre">hashlib</span></tt></a>
already do this.</p>
<p>It has been suggested that the GIL should be a per-interpreter-state lock rather
than truly global; interpreters then wouldn’t be able to share objects.
Unfortunately, this isn’t likely to happen either. It would be a tremendous
amount of work, because many object implementations currently have global state.
For example, small integers and short strings are cached; these caches would
have to be moved to the interpreter state. Other object types have their own
free list; these free lists would have to be moved to the interpreter state.
And so on.</p>
<p>And I doubt that it can even be done in finite time, because the same problem
exists for 3rd party extensions. It is likely that 3rd party extensions are
being written at a faster rate than you can convert them to store all their
global state in the interpreter state.</p>
<p>And finally, once you have multiple interpreters not sharing any state, what
have you gained over running each interpreter in a separate process?</p>
</div>
</div>
<div class="section" id="input-and-output">
<h2><a class="toc-backref" href="#id20">Input and Output</a><a class="headerlink" href="#input-and-output" title="Permalink to this headline">¶</a></h2>
<div class="section" id="how-do-i-delete-a-file-and-other-file-questions">
<h3><a class="toc-backref" href="#id21">How do I delete a file? (And other file questions...)</a><a class="headerlink" href="#how-do-i-delete-a-file-and-other-file-questions" title="Permalink to this headline">¶</a></h3>
<p>Use <tt class="docutils literal"><span class="pre">os.remove(filename)</span></tt> or <tt class="docutils literal"><span class="pre">os.unlink(filename)</span></tt>; for documentation, see
the <a class="reference internal" href="../library/os.html#module-os" title="os: Miscellaneous operating system interfaces."><tt class="xref py py-mod docutils literal"><span class="pre">os</span></tt></a> module. The two functions are identical; <a class="reference internal" href="../library/os.html#os.unlink" title="os.unlink"><tt class="xref py py-func docutils literal"><span class="pre">unlink()</span></tt></a> is simply
the name of the Unix system call for this function.</p>
<p>To remove a directory, use <a class="reference internal" href="../library/os.html#os.rmdir" title="os.rmdir"><tt class="xref py py-func docutils literal"><span class="pre">os.rmdir()</span></tt></a>; use <a class="reference internal" href="../library/os.html#os.mkdir" title="os.mkdir"><tt class="xref py py-func docutils literal"><span class="pre">os.mkdir()</span></tt></a> to create one.
<tt class="docutils literal"><span class="pre">os.makedirs(path)</span></tt> will create any intermediate directories in <tt class="docutils literal"><span class="pre">path</span></tt> that
don’t exist. <tt class="docutils literal"><span class="pre">os.removedirs(path)</span></tt> will remove intermediate directories as
long as they’re empty; if you want to delete an entire directory tree and its
contents, use <a class="reference internal" href="../library/shutil.html#shutil.rmtree" title="shutil.rmtree"><tt class="xref py py-func docutils literal"><span class="pre">shutil.rmtree()</span></tt></a>.</p>
<p>To rename a file, use <tt class="docutils literal"><span class="pre">os.rename(old_path,</span> <span class="pre">new_path)</span></tt>.</p>
<p>To truncate a file, open it using <tt class="docutils literal"><span class="pre">f</span> <span class="pre">=</span> <span class="pre">open(filename,</span> <span class="pre">"rb+")</span></tt>, and use
<tt class="docutils literal"><span class="pre">f.truncate(offset)</span></tt>; offset defaults to the current seek position. There’s
also <tt class="docutils literal"><span class="pre">os.ftruncate(fd,</span> <span class="pre">offset)</span></tt> for files opened with <a class="reference internal" href="../library/os.html#os.open" title="os.open"><tt class="xref py py-func docutils literal"><span class="pre">os.open()</span></tt></a>, where
<tt class="docutils literal"><span class="pre">fd</span></tt> is the file descriptor (a small integer).</p>
