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<div class="section" id="library-and-extension-faq">
<h1><a class="toc-backref" href="#id1">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="id1">Library and Extension FAQ</a><ul>
<li><a class="reference internal" href="#general-library-questions" id="id2">General Library Questions</a><ul>
<li><a class="reference internal" href="#how-do-i-find-a-module-or-application-to-perform-task-x" id="id3">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="id4">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="id5">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="id6">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="id7">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="id8">Why don’t my signal handlers work?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#common-tasks" id="id9">Common tasks</a><ul>
<li><a class="reference internal" href="#how-do-i-test-a-python-program-or-component" id="id10">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="id11">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="id12">How do I get a single keypress at a time?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#threads" id="id13">Threads</a><ul>
<li><a class="reference internal" href="#how-do-i-program-using-threads" id="id14">How do I program using threads?</a></li>
<li><a class="reference internal" href="#none-of-my-threads-seem-to-run-why" id="id15">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="id16">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="id17">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="id18">Can’t we get rid of the Global Interpreter Lock?</a></li>
</ul>
</li>
<li><a class="reference internal" href="#input-and-output" id="id19">Input and Output</a><ul>
<li><a class="reference internal" href="#how-do-i-delete-a-file-and-other-file-questions" id="id20">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="id21">How do I copy a file?</a></li>
<li><a class="reference internal" href="#how-do-i-read-or-write-binary-data" id="id22">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="id23">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-run-a-subprocess-with-pipes-connected-to-both-input-and-output" id="id24">How do I run a subprocess with pipes connected to both input and output?</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="#why-is-cpickle-so-slow" id="id36">Why is cPickle so slow?</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="id37">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="id38">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="id39">Mathematics and Numerics</a><ul>
<li><a class="reference internal" href="#how-do-i-generate-random-numbers-in-python" id="id40">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="#id2">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="#id3">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 external" 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="#id4">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-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">sys</span>
<span class="k">print</span> <span class="n">sys</span><span class="o">.</span><span class="n">builtin_module_names</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="#id5">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-python"><div class="highlight"><pre><span class="c">#!/usr/bin/python</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-python"><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-python"><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-python"><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="#id6">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="#id7">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 title="Register and execute cleanup functions." class="reference external" href="../library/atexit.html#module-atexit"><tt class="xref 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="#id8">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-python"><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-python"><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="#id9">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="#id10">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 title="Test pieces of code within docstrings." class="reference external" href="../library/doctest.html#module-doctest"><tt class="xref 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 title="Unit testing framework for Python." class="reference external" href="../library/unittest.html#module-unittest"><tt class="xref 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-python"><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-python"><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="#id11">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 title="Documentation generator and online help system." class="reference external" href="../library/pydoc.html#module-pydoc"><tt class="xref 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="#id12">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. Here’s a solution
without curses:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">termios</span><span class="o">,</span> <span class="nn">fcntl</span><span class="o">,</span> <span class="nn">sys</span><span class="o">,</span> <span class="nn">os</span>
