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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">
Contents
<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>
Check <a class="reference internal" href="../library/index.html#library-index">the Library Reference</a> to see if there’s a relevant
standard library module. (Eventually you’ll learn what’s in the standard
library and will be able to skip this step.)
For third-party packages, search the <a class="reference external" href="https://pypi.org">Python Package Index</a> or try <a class="reference external" href="https://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.
</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>
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
<code class="file docutils literal notranslate">mathmodule.c</code>, somewhere in a C source directory (not on the Python Path).
There are (at least) three kinds of modules in Python:
<ol class="arabic">
<li>modules written in Python (.py);
</li>
<li>modules written in C and dynamically loaded (.dll, .pyd, .so, .sl, etc);
</li>
<li>modules written in C and linked with the interpreter; to get a list of these,
type:
<div class="highlight-default notranslate"><div class="highlight"><pre>import sys
print sys.builtin_module_names
</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>
You need to do two things: the script file’s mode must be executable and the
first line must begin with <code class="docutils literal notranslate">#!</code> followed by the path of the Python
interpreter.
The first is done by executing <code class="docutils literal notranslate">chmod +x scriptfile</code> or perhaps <code class="docutils literal notranslate">chmod 755
scriptfile</code>.
The second can be done in a number of ways. The most straightforward way is to
write
<div class="highlight-default notranslate"><div class="highlight"><pre>#!/usr/local/bin/python
</pre></div>
</div>
as the very first line of your file, using the pathname for where the Python
interpreter is installed on your platform.
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
<code class="xref std std-envvar docutils literal notranslate">PATH</code>:
<div class="highlight-default notranslate"><div class="highlight"><pre>#!/usr/bin/env python
</pre></div>
</div>
Don’t do this for CGI scripts. The <code class="xref std std-envvar docutils literal notranslate">PATH</code> variable for CGI scripts is
often very minimal, so you need to use the actual absolute pathname of the
interpreter.
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):
<div class="highlight-default notranslate"><div class="highlight"><pre>#! /bin/sh
&quot;&quot;&quot;:&quot;
exec python $0 ${1+&quot;$@&quot;}
&quot;&quot;&quot;
</pre></div>
</div>
The minor disadvantage is that this defines the script’s __doc__ string.
However, you can fix that by adding
<div class="highlight-default notranslate"><div class="highlight"><pre>__doc__ = &quot;&quot;&quot;...Whatever...&quot;&quot;&quot;
</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>
For Unix variants the standard Python source distribution comes with a curses
module in the <a class="reference external" href="https://github.com/python/cpython/tree/2.7/Modules">Modules</a> 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.)
The <a class="reference internal" href="../library/curses.html#module-curses" title="curses: An interface to the curses library, providing portable terminal handling. (Unix)"><code class="xref py py-mod docutils literal notranslate">curses</code></a> 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.
For Windows: use <a class="reference external" href="http://effbot.org/zone/console-index.htm">the consolelib module</a>.
</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>
The <a class="reference internal" href="../library/atexit.html#module-atexit" title="atexit: Register and execute cleanup functions."><code class="xref py py-mod docutils literal notranslate">atexit</code></a> module provides a register function that is similar to C’s
<code class="xref c c-func docutils literal notranslate">onexit()</code>.
</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>
The most common problem is that the signal handler is declared with the wrong
argument list. It is called as
<div class="highlight-default notranslate"><div class="highlight"><pre>handler(signum, frame)
</pre></div>
</div>
so it should be declared with two arguments:
<div class="highlight-default notranslate"><div class="highlight"><pre>def handler(signum, frame):
 ...
</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>
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."><code class="xref py py-mod docutils literal notranslate">doctest</code></a> module finds
examples in the docstrings for a module and runs them, comparing the output with
the expected output given in the docstring.
The <a class="reference internal" href="../library/unittest.html#module-unittest" title="unittest: Unit testing framework for Python."><code class="xref py py-mod docutils literal notranslate">unittest</code></a> module is a fancier testing framework modelled on Java and
Smalltalk testing frameworks.
To make testing easier, you should use good modular design in your program.
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.
The “global main logic” of your program may be as simple as
<div class="highlight-default notranslate"><div class="highlight"><pre>if __name__ == &quot;__main__&quot;:
 main_logic()
</pre></div>
</div>
at the bottom of the main module of your program.
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 that automates a sequence of tests can be associated with each module.
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.
“Support modules” that are not intended to be the main module of a program may
include a self-test of the module.
<div class="highlight-default notranslate"><div class="highlight"><pre>if __name__ == &quot;__main__&quot;:
 self_test()
</pre></div>
</div>
Even programs that interact with complex external interfaces may be tested when
the external interfaces are unavailable by using “fake” interfaces implemented
in Python.
</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>
The <a class="reference internal" href="../library/pydoc.html#module-pydoc" title="pydoc: Documentation generator and online help system."><code class="xref py py-mod docutils literal notranslate">pydoc</code></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.sourceforge.net/">epydoc</a>. <a class="reference external" href="http://sphinx-doc.org">Sphinx</a> can also include docstring content.
</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>
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:
<div class="highlight-default notranslate"><div class="highlight"><pre>import termios, fcntl, sys, os
fd = sys.stdin.fileno()