<p>The <a class="reference internal" href="../library/shutil.html#module-shutil" title="shutil: High-level file operations, including copying."><tt class="xref py py-mod docutils literal"><span class="pre">shutil</span></tt></a> module also contains a number of functions to work on files
including <a class="reference internal" href="../library/shutil.html#shutil.copyfile" title="shutil.copyfile"><tt class="xref py py-func docutils literal"><span class="pre">copyfile()</span></tt></a>, <a class="reference internal" href="../library/shutil.html#shutil.copytree" title="shutil.copytree"><tt class="xref py py-func docutils literal"><span class="pre">copytree()</span></tt></a>, and
<a class="reference internal" href="../library/shutil.html#shutil.rmtree" title="shutil.rmtree"><tt class="xref py py-func docutils literal"><span class="pre">rmtree()</span></tt></a>.</p>
</div>
<div class="section" id="how-do-i-copy-a-file">
<h3><a class="toc-backref" href="#id22">How do I copy a file?</a><a class="headerlink" href="#how-do-i-copy-a-file" title="Permalink to this headline">¶</a></h3>
<p>The <a class="reference internal" href="../library/shutil.html#module-shutil" title="shutil: High-level file operations, including copying."><tt class="xref py py-mod docutils literal"><span class="pre">shutil</span></tt></a> module contains a <a class="reference internal" href="../library/shutil.html#shutil.copyfile" title="shutil.copyfile"><tt class="xref py py-func docutils literal"><span class="pre">copyfile()</span></tt></a> function. Note
that on MacOS 9 it doesn’t copy the resource fork and Finder info.</p>
</div>
<div class="section" id="how-do-i-read-or-write-binary-data">
<h3><a class="toc-backref" href="#id23">How do I read (or write) binary data?</a><a class="headerlink" href="#how-do-i-read-or-write-binary-data" title="Permalink to this headline">¶</a></h3>
<p>To read or write complex binary data formats, it’s best to use the <a class="reference internal" href="../library/struct.html#module-struct" title="struct: Interpret bytes as packed binary data."><tt class="xref py py-mod docutils literal"><span class="pre">struct</span></tt></a>
module. It allows you to take a string containing binary data (usually numbers)
and convert it to Python objects; and vice versa.</p>
<p>For example, the following code reads two 2-byte integers and one 4-byte integer
in big-endian format from a file:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">struct</span>
<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="s">"rb"</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
<span class="n">s</span> <span class="o">=</span> <span class="n">f</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="mi">8</span><span class="p">)</span>
<span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">z</span> <span class="o">=</span> <span class="n">struct</span><span class="o">.</span><span class="n">unpack</span><span class="p">(</span><span class="s">">hhl"</span><span class="p">,</span> <span class="n">s</span><span class="p">)</span>
</pre></div>
</div>
<p>The ‘>’ in the format string forces big-endian data; the letter ‘h’ reads one
“short integer” (2 bytes), and ‘l’ reads one “long integer” (4 bytes) from the
string.</p>
<p>For data that is more regular (e.g. a homogeneous list of ints or thefloats),
you can also use the <a class="reference internal" href="../library/array.html#module-array" title="array: Space efficient arrays of uniformly typed numeric values."><tt class="xref py py-mod docutils literal"><span class="pre">array</span></tt></a> module.</p>
<blockquote>
<div><div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">To read and write binary data, it is mandatory to open the file in
binary mode (here, passing <tt class="docutils literal"><span class="pre">"rb"</span></tt> to <a class="reference internal" href="../library/functions.html#open" title="open"><tt class="xref py py-func docutils literal"><span class="pre">open()</span></tt></a>). If you use
<tt class="docutils literal"><span class="pre">"r"</span></tt> instead (the default), the file will be open in text mode