<span class="n">fd</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">fileno</span><span class="p">()</span>
<span class="n">oldterm</span> <span class="o">=</span> <span class="n">termios</span><span class="o">.</span><span class="n">tcgetattr</span><span class="p">(</span><span class="n">fd</span><span class="p">)</span>
<span class="n">newattr</span> <span class="o">=</span> <span class="n">termios</span><span class="o">.</span><span class="n">tcgetattr</span><span class="p">(</span><span class="n">fd</span><span class="p">)</span>
<span class="n">newattr</span><span class="p">[</span><span class="mi">3</span><span class="p">]</span> <span class="o">=</span> <span class="n">newattr</span><span class="p">[</span><span class="mi">3</span><span class="p">]</span> <span class="o">&</span> <span class="o">~</span><span class="n">termios</span><span class="o">.</span><span class="n">ICANON</span> <span class="o">&</span> <span class="o">~</span><span class="n">termios</span><span class="o">.</span><span class="n">ECHO</span>
<span class="n">termios</span><span class="o">.</span><span class="n">tcsetattr</span><span class="p">(</span><span class="n">fd</span><span class="p">,</span> <span class="n">termios</span><span class="o">.</span><span class="n">TCSANOW</span><span class="p">,</span> <span class="n">newattr</span><span class="p">)</span>
<span class="n">oldflags</span> <span class="o">=</span> <span class="n">fcntl</span><span class="o">.</span><span class="n">fcntl</span><span class="p">(</span><span class="n">fd</span><span class="p">,</span> <span class="n">fcntl</span><span class="o">.</span><span class="n">F_GETFL</span><span class="p">)</span>
<span class="n">fcntl</span><span class="o">.</span><span class="n">fcntl</span><span class="p">(</span><span class="n">fd</span><span class="p">,</span> <span class="n">fcntl</span><span class="o">.</span><span class="n">F_SETFL</span><span class="p">,</span> <span class="n">oldflags</span> <span class="o">|</span> <span class="n">os</span><span class="o">.</span><span class="n">O_NONBLOCK</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="k">while</span> <span class="mi">1</span><span class="p">:</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">c</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">read</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="k">print</span> <span class="s">"Got character"</span><span class="p">,</span> <span class="sb">`c`</span>
<span class="k">except</span> <span class="ne">IOError</span><span class="p">:</span> <span class="k">pass</span>
<span class="k">finally</span><span class="p">:</span>
<span class="n">termios</span><span class="o">.</span><span class="n">tcsetattr</span><span class="p">(</span><span class="n">fd</span><span class="p">,</span> <span class="n">termios</span><span class="o">.</span><span class="n">TCSAFLUSH</span><span class="p">,</span> <span class="n">oldterm</span><span class="p">)</span>
<span class="n">fcntl</span><span class="o">.</span><span class="n">fcntl</span><span class="p">(</span><span class="n">fd</span><span class="p">,</span> <span class="n">fcntl</span><span class="o">.</span><span class="n">F_SETFL</span><span class="p">,</span> <span class="n">oldflags</span><span class="p">)</span>
</pre></div>
</div>
<p>You need the <a title="(Unix) POSIX style tty control." class="reference external" href="../library/termios.html#module-termios"><tt class="xref docutils literal"><span class="pre">termios</span></tt></a> and the <a title="(Unix) The fcntl() and ioctl() system calls." class="reference external" href="../library/fcntl.html#module-fcntl"><tt class="xref docutils literal"><span class="pre">fcntl</span></tt></a> module for any of this to work,
and I’ve only tried it on Linux, though it should work elsewhere. In this code,
characters are read and printed one at a time.</p>
<p><a title="termios.tcsetattr" class="reference external" href="../library/termios.html#termios.tcsetattr"><tt class="xref docutils literal"><span class="pre">termios.tcsetattr()</span></tt></a> turns off stdin’s echoing and disables canonical mode.
<tt class="xref docutils literal"><span class="pre">fcntl.fnctl()</span></tt> is used to obtain stdin’s file descriptor flags and modify
them for non-blocking mode. Since reading stdin when it is empty results in an
<a title="exceptions.IOError" class="reference external" href="../library/exceptions.html#exceptions.IOError"><tt class="xref docutils literal"><span class="pre">IOError</span></tt></a>, this error is caught and ignored.</p>
</div>
</div>
<div class="section" id="threads">
<h2><a class="toc-backref" href="#id13">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="#id14">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 title="Higher-level threading interface." class="reference external" href="../library/threading.html#module-threading"><tt class="xref docutils literal"><span class="pre">threading</span></tt></a> module and not the <a title="Create multiple threads of control within one interpreter." class="reference external" href="../library/thread.html#module-thread"><tt class="xref docutils literal"><span class="pre">thread</span></tt></a> module.