oldterm = termios.tcgetattr(fd)
newattr = termios.tcgetattr(fd)
newattr[3] = newattr[3] &amp; ~termios.ICANON &amp; ~termios.ECHO
termios.tcsetattr(fd, termios.TCSANOW, newattr)

oldflags = fcntl.fcntl(fd, fcntl.F_GETFL)
fcntl.fcntl(fd, fcntl.F_SETFL, oldflags | os.O_NONBLOCK)

try:
 while 1:
 try:
 c = sys.stdin.read(1)
 print &quot;Got character&quot;, repr(c)
 except IOError: pass
finally:
 termios.tcsetattr(fd, termios.TCSAFLUSH, oldterm)
 fcntl.fcntl(fd, fcntl.F_SETFL, oldflags)
</pre></div>
</div>
You need the <a class="reference internal" href="../library/termios.html#module-termios" title="termios: POSIX style tty control. (Unix)"><code class="xref py py-mod docutils literal notranslate">termios</code></a> and the <a class="reference internal" href="../library/fcntl.html#module-fcntl" title="fcntl: The fcntl() and ioctl() system calls. (Unix)"><code class="xref py py-mod docutils literal notranslate">fcntl</code></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.
<a class="reference internal" href="../library/termios.html#termios.tcsetattr" title="termios.tcsetattr"><code class="xref py py-func docutils literal notranslate">termios.tcsetattr()</code></a> turns off stdin’s echoing and disables canonical mode.
<code class="xref py py-func docutils literal notranslate">fcntl.fnctl()</code> 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 class="reference internal" href="../library/exceptions.html#exceptions.IOError" title="exceptions.IOError"><code class="xref py py-exc docutils literal notranslate">IOError</code></a>, this error is caught and ignored.
</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>
Be sure to use the <a class="reference internal" href="../library/threading.html#module-threading" title="threading: Higher-level threading interface."><code class="xref py py-mod docutils literal notranslate">threading</code></a> module and not the <a class="reference internal" href="../library/thread.html#module-thread" title="thread: Create multiple threads of control within one interpreter."><code class="xref py py-mod docutils literal notranslate">thread</code></a> module.
The <a class="reference internal" href="../library/threading.html#module-threading" title="threading: Higher-level threading interface."><code class="xref py py-mod docutils literal notranslate">threading</code></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: Create multiple threads of control within one interpreter."><code class="xref py py-mod docutils literal notranslate">thread</code></a> module.
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>.
</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>
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.
A simple fix is to add a sleep to the end of the program that’s long enough for
all the threads to finish:
<div class="highlight-default notranslate"><div class="highlight"><pre>import threading, time

def thread_task(name, n):
 for i in range(n): print name, i

for i in range(10):
 T = threading.Thread(target=thread_task, args=(str(i), i))
 T.start()

time.sleep(10) # &lt;----------------------------!
</pre></div>
</div>
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.
A simple fix is to add a tiny sleep to the start of the run function:
<div class="highlight-default notranslate"><div class="highlight"><pre>def thread_task(name, n):
 time.sleep(0.001) # &lt;---------------------!
 for i in range(n): print name, i

time.sleep(10)
</pre></div>
</div>
Instead of trying to guess a good delay value for <a class="reference internal" href="../library/time.html#time.sleep" title="time.sleep"><code class="xref py py-func docutils literal notranslate">time.sleep()</code></a>,
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."><code class="xref py py-mod docutils literal notranslate">Queue</code></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.
</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>
Use the <a class="reference internal" href="../library/queue.html#module-Queue" title="Queue: A synchronized queue class."><code class="xref py py-mod docutils literal notranslate">Queue</code></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"><code class="xref py py-class docutils literal notranslate">Queue</code></a> class maintains a list of objects and has a <code class="docutils literal notranslate">.put(obj)</code>
method that adds items to the queue and a <code class="docutils literal notranslate">.get()</code> method to return them.
The class will take care of the locking necessary to ensure that each job is
handed out exactly once.
Here’s a trivial example:
<div class="highlight-default notranslate"><div class="highlight"><pre>import threading, Queue, time