and <tt class="docutils literal"><span class="pre">f.read()</span></tt> will return <a class="reference internal" href="../library/functions.html#str" title="str"><tt class="xref py py-class docutils literal"><span class="pre">str</span></tt></a> objects rather than
<a class="reference internal" href="../library/functions.html#bytes" title="bytes"><tt class="xref py py-class docutils literal"><span class="pre">bytes</span></tt></a> objects.</p>
</div>
</div></blockquote>
</div>
<div class="section" id="i-can-t-seem-to-use-os-read-on-a-pipe-created-with-os-popen-why">
<h3><a class="toc-backref" href="#id24">I can’t seem to use os.read() on a pipe created with os.popen(); why?</a><a class="headerlink" href="#i-can-t-seem-to-use-os-read-on-a-pipe-created-with-os-popen-why" title="Permalink to this headline">¶</a></h3>
<p><a class="reference internal" href="../library/os.html#os.read" title="os.read"><tt class="xref py py-func docutils literal"><span class="pre">os.read()</span></tt></a> is a low-level function which takes a file descriptor, a small
integer representing the opened file. <tt class="xref py py-func docutils literal"><span class="pre">os.popen()</span></tt> creates a high-level
file object, the same type returned by the built-in <a class="reference internal" href="../library/functions.html#open" title="open"><tt class="xref py py-func docutils literal"><span class="pre">open()</span></tt></a> function.
Thus, to read n bytes from a pipe p created with <tt class="xref py py-func docutils literal"><span class="pre">os.popen()</span></tt>, you need to
use <tt class="docutils literal"><span class="pre">p.read(n)</span></tt>.</p>
</div>
<div class="section" id="how-do-i-access-the-serial-rs232-port">
<h3><a class="toc-backref" href="#id25">How do I access the serial (RS232) port?</a><a class="headerlink" href="#how-do-i-access-the-serial-rs232-port" title="Permalink to this headline">¶</a></h3>
<p>For Win32, POSIX (Linux, BSD, etc.), Jython:</p>
<blockquote>
<div><a class="reference external" href="http://pyserial.sourceforge.net">http://pyserial.sourceforge.net</a></div></blockquote>
<p>For Unix, see a Usenet post by Mitch Chapman:</p>
<blockquote>
<div><a class="reference external" href="http://groups.google.com/groups?selm=34A04430.CF9@ohioee.com">http://groups.google.com/groups?selm=34A04430.CF9@ohioee.com</a></div></blockquote>
</div>
<div class="section" id="why-doesn-t-closing-sys-stdout-stdin-stderr-really-close-it">
<h3><a class="toc-backref" href="#id26">Why doesn’t closing sys.stdout (stdin, stderr) really close it?</a><a class="headerlink" href="#why-doesn-t-closing-sys-stdout-stdin-stderr-really-close-it" title="Permalink to this headline">¶</a></h3>
<p>Python <a class="reference internal" href="../glossary.html#term-file-object"><em class="xref std std-term">file objects</em></a> are a high-level layer of
abstraction on low-level C file descriptors.</p>
<p>For most file objects you create in Python via the built-in <a class="reference internal" href="../library/functions.html#open" title="open"><tt class="xref py py-func docutils literal"><span class="pre">open()</span></tt></a>
function, <tt class="docutils literal"><span class="pre">f.close()</span></tt> marks the Python file object as being closed from
Python’s point of view, and also arranges to close the underlying C file
descriptor. This also happens automatically in <tt class="docutils literal"><span class="pre">f</span></tt>‘s destructor, when
<tt class="docutils literal"><span class="pre">f</span></tt> becomes garbage.</p>
<p>But stdin, stdout and stderr are treated specially by Python, because of the
special status also given to them by C. Running <tt class="docutils literal"><span class="pre">sys.stdout.close()</span></tt> marks
the Python-level file object as being closed, but does <em>not</em> close the
associated C file descriptor.</p>
<p>To close the underlying C file descriptor for one of these three, you should
first be sure that’s what you really want to do (e.g., you may confuse