The <a title="Higher-level threading interface." class="reference external" href="../library/threading.html#module-threading"><tt class="xref docutils literal"><span class="pre">threading</span></tt></a> module builds convenient abstractions on top of the
low-level primitives provided by the <a title="Create multiple threads of control within one interpreter." class="reference external" href="../library/thread.html#module-thread"><tt class="xref 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="#id15">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-python"><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="k">print</span> <span class="n">name</span><span class="p">,</span> <span class="n">i</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-python"><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="k">print</span> <span class="n">name</span><span class="p">,</span> <span class="n">i</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 title="time.sleep" class="reference external" href="../library/time.html#time.sleep"><tt class="xref 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 title="A synchronized queue class." class="reference external" href="../library/queue.html#module-Queue"><tt class="xref 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="#id16">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>Use the <a title="A synchronized queue class." class="reference external" href="../library/queue.html#module-Queue"><tt class="xref docutils literal"><span class="pre">Queue</span></tt></a> module to create a queue containing a list of jobs. The
<a title="Queue.Queue" class="reference external" href="../library/queue.html#Queue.Queue"><tt class="xref 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-python"><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="k">print</span> <span class="s">'Running worker'</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="bp">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="bp">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="k">print</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="k">print</span> <span class="s">'queue empty'</span>
<span class="k">break</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">print</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="k">print</span> <span class="s">'running with argument'</span><span class="p">,</span> <span class="n">arg</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 </span><span class="si">%i</span><span class="s">'</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="k">print</span> <span class="s">'Main thread sleeping'</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>
<blockquote>
Running worker
Running worker
Running worker
Running worker
Running worker
Main thread sleeping
Worker <Thread(worker 1, started)> running with argument 0
Worker <Thread(worker 2, started)> running with argument 1
Worker <Thread(worker 3, started)> running with argument 2
Worker <Thread(worker 4, started)> running with argument 3
Worker <Thread(worker 5, started)> running with argument 4
Worker <Thread(worker 1, started)> running with argument 5
...</blockquote>
<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="#id17">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 global interpreter lock (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 title="sys.setcheckinterval" class="reference external" href="../library/sys.html#sys.setcheckinterval"><tt class="xref docutils literal"><span class="pre">sys.setcheckinterval()</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-python"><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-python"><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 title="object.__del__" class="reference external" href="../reference/datamodel.html#object.__del__"><tt class="xref 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="#id18">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 Global Interpreter Lock (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. Unfortunately, even on Windows (where locks are very
efficient) this ran ordinary Python code about twice as slow as the interpreter
using the GIL. On Linux the performance loss was even worse because pthread
locks aren’t as efficient.</p>
<p>Since then, the idea of getting rid of the GIL has occasionally come up but
nobody has found a way to deal with the expected slowdown, and users who don’t
use threads would not be happy if their code ran at half at the speed. Greg’s
free threading patch set has not been kept up-to-date for later Python versions.</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>. 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.</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="#id19">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="#id20">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 title="Miscellaneous operating system interfaces." class="reference external" href="../library/os.html#module-os"><tt class="xref docutils literal"><span class="pre">os</span></tt></a> module. The two functions are identical; <tt class="xref docutils literal"><span class="pre">unlink()</span></tt> is simply
the name of the Unix system call for this function.</p>
<p>To remove a directory, use <a title="os.rmdir" class="reference external" href="../library/os.html#os.rmdir"><tt class="xref docutils literal"><span class="pre">os.rmdir()</span></tt></a>; use <a title="os.mkdir" class="reference external" href="../library/os.html#os.mkdir"><tt class="xref 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 title="shutil.rmtree" class="reference external" href="../library/shutil.html#shutil.rmtree"><tt class="xref 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">"r+")</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 title="os.open" class="reference external" href="../library/os.html#os.open"><tt class="xref 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 title="High-level file operations, including copying." class="reference external" href="../library/shutil.html#module-shutil"><tt class="xref docutils literal"><span class="pre">shutil</span></tt></a> module also contains a number of functions to work on files
including <a title="shutil.copyfile" class="reference external" href="../library/shutil.html#shutil.copyfile"><tt class="xref docutils literal"><span class="pre">copyfile()</span></tt></a>, <a title="shutil.copytree" class="reference external" href="../library/shutil.html#shutil.copytree"><tt class="xref docutils literal"><span class="pre">copytree()</span></tt></a>, and
<a title="shutil.rmtree" class="reference external" href="../library/shutil.html#shutil.rmtree"><tt class="xref 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="#id21">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 title="High-level file operations, including copying." class="reference external" href="../library/shutil.html#module-shutil"><tt class="xref docutils literal"><span class="pre">shutil</span></tt></a> module contains a <a title="shutil.copyfile" class="reference external" href="../library/shutil.html#shutil.copyfile"><tt class="xref 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="#id22">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 title="Interpret strings as packed binary data." class="reference external" href="../library/struct.html#module-struct"><tt class="xref 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-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">struct</span>
<span class="n">f</span> <span class="o">=</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="c"># Open in binary mode for portability</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 title="Space efficient arrays of uniformly typed numeric values." class="reference external" href="../library/array.html#module-array"><tt class="xref docutils literal"><span class="pre">array</span></tt></a> module.</p>
</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="#id23">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 title="os.read" class="reference external" href="../library/os.html#os.read"><tt class="xref 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. <a title="os.popen" class="reference external" href="../library/os.html#os.popen"><tt class="xref docutils literal"><span class="pre">os.popen()</span></tt></a> creates a high-level
file object, the same type returned by the built-in <a title="open" class="reference external" href="../library/functions.html#open"><tt class="xref docutils literal"><span class="pre">open()</span></tt></a> function.