# The worker thread gets jobs off the queue. When the queue is empty, it
# assumes there will be no more work and exits.
# (Realistically workers will run until terminated.)
def worker():
 print &#39;Running worker&#39;
 time.sleep(0.1)
 while True:
 try:
 arg = q.get(block=False)
 except Queue.Empty:
 print &#39;Worker&#39;, threading.currentThread(),
 print &#39;queue empty&#39;
 break
 else:
 print &#39;Worker&#39;, threading.currentThread(),
 print &#39;running with argument&#39;, arg
 time.sleep(0.5)

# Create queue
q = Queue.Queue()

# Start a pool of 5 workers
for i in range(5):
 t = threading.Thread(target=worker, name=&#39;worker %i&#39; % (i+1))
 t.start()

# Begin adding work to the queue
for i in range(50):
 q.put(i)

# Give threads time to run
print &#39;Main thread sleeping&#39;
time.sleep(5)
</pre></div>
</div>
When run, this will produce the following output:
<div class="highlight-none notranslate"><div class="highlight"><pre>Running worker
Running worker
Running worker
Running worker
Running worker
Main thread sleeping
Worker &lt;Thread(worker 1, started)&gt; running with argument 0
Worker &lt;Thread(worker 2, started)&gt; running with argument 1
Worker &lt;Thread(worker 3, started)&gt; running with argument 2
Worker &lt;Thread(worker 4, started)&gt; running with argument 3
Worker &lt;Thread(worker 5, started)&gt; running with argument 4
Worker &lt;Thread(worker 1, started)&gt; running with argument 5
...
</pre></div>
</div>
Consult the module’s documentation for more details; the <a class="reference internal" href="../library/queue.html#Queue.Queue" title="Queue.Queue"><code class="xref py py-class docutils literal notranslate">Queue</code></a>
class provides a featureful interface.
</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>
A <a class="reference internal" href="../glossary.html#term-global-interpreter-lock">global interpreter lock</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.setcheckinterval" title="sys.setcheckinterval"><code class="xref py py-func docutils literal notranslate">sys.setcheckinterval()</code></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.
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.
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):
<div class="highlight-default notranslate"><div class="highlight"><pre>L.append(x)
L1.extend(L2)
x = L[i]
x = L.pop()
L1[i:j] = L2
L.sort()
x = y
x.field = y
D[x] = y
D1.update(D2)
D.keys()
</pre></div>
</div>
These aren’t:
<div class="highlight-default notranslate"><div class="highlight"><pre>i = i+1
L.append(L[-1])
L[i] = L[j]
D[x] = D[x] + 1
</pre></div>
</div>
Operations that replace other objects may invoke those other objects’
<a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate">__del__()</code></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!
</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>
The <a class="reference internal" href="../glossary.html#term-global-interpreter-lock">global interpreter lock</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.
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.
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 the speed. Greg’s
free threading patch set has not been kept up-to-date for later Python versions.
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
processes rather than multiple threads. 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.
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.
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.
And finally, once you have multiple interpreters not sharing any state, what
have you gained over running each interpreter in a separate process?
</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>
Use <code class="docutils literal notranslate">os.remove(filename)</code> or <code class="docutils literal notranslate">os.unlink(filename)</code>; for documentation, see
the <a class="reference internal" href="../library/os.html#module-os" title="os: Miscellaneous operating system interfaces."><code class="xref py py-mod docutils literal notranslate">os</code></a> module. The two functions are identical; <code class="xref py py-func docutils literal notranslate">unlink()</code> is simply
the name of the Unix system call for this function.
To remove a directory, use <a class="reference internal" href="../library/os.html#os.rmdir" title="os.rmdir"><code class="xref py py-func docutils literal notranslate">os.rmdir()</code></a>; use <a class="reference internal" href="../library/os.html#os.mkdir" title="os.mkdir"><code class="xref py py-func docutils literal notranslate">os.mkdir()</code></a> to create one.
<code class="docutils literal notranslate">os.makedirs(path)</code> will create any intermediate directories in <code class="docutils literal notranslate">path</code> that
don’t exist. <code class="docutils literal notranslate">os.removedirs(path)</code> 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"><code class="xref py py-func docutils literal notranslate">shutil.rmtree()</code></a>.
To rename a file, use <code class="docutils literal notranslate">os.rename(old_path, new_path)</code>.
To truncate a file, open it using <code class="docutils literal notranslate">f = open(filename, &quot;r+&quot;)</code>, and use
<code class="docutils literal notranslate">f.truncate(offset)</code>; offset defaults to the current seek position. There’s
also <code class="docutils literal notranslate">os.ftruncate(fd, offset)</code> for files opened with <a class="reference internal" href="../library/os.html#os.open" title="os.open"><code class="xref py py-func docutils literal notranslate">os.open()</code></a>, where
fd is the file descriptor (a small integer).
The <a class="reference internal" href="../library/shutil.html#module-shutil" title="shutil: High-level file operations, including copying."><code class="xref py py-mod docutils literal notranslate">shutil</code></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"><code class="xref py py-func docutils literal notranslate">copyfile()</code></a>, <a class="reference internal" href="../library/shutil.html#shutil.copytree" title="shutil.copytree"><code class="xref py py-func docutils literal notranslate">copytree()</code></a>, and
<a class="reference internal" href="../library/shutil.html#shutil.rmtree" title="shutil.rmtree"><code class="xref py py-func docutils literal notranslate">rmtree()</code></a>.
</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>
The <a class="reference internal" href="../library/shutil.html#module-shutil" title="shutil: High-level file operations, including copying."><code class="xref py py-mod docutils literal notranslate">shutil</code></a> module contains a <a class="reference internal" href="../library/shutil.html#shutil.copyfile" title="shutil.copyfile"><code class="xref py py-func docutils literal notranslate">copyfile()</code></a> function. Note
that on MacOS 9 it doesn’t copy the resource fork and Finder info.
</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>
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 strings as packed binary data."><code class="xref py py-mod docutils literal notranslate">struct</code></a>
module. It allows you to take a string containing binary data (usually numbers)
and convert it to Python objects; and vice versa.
For example, the following code reads two 2-byte integers and one 4-byte integer
in big-endian format from a file:
<div class="highlight-default notranslate"><div class="highlight"><pre>import struct