extension modules trying to do I/O). If it is, use <a class="reference internal" href="../library/os.html#os.close" title="os.close"><tt class="xref py py-func docutils literal"><span class="pre">os.close()</span></tt></a>:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="n">os</span><span class="o">.</span><span class="n">close</span><span class="p">(</span><span class="n">stdin</span><span class="o">.</span><span class="n">fileno</span><span class="p">())</span>
<span class="n">os</span><span class="o">.</span><span class="n">close</span><span class="p">(</span><span class="n">stdout</span><span class="o">.</span><span class="n">fileno</span><span class="p">())</span>
<span class="n">os</span><span class="o">.</span><span class="n">close</span><span class="p">(</span><span class="n">stderr</span><span class="o">.</span><span class="n">fileno</span><span class="p">())</span>
</pre></div>
</div>
<p>Or you can use the numeric constants 0, 1 and 2, respectively.</p>
</div>
</div>
<div class="section" id="network-internet-programming">
<h2><a class="toc-backref" href="#id27">Network/Internet Programming</a><a class="headerlink" href="#network-internet-programming" title="Permalink to this headline">¶</a></h2>
<div class="section" id="what-www-tools-are-there-for-python">
<h3><a class="toc-backref" href="#id28">What WWW tools are there for Python?</a><a class="headerlink" href="#what-www-tools-are-there-for-python" title="Permalink to this headline">¶</a></h3>
<p>See the chapters titled <a class="reference internal" href="../library/internet.html#internet"><em>Internet Protocols and Support</em></a> and <a class="reference internal" href="../library/netdata.html#netdata"><em>Internet Data Handling</em></a> in the Library
Reference Manual. Python has many modules that will help you build server-side
and client-side web systems.</p>
<p>A summary of available frameworks is maintained by Paul Boddie at
<a class="reference external" href="http://wiki.python.org/moin/WebProgramming">http://wiki.python.org/moin/WebProgramming</a> .</p>
<p>Cameron Laird maintains a useful set of pages about Python web technologies at
<a class="reference external" href="http://phaseit.net/claird/comp.lang.python/web_python">http://phaseit.net/claird/comp.lang.python/web_python</a>.</p>
</div>
<div class="section" id="how-can-i-mimic-cgi-form-submission-method-post">
<h3><a class="toc-backref" href="#id29">How can I mimic CGI form submission (METHOD=POST)?</a><a class="headerlink" href="#how-can-i-mimic-cgi-form-submission-method-post" title="Permalink to this headline">¶</a></h3>
<p>I would like to retrieve web pages that are the result of POSTing a form. Is
there existing code that would let me do this easily?</p>
<p>Yes. Here’s a simple example that uses urllib.request:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="c">#!/usr/bin/python3</span>
<span class="kn">import</span> <span class="nn">urllib.request</span>
<span class="c">### build the query string</span>
<span class="n">qs</span> <span class="o">=</span> <span class="s">"First=Josephine&MI=Q&Last=Public"</span>
<span class="c">### connect and send the server a path</span>
<span class="n">req</span> <span class="o">=</span> <span class="n">urllib</span><span class="o">.</span><span class="n">request</span><span class="o">.</span><span class="n">urlopen</span><span class="p">(</span><span class="s">'http://www.some-server.out-there'</span>
<span class="s">'/cgi-bin/some-cgi-script'</span><span class="p">,</span> <span class="n">data</span><span class="o">=</span><span class="n">qs</span><span class="p">)</span>
<span class="n">msg</span><span class="p">,</span> <span class="n">hdrs</span> <span class="o">=</span> <span class="n">req</span><span class="o">.</span><span class="n">read</span><span class="p">(),</span> <span class="n">req</span><span class="o">.</span><span class="n">info</span><span class="p">()</span>
</pre></div>
</div>
<p>Note that in general for percent-encoded POST operations, query strings must be
quoted using <a class="reference internal" href="../library/urllib.parse.html#urllib.parse.urlencode" title="urllib.parse.urlencode"><tt class="xref py py-func docutils literal"><span class="pre">urllib.parse.urlencode()</span></tt></a>. For example to send name=”Guy Steele,
Jr.”:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">urllib.parse</span>