Thus, to read n bytes from a pipe p created with <a title="os.popen" class="reference external" href="../library/os.html#os.popen"><tt class="xref docutils literal"><span class="pre">os.popen()</span></tt></a>, 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-run-a-subprocess-with-pipes-connected-to-both-input-and-output">
<h3><a class="toc-backref" href="#id24">How do I run a subprocess with pipes connected to both input and output?</a><a class="headerlink" href="#how-do-i-run-a-subprocess-with-pipes-connected-to-both-input-and-output" title="Permalink to this headline">¶</a></h3>
<p>Use the <a title="Subprocesses with accessible standard I/O streams. (deprecated)" class="reference external" href="../library/popen2.html#module-popen2"><tt class="xref docutils literal"><span class="pre">popen2</span></tt></a> module. For example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">popen2</span>
<span class="n">fromchild</span><span class="p">,</span> <span class="n">tochild</span> <span class="o">=</span> <span class="n">popen2</span><span class="o">.</span><span class="n">popen2</span><span class="p">(</span><span class="s">"command"</span><span class="p">)</span>
<span class="n">tochild</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">"input</span><span class="se">\n</span><span class="s">"</span><span class="p">)</span>
<span class="n">tochild</span><span class="o">.</span><span class="n">flush</span><span class="p">()</span>
<span class="n">output</span> <span class="o">=</span> <span class="n">fromchild</span><span class="o">.</span><span class="n">readline</span><span class="p">()</span>
</pre></div>
</div>
<p>Warning: in general it is unwise to do this because you can easily cause a
deadlock where your process is blocked waiting for output from the child while
the child is blocked waiting for input from you. This can be caused because the
parent expects the child to output more text than it does, or it can be caused
by data being stuck in stdio buffers due to lack of flushing. The Python parent
can of course explicitly flush the data it sends to the child before it reads
any output, but if the child is a naive C program it may have been written to
never explicitly flush its output, even if it is interactive, since flushing is
normally automatic.</p>
<p>Note that a deadlock is also possible if you use <tt class="xref docutils literal"><span class="pre">popen3()</span></tt> to read stdout
and stderr. If one of the two is too large for the internal buffer (increasing
the buffer size does not help) and you <tt class="docutils literal"><span class="pre">read()</span></tt> the other one first, there is
a deadlock, too.</p>
<p>Note on a bug in popen2: unless your program calls <tt class="docutils literal"><span class="pre">wait()</span></tt> or <tt class="docutils literal"><span class="pre">waitpid()</span></tt>,
finished child processes are never removed, and eventually calls to popen2 will
fail because of a limit on the number of child processes. Calling
<a title="os.waitpid" class="reference external" href="../library/os.html#os.waitpid"><tt class="xref docutils literal"><span class="pre">os.waitpid()</span></tt></a> with the <a title="os.WNOHANG" class="reference external" href="../library/os.html#os.WNOHANG"><tt class="xref docutils literal"><span class="pre">os.WNOHANG</span></tt></a> option can prevent this; a good
place to insert such a call would be before calling <tt class="docutils literal"><span class="pre">popen2</span></tt> again.</p>
<p>In many cases, all you really need is to run some data through a command and get
the result back. Unless the amount of data is very large, the easiest way to do
this is to write it to a temporary file and run the command with that temporary
file as input. The standard module <a title="Generate temporary files and directories." class="reference external" href="../library/tempfile.html#module-tempfile"><tt class="xref docutils literal"><span class="pre">tempfile</span></tt></a> exports a <tt class="docutils literal"><span class="pre">mktemp()</span></tt>
function to generate unique temporary file names.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">tempfile</span>
<span class="kn">import</span> <span class="nn">os</span>
<span class="k">class</span> <span class="nc">Popen3</span><span class="p">:</span>
<span class="sd">"""</span>
<span class="sd"> This is a deadlock-safe version of popen that returns</span>
<span class="sd"> an object with errorlevel, out (a string) and err (a string).</span>
<span class="sd"> (capturestderr may not work under windows.)</span>
<span class="sd"> Example: print Popen3('grep spam','\n\nhere spam\n\n').out</span>