f = open(filename, &quot;rb&quot;) # Open in binary mode for portability
s = f.read(8)
x, y, z = struct.unpack(&quot;&gt;hhl&quot;, s)
</pre></div>
</div>
The ‘&gt;’ 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.
For data that is more regular (e.g. a homogeneous list of ints or floats),
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."><code class="xref py py-mod docutils literal notranslate">array</code></a> module.
</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>
<a class="reference internal" href="../library/os.html#os.read" title="os.read"><code class="xref py py-func docutils literal notranslate">os.read()</code></a> is a low-level function which takes a file descriptor, a small
integer representing the opened file. <a class="reference internal" href="../library/os.html#os.popen" title="os.popen"><code class="xref py py-func docutils literal notranslate">os.popen()</code></a> 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"><code class="xref py py-func docutils literal notranslate">open()</code></a> function.
Thus, to read n bytes from a pipe p created with <a class="reference internal" href="../library/os.html#os.popen" title="os.popen"><code class="xref py py-func docutils literal notranslate">os.popen()</code></a>, you need to
use <code class="docutils literal notranslate">p.read(n)</code>.
</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>
Use the <a class="reference internal" href="../library/popen2.html#module-popen2" title="popen2: Subprocesses with accessible standard I/O streams. (deprecated)"><code class="xref py py-mod docutils literal notranslate">popen2</code></a> module. For example:
<div class="highlight-default notranslate"><div class="highlight"><pre>import popen2
fromchild, tochild = popen2.popen2(&quot;command&quot;)
tochild.write(&quot;input\n&quot;)
tochild.flush()
output = fromchild.readline()
</pre></div>
</div>
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 by the
parent expecting the child to output more text than it does or 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.
Note that a deadlock is also possible if you use <code class="xref py py-func docutils literal notranslate">popen3()</code> 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 <code class="docutils literal notranslate">read()</code> the other one first, there is
a deadlock, too.
Note on a bug in popen2: unless your program calls <code class="docutils literal notranslate">wait()</code> or <code class="docutils literal notranslate">waitpid()</code>,
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 class="reference internal" href="../library/os.html#os.waitpid" title="os.waitpid"><code class="xref py py-func docutils literal notranslate">os.waitpid()</code></a> with the <a class="reference internal" href="../library/os.html#os.WNOHANG" title="os.WNOHANG"><code class="xref py py-data docutils literal notranslate">os.WNOHANG</code></a> option can prevent this; a good
place to insert such a call would be before calling <code class="docutils literal notranslate">popen2</code> again.
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 class="reference internal" href="../library/tempfile.html#module-tempfile" title="tempfile: Generate temporary files and directories."><code class="xref py py-mod docutils literal notranslate">tempfile</code></a> exports a
<a class="reference internal" href="../library/tempfile.html#tempfile.mktemp" title="tempfile.mktemp"><code class="xref py py-func docutils literal notranslate">mktemp()</code></a> function to generate unique temporary file names.
<div class="highlight-default notranslate"><div class="highlight"><pre>import tempfile
import os