<span class="gp">>>> </span><span class="n">urllib</span><span class="o">.</span><span class="n">parse</span><span class="o">.</span><span class="n">urlencode</span><span class="p">({</span><span class="s">'name'</span><span class="p">:</span> <span class="s">'Guy Steele, Jr.'</span><span class="p">})</span>
<span class="go">'name=Guy+Steele%2C+Jr.'</span>
</pre></div>
</div>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference internal" href="../howto/urllib2.html#urllib-howto"><em>HOWTO Fetch Internet Resources Using The urllib Package</em></a> for extensive examples.</p>
</div>
</div>
<div class="section" id="what-module-should-i-use-to-help-with-generating-html">
<h3><a class="toc-backref" href="#id30">What module should I use to help with generating HTML?</a><a class="headerlink" href="#what-module-should-i-use-to-help-with-generating-html" title="Permalink to this headline">¶</a></h3>
<p>There are many different modules available:</p>
<ul class="simple">
<li>HTMLgen is a class library of objects corresponding to all the HTML 3.2 markup
tags. It’s used when you are writing in Python and wish to synthesize HTML
pages for generating a web or for CGI forms, etc.</li>
<li>DocumentTemplate and Zope Page Templates are two different systems that are
part of Zope.</li>
<li>Quixote’s PTL uses Python syntax to assemble strings of text.</li>
</ul>
<p>Consult the <a class="reference external" href="http://wiki.python.org/moin/WebProgramming">Web Programming wiki pages</a> for more links.</p>
</div>
<div class="section" id="how-do-i-send-mail-from-a-python-script">
<h3><a class="toc-backref" href="#id31">How do I send mail from a Python script?</a><a class="headerlink" href="#how-do-i-send-mail-from-a-python-script" title="Permalink to this headline">¶</a></h3>
<p>Use the standard library module <a class="reference internal" href="../library/smtplib.html#module-smtplib" title="smtplib: SMTP protocol client (requires sockets)."><tt class="xref py py-mod docutils literal"><span class="pre">smtplib</span></tt></a>.</p>
<p>Here’s a very simple interactive mail sender that uses it. This method will
work on any host that supports an SMTP listener.</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">sys</span><span class="o">,</span> <span class="nn">smtplib</span>
<span class="n">fromaddr</span> <span class="o">=</span> <span class="nb">input</span><span class="p">(</span><span class="s">"From: "</span><span class="p">)</span>
<span class="n">toaddrs</span> <span class="o">=</span> <span class="nb">input</span><span class="p">(</span><span class="s">"To: "</span><span class="p">)</span><span class="o">.</span><span class="n">split</span><span class="p">(</span><span class="s">','</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"Enter message, end with ^D:"</span><span class="p">)</span>
<span class="n">msg</span> <span class="o">=</span> <span class="s">''</span>
<span class="k">while</span> <span class="k">True</span><span class="p">:</span>
<span class="n">line</span> <span class="o">=</span> <span class="n">sys</span><span class="o">.</span><span class="n">stdin</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">line</span><span class="p">:</span>
<span class="k">break</span>
<span class="n">msg</span> <span class="o">+=</span> <span class="n">line</span>
<span class="c"># The actual mail send</span>
<span class="n">server</span> <span class="o">=</span> <span class="n">smtplib</span><span class="o">.</span><span class="n">SMTP</span><span class="p">(</span><span class="s">'localhost'</span><span class="p">)</span>
<span class="n">server</span><span class="o">.</span><span class="n">sendmail</span><span class="p">(</span><span class="n">fromaddr</span><span class="p">,</span> <span class="n">toaddrs</span><span class="p">,</span> <span class="n">msg</span><span class="p">)</span>
<span class="n">server</span><span class="o">.</span><span class="n">quit</span><span class="p">()</span>
</pre></div>
</div>
<p>A Unix-only alternative uses sendmail. The location of the sendmail program
varies between systems; sometimes it is <tt class="docutils literal"><span class="pre">/usr/lib/sendmail</span></tt>, sometime