<span class="sd"> """</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">command</span><span class="p">,</span><span class="nb">input</span><span class="o">=</span><span class="bp">None</span><span class="p">,</span><span class="n">capturestderr</span><span class="o">=</span><span class="bp">None</span><span class="p">):</span>
<span class="n">outfile</span><span class="o">=</span><span class="n">tempfile</span><span class="o">.</span><span class="n">mktemp</span><span class="p">()</span>
<span class="n">command</span><span class="o">=</span><span class="s">"( </span><span class="si">%s</span><span class="s"> ) > </span><span class="si">%s</span><span class="s">"</span> <span class="o">%</span> <span class="p">(</span><span class="n">command</span><span class="p">,</span><span class="n">outfile</span><span class="p">)</span>
<span class="k">if</span> <span class="nb">input</span><span class="p">:</span>
<span class="n">infile</span><span class="o">=</span><span class="n">tempfile</span><span class="o">.</span><span class="n">mktemp</span><span class="p">()</span>
<span class="nb">open</span><span class="p">(</span><span class="n">infile</span><span class="p">,</span><span class="s">"w"</span><span class="p">)</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="nb">input</span><span class="p">)</span>
<span class="n">command</span><span class="o">=</span><span class="n">command</span><span class="o">+</span><span class="s">" <"</span><span class="o">+</span><span class="n">infile</span>
<span class="k">if</span> <span class="n">capturestderr</span><span class="p">:</span>
<span class="n">errfile</span><span class="o">=</span><span class="n">tempfile</span><span class="o">.</span><span class="n">mktemp</span><span class="p">()</span>
<span class="n">command</span><span class="o">=</span><span class="n">command</span><span class="o">+</span><span class="s">" 2>"</span><span class="o">+</span><span class="n">errfile</span>
<span class="bp">self</span><span class="o">.</span><span class="n">errorlevel</span><span class="o">=</span><span class="n">os</span><span class="o">.</span><span class="n">system</span><span class="p">(</span><span class="n">command</span><span class="p">)</span> <span class="o">>></span> <span class="mi">8</span>
<span class="bp">self</span><span class="o">.</span><span class="n">out</span><span class="o">=</span><span class="nb">open</span><span class="p">(</span><span class="n">outfile</span><span class="p">,</span><span class="s">"r"</span><span class="p">)</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="n">os</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="n">outfile</span><span class="p">)</span>
<span class="k">if</span> <span class="nb">input</span><span class="p">:</span>
<span class="n">os</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="n">infile</span><span class="p">)</span>
<span class="k">if</span> <span class="n">capturestderr</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">err</span><span class="o">=</span><span class="nb">open</span><span class="p">(</span><span class="n">errfile</span><span class="p">,</span><span class="s">"r"</span><span class="p">)</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="n">os</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="n">errfile</span><span class="p">)</span>
</pre></div>
</div>
<p>Note that many interactive programs (e.g. vi) don’t work well with pipes
substituted for standard input and output. You will have to use pseudo ttys
(“ptys”) instead of pipes. Or you can use a Python interface to Don Libes’
“expect” library. A Python extension that interfaces to expect is called “expy”
and available from <a class="reference external" href="http://expectpy.sourceforge.net">http://expectpy.sourceforge.net</a>. A pure Python solution that
works like expect is <a class="reference external" href="http://pypi.python.org/pypi/pexpect/">pexpect</a>.</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>
<a class="reference external" href="http://pyserial.sourceforge.net">http://pyserial.sourceforge.net</a></blockquote>
<p>For Unix, see a Usenet post by Mitch Chapman:</p>
<blockquote>
<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></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 file objects are a high-level layer of abstraction on top of C streams,
which in turn are a medium-level layer of abstraction on top of (among other
things) low-level C file descriptors.</p>
<p>For most file objects you create in Python via the built-in <tt class="docutils literal"><span class="pre">file</span></tt>
constructor, <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 stream.
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 stream.</p>
<p>To close the underlying C stream 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 os.close:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">os</span><span class="o">.</span><span class="n">close</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span> <span class="c"># close C's stdin stream</span>
<span class="n">os</span><span class="o">.</span><span class="n">close</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span> <span class="c"># close C's stdout stream</span>
<span class="n">os</span><span class="o">.</span><span class="n">close</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span> <span class="c"># close C's stderr stream</span>
</pre></div>
</div>
</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 external" href="../library/internet.html#internet"><em>Internet Protocols and Support</em></a> and <a class="reference external" 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 httplib:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c">#!/usr/bin/python</span>
<span class="kn">import</span> <span class="nn">httplib</span><span class="o">,</span> <span class="nn">sys</span><span class="o">,</span> <span class="nn">time</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">httpobj</span> <span class="o">=</span> <span class="n">httplib</span><span class="o">.</span><span class="n">HTTP</span><span class="p">(</span><span class="s">'www.some-server.out-there'</span><span class="p">,</span> <span class="mi">80</span><span class="p">)</span>
<span class="n">httpobj</span><span class="o">.</span><span class="n">putrequest</span><span class="p">(</span><span class="s">'POST'</span><span class="p">,</span> <span class="s">'/cgi-bin/some-cgi-script'</span><span class="p">)</span>
<span class="c">### now generate the rest of the HTTP headers...</span>
<span class="n">httpobj</span><span class="o">.</span><span class="n">putheader</span><span class="p">(</span><span class="s">'Accept'</span><span class="p">,</span> <span class="s">'*/*'</span><span class="p">)</span>
<span class="n">httpobj</span><span class="o">.</span><span class="n">putheader</span><span class="p">(</span><span class="s">'Connection'</span><span class="p">,</span> <span class="s">'Keep-Alive'</span><span class="p">)</span>
<span class="n">httpobj</span><span class="o">.</span><span class="n">putheader</span><span class="p">(</span><span class="s">'Content-type'</span><span class="p">,</span> <span class="s">'application/x-www-form-urlencoded'</span><span class="p">)</span>
<span class="n">httpobj</span><span class="o">.</span><span class="n">putheader</span><span class="p">(</span><span class="s">'Content-length'</span><span class="p">,</span> <span class="s">'</span><span class="si">%d</span><span class="s">'</span> <span class="o">%</span> <span class="nb">len</span><span class="p">(</span><span class="n">qs</span><span class="p">))</span>
<span class="n">httpobj</span><span class="o">.</span><span class="n">endheaders</span><span class="p">()</span>
<span class="n">httpobj</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="n">qs</span><span class="p">)</span>
<span class="c">### find out what the server said in response...</span>
<span class="n">reply</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">httpobj</span><span class="o">.</span><span class="n">getreply</span><span class="p">()</span>
<span class="k">if</span> <span class="n">reply</span> <span class="o">!=</span> <span class="mi">200</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">httpobj</span><span class="o">.</span><span class="n">getfile</span><span class="p">()</span><span class="o">.</span><span class="n">read</span><span class="p">())</span>
</pre></div>
</div>
<p>Note that in general for URL-encoded POST operations, query strings must be
quoted by using <a title="urllib.quote" class="reference external" href="../library/urllib.html#urllib.quote"><tt class="xref docutils literal"><span class="pre">urllib.quote()</span></tt></a>. For example to send name=”Guy Steele,
Jr.”:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">urllib</span> <span class="kn">import</span> <span class="n">quote</span>
<span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="n">quote</span><span class="p">(</span><span class="s">"Guy Steele, Jr."</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">x</span>
<span class="go">'Guy%20Steele,%20Jr.'</span>
<span class="gp">>>> </span><span class="n">query_string</span> <span class="o">=</span> <span class="s">"name="</span><span class="o">+</span><span class="n">x</span>
<span class="gp">>>> </span><span class="n">query_string</span>
<span class="go">'name=Guy%20Steele,%20Jr.'</span>
</pre></div>