<tt class="docutils literal"><span class="pre">/usr/sbin/sendmail</span></tt>. The sendmail manual page will help you out. Here’s
some sample code:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="n">SENDMAIL</span> <span class="o">=</span> <span class="s">"/usr/sbin/sendmail"</span> <span class="c"># sendmail location</span>
<span class="kn">import</span> <span class="nn">os</span>
<span class="n">p</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">popen</span><span class="p">(</span><span class="s">"%s -t -i"</span> <span class="o">%</span> <span class="n">SENDMAIL</span><span class="p">,</span> <span class="s">"w"</span><span class="p">)</span>
<span class="n">p</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">"To: receiver@example.com</span><span class="se">\n</span><span class="s">"</span><span class="p">)</span>
<span class="n">p</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">"Subject: test</span><span class="se">\n</span><span class="s">"</span><span class="p">)</span>
<span class="n">p</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">"</span><span class="se">\n</span><span class="s">"</span><span class="p">)</span> <span class="c"># blank line separating headers from body</span>
<span class="n">p</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">"Some text</span><span class="se">\n</span><span class="s">"</span><span class="p">)</span>
<span class="n">p</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">"some more text</span><span class="se">\n</span><span class="s">"</span><span class="p">)</span>
<span class="n">sts</span> <span class="o">=</span> <span class="n">p</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
<span class="k">if</span> <span class="n">sts</span> <span class="o">!=</span> <span class="mi">0</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s">"Sendmail exit status"</span><span class="p">,</span> <span class="n">sts</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="how-do-i-avoid-blocking-in-the-connect-method-of-a-socket">
<h3><a class="toc-backref" href="#id32">How do I avoid blocking in the connect() method of a socket?</a><a class="headerlink" href="#how-do-i-avoid-blocking-in-the-connect-method-of-a-socket" title="Permalink to this headline">¶</a></h3>
<p>The <a class="reference internal" href="../library/select.html#module-select" title="select: Wait for I/O completion on multiple streams."><tt class="xref py py-mod docutils literal"><span class="pre">select</span></tt></a> module is commonly used to help with asynchronous I/O on
sockets.</p>
<p>To prevent the TCP connect from blocking, you can set the socket to non-blocking
mode. Then when you do the <tt class="docutils literal"><span class="pre">connect()</span></tt>, you will either connect immediately
(unlikely) or get an exception that contains the error number as <tt class="docutils literal"><span class="pre">.errno</span></tt>.
<tt class="docutils literal"><span class="pre">errno.EINPROGRESS</span></tt> indicates that the connection is in progress, but hasn’t
finished yet. Different OSes will return different values, so you’re going to
have to check what’s returned on your system.</p>
<p>You can use the <tt class="docutils literal"><span class="pre">connect_ex()</span></tt> method to avoid creating an exception. It will
just return the errno value. To poll, you can call <tt class="docutils literal"><span class="pre">connect_ex()</span></tt> again later
– <tt class="docutils literal"><span class="pre">0</span></tt> or <tt class="docutils literal"><span class="pre">errno.EISCONN</span></tt> indicate that you’re connected – or you can pass this
socket to select to check if it’s writable.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The <a class="reference internal" href="../library/asyncore.html#module-asyncore" title="asyncore: A base class for developing asynchronous socket handling services."><tt class="xref py py-mod docutils literal"><span class="pre">asyncore</span></tt></a> module presents a framework-like approach to the problem
of writing non-blocking networking code.
The third-party <a class="reference external" href="http://twistedmatrix.com/">Twisted</a> library is
a popular and feature-rich alternative.</p>
</div>
</div>
</div>
<div class="section" id="databases">
<h2><a class="toc-backref" href="#id33">Databases</a><a class="headerlink" href="#databases" title="Permalink to this headline">¶</a></h2>
<div class="section" id="are-there-any-interfaces-to-database-packages-in-python">
<h3><a class="toc-backref" href="#id34">Are there any interfaces to database packages in Python?</a><a class="headerlink" href="#are-there-any-interfaces-to-database-packages-in-python" title="Permalink to this headline">¶</a></h3>
<p>Yes.</p>
<p>Interfaces to disk-based hashes such as <a class="reference internal" href="../library/dbm.html#module-dbm.ndbm" title="dbm.ndbm: The standard "database" interface, based on ndbm. (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">DBM</span></tt></a> and <a class="reference internal" href="../library/dbm.html#module-dbm.gnu" title="dbm.gnu: GNU's reinterpretation of dbm. (Unix)"><tt class="xref py py-mod docutils literal"><span class="pre">GDBM</span></tt></a> are also included with standard Python. There is also the
<a class="reference internal" href="../library/sqlite3.html#module-sqlite3" title="sqlite3: A DB-API 2.0 implementation using SQLite 3.x."><tt class="xref py py-mod docutils literal"><span class="pre">sqlite3</span></tt></a> module, which provides a lightweight disk-based relational
database.</p>
<p>Support for most relational databases is available. See the
<a class="reference external" href="http://wiki.python.org/moin/DatabaseProgramming">DatabaseProgramming wiki page</a> for details.</p>
</div>
<div class="section" id="how-do-you-implement-persistent-objects-in-python">
<h3><a class="toc-backref" href="#id35">How do you implement persistent objects in Python?</a><a class="headerlink" href="#how-do-you-implement-persistent-objects-in-python" title="Permalink to this headline">¶</a></h3>
<p>The <a class="reference internal" href="../library/pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><tt class="xref py py-mod docutils literal"><span class="pre">pickle</span></tt></a> library module solves this in a very general way (though you
still can’t store things like open files, sockets or windows), and the
<a class="reference internal" href="../library/shelve.html#module-shelve" title="shelve: Python object persistence."><tt class="xref py py-mod docutils literal"><span class="pre">shelve</span></tt></a> library module uses pickle and (g)dbm to create persistent
mappings containing arbitrary Python objects.</p>
<p>A more awkward way of doing things is to use pickle’s little sister, marshal.
The <a class="reference internal" href="../library/marshal.html#module-marshal" title="marshal: Convert Python objects to streams of bytes and back (with different constraints)."><tt class="xref py py-mod docutils literal"><span class="pre">marshal</span></tt></a> module provides very fast ways to store noncircular basic
Python types to files and strings, and back again. Although marshal does not do
fancy things like store instances or handle shared references properly, it does
run extremely fast. For example loading a half megabyte of data may take less
than a third of a second. This often beats doing something more complex and
general such as using gdbm with pickle/shelve.</p>
</div>
<div class="section" id="if-my-program-crashes-with-a-bsddb-or-anydbm-database-open-it-gets-corrupted-how-come">
<h3><a class="toc-backref" href="#id36">If my program crashes with a bsddb (or anydbm) database open, it gets corrupted. How come?</a><a class="headerlink" href="#if-my-program-crashes-with-a-bsddb-or-anydbm-database-open-it-gets-corrupted-how-come" title="Permalink to this headline">¶</a></h3>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The bsddb module is now available as a standalone package <a class="reference external" href="http://www.jcea.es/programacion/pybsddb.htm">pybsddb</a>.</p>
</div>
<p>Databases opened for write access with the bsddb module (and often by the anydbm
module, since it will preferentially use bsddb) must explicitly be closed using
the <tt class="docutils literal"><span class="pre">.close()</span></tt> method of the database. The underlying library caches database