</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 title="SMTP protocol client (requires sockets)." class="reference external" href="../library/smtplib.html#module-smtplib"><tt class="xref 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-python"><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">raw_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">raw_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="k">print</span> <span class="s">"Enter message, end with ^D:"</span>
<span class="n">msg</span> <span class="o">=</span> <span class="s">''</span>
<span class="k">while</span> <span class="bp">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-python"><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">"</span><span class="si">%s</span><span class="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="k">print</span> <span class="s">"Sendmail exit status"</span><span class="p">,</span> <span class="n">sts</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 select 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
– 0 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>
</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>Python 2.3 includes the <a title="Interface to Berkeley DB database library" class="reference external" href="../library/bsddb.html#module-bsddb"><tt class="xref docutils literal"><span class="pre">bsddb</span></tt></a> package which provides an interface to the
BerkeleyDB library. Interfaces to disk-based hashes such as <a title="(Unix) The standard "database" interface, based on ndbm." class="reference external" href="../library/dbm.html#module-dbm"><tt class="xref docutils literal"><span class="pre">DBM</span></tt></a>
and <a title="(Unix) GNU's reinterpretation of dbm." class="reference external" href="../library/gdbm.html#module-gdbm"><tt class="xref docutils literal"><span class="pre">GDBM</span></tt></a> are also included with standard Python.</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 title="Convert Python objects to streams of bytes and back." class="reference external" href="../library/pickle.html#module-pickle"><tt class="xref 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 title="Python object persistence." class="reference external" href="../library/shelve.html#module-shelve"><tt class="xref docutils literal"><span class="pre">shelve</span></tt></a> library module uses pickle and (g)dbm to create persistent
mappings containing arbitrary Python objects. For better performance, you can
use the <a title="Faster version of pickle, but not subclassable." class="reference external" href="../library/pickle.html#module-cPickle"><tt class="xref docutils literal"><span class="pre">cPickle</span></tt></a> module.</p>
<p>A more awkward way of doing things is to use pickle’s little sister, marshal.
The <a title="Convert Python objects to streams of bytes and back (with different constraints)." class="reference external" href="../library/marshal.html#module-marshal"><tt class="xref 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="why-is-cpickle-so-slow">
<h3><a class="toc-backref" href="#id36">Why is cPickle so slow?</a><a class="headerlink" href="#why-is-cpickle-so-slow" title="Permalink to this headline">¶</a></h3>
<p>The default format used by the pickle module is a slow one that results in
readable pickles. Making it the default, but it would break backward
compatibility:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">largeString</span> <span class="o">=</span> <span class="s">'z'</span> <span class="o">*</span> <span class="p">(</span><span class="mi">100</span> <span class="o">*</span> <span class="mi">1024</span><span class="p">)</span>
<span class="n">myPickle</span> <span class="o">=</span> <span class="n">cPickle</span><span class="o">.</span><span class="n">dumps</span><span class="p">(</span><span class="n">largeString</span><span class="p">,</span> <span class="n">protocol</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
</pre></div>
</div>
</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="#id37">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>
<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="#id38">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>
<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="#id39">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="#id40">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 title="Generate pseudo-random numbers with various common distributions." class="reference external" href="../library/random.html#module-random"><tt class="xref docutils literal"><span class="pre">random</span></tt></a> implements a random number generator. Usage
is simple:</p>
<div class="highlight-python"><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>
</div>
</div>
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<ul>
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<li><a class="reference external" href="#common-tasks">Common tasks</a></li>
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<li><a class="reference external" href="#databases">Databases</a></li>
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