contents which need to be converted to on-disk form and written.</p>
<p>If you have initialized a new bsddb database but not written anything to it
before the program crashes, you will often wind up with a zero-length file and
encounter an exception the next time the file is opened.</p>
</div>
<div class="section" id="i-tried-to-open-berkeley-db-file-but-bsddb-produces-bsddb-error-22-invalid-argument-help-how-can-i-restore-my-data">
<h3><a class="toc-backref" href="#id37">I tried to open Berkeley DB file, but bsddb produces bsddb.error: (22, ‘Invalid argument’). Help! How can I restore my data?</a><a class="headerlink" href="#i-tried-to-open-berkeley-db-file-but-bsddb-produces-bsddb-error-22-invalid-argument-help-how-can-i-restore-my-data" title="Permalink to this headline">¶</a></h3>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The bsddb module is now available as a standalone package <a class="reference external" href="http://www.jcea.es/programacion/pybsddb.htm">pybsddb</a>.</p>
</div>
<p>Don’t panic! Your data is probably intact. The most frequent cause for the error
is that you tried to open an earlier Berkeley DB file with a later version of
the Berkeley DB library.</p>
<p>Many Linux systems now have all three versions of Berkeley DB available. If you
are migrating from version 1 to a newer version use db_dump185 to dump a plain
text version of the database. If you are migrating from version 2 to version 3
use db2_dump to create a plain text version of the database. In either case,
use db_load to create a new native database for the latest version installed on
your computer. If you have version 3 of Berkeley DB installed, you should be
able to use db2_load to create a native version 2 database.</p>
<p>You should move away from Berkeley DB version 1 files because the hash file code
contains known bugs that can corrupt your data.</p>
</div>
</div>
<div class="section" id="mathematics-and-numerics">
<h2><a class="toc-backref" href="#id38">Mathematics and Numerics</a><a class="headerlink" href="#mathematics-and-numerics" title="Permalink to this headline">¶</a></h2>
<div class="section" id="how-do-i-generate-random-numbers-in-python">
<h3><a class="toc-backref" href="#id39">How do I generate random numbers in Python?</a><a class="headerlink" href="#how-do-i-generate-random-numbers-in-python" title="Permalink to this headline">¶</a></h3>
<p>The standard module <a class="reference internal" href="../library/random.html#module-random" title="random: Generate pseudo-random numbers with various common distributions."><tt class="xref py py-mod docutils literal"><span class="pre">random</span></tt></a> implements a random number generator. Usage
is simple:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">random</span>
<span class="n">random</span><span class="o">.</span><span class="n">random</span><span class="p">()</span>
</pre></div>
</div>
<p>This returns a random floating point number in the range [0, 1).</p>
<p>There are also many other specialized generators in this module, such as:</p>
<ul class="simple">
<li><tt class="docutils literal"><span class="pre">randrange(a,</span> <span class="pre">b)</span></tt> chooses an integer in the range [a, b).</li>
<li><tt class="docutils literal"><span class="pre">uniform(a,</span> <span class="pre">b)</span></tt> chooses a floating point number in the range [a, b).</li>
<li><tt class="docutils literal"><span class="pre">normalvariate(mean,</span> <span class="pre">sdev)</span></tt> samples the normal (Gaussian) distribution.</li>
</ul>
<p>Some higher-level functions operate on sequences directly, such as:</p>
<ul class="simple">
<li><tt class="docutils literal"><span class="pre">choice(S)</span></tt> chooses random element from a given sequence</li>
<li><tt class="docutils literal"><span class="pre">shuffle(L)</span></tt> shuffles a list in-place, i.e. permutes it randomly</li>
</ul>
<p>There’s also a <tt class="docutils literal"><span class="pre">Random</span></tt> class you can instantiate to create independent
multiple random number generators.</p>
</div>
</div>
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<li><a class="reference internal" href="#common-tasks">Common tasks</a></li>
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