class Popen3:
 &quot;&quot;&quot;
 This is a deadlock-safe version of popen that returns
 an object with errorlevel, out (a string) and err (a string).
 (capturestderr may not work under windows.)
 Example: print Popen3(&#39;grep spam&#39;,&#39;\n\nhere spam\n\n&#39;).out
 &quot;&quot;&quot;
 def __init__(self,command,input=None,capturestderr=None):
 outfile=tempfile.mktemp()
 command=&quot;( %s ) &gt; %s&quot; % (command,outfile)
 if input:
 infile=tempfile.mktemp()
 open(infile,&quot;w&quot;).write(input)
 command=command+&quot; &lt;&quot;+infile
 if capturestderr:
 errfile=tempfile.mktemp()
 command=command+&quot; 2&gt;&quot;+errfile
 self.errorlevel=os.system(command) &gt;&gt; 8
 self.out=open(outfile,&quot;r&quot;).read()
 os.remove(outfile)
 if input:
 os.remove(infile)
 if capturestderr:
 self.err=open(errfile,&quot;r&quot;).read()
 os.remove(errfile)
</pre></div>
</div>
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="https://pypi.org/project/pexpect/">pexpect</a>.
</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>
For Win32, POSIX (Linux, BSD, etc.), Jython:
<blockquote>
<div><a class="reference external" href="http://pyserial.sourceforge.net">http://pyserial.sourceforge.net</a></div></blockquote>
For Unix, see a Usenet post by Mitch Chapman:
<blockquote>
<div><a class="reference external" href="https://groups.google.com/groups?selm=34A04430.CF9&#64;ohioee.com">https://groups.google.com/groups?selm=34A04430.CF9&#64;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>
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.
For most file objects you create in Python via the built-in <code class="docutils literal notranslate">file</code>
constructor, <code class="docutils literal notranslate">f.close()</code> 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 <code class="docutils literal notranslate">f</code>’s destructor, when <code class="docutils literal notranslate">f</code> becomes
garbage.
But stdin, stdout and stderr are treated specially by Python, because of the
special status also given to them by C. Running <code class="docutils literal notranslate">sys.stdout.close()</code> marks
the Python-level file object as being closed, but does not close the
associated C stream.
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:
<div class="highlight-default notranslate"><div class="highlight"><pre>os.close(0) # close C&#39;s stdin stream
os.close(1) # close C&#39;s stdout stream
os.close(2) # close C&#39;s stderr stream
</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>
See the chapters titled <a class="reference internal" href="../library/internet.html#internet">Internet Protocols and Support</a> and <a class="reference internal" href="../library/netdata.html#netdata">Internet Data Handling</a> in the Library
Reference Manual. Python has many modules that will help you build server-side
and client-side web systems.
A summary of available frameworks is maintained by Paul Boddie at
<a class="reference external" href="https://wiki.python.org/moin/WebProgramming">https://wiki.python.org/moin/WebProgramming</a>.
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>.
</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>
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?
Yes. Here’s a simple example that uses httplib:
<div class="highlight-default notranslate"><div class="highlight"><pre>#!/usr/local/bin/python

import httplib, sys, time

# build the query string
qs = &quot;First=Josephine&amp;MI=Q&amp;Last=Public&quot;

# connect and send the server a path
httpobj = httplib.HTTP(&#39;www.some-server.out-there&#39;, 80)
httpobj.putrequest(&#39;POST&#39;, &#39;/cgi-bin/some-cgi-script&#39;)
# now generate the rest of the HTTP headers...
httpobj.putheader(&#39;Accept&#39;, &#39;*/*&#39;)
httpobj.putheader(&#39;Connection&#39;, &#39;Keep-Alive&#39;)
httpobj.putheader(&#39;Content-type&#39;, &#39;application/x-www-form-urlencoded&#39;)
httpobj.putheader(&#39;Content-length&#39;, &#39;%d&#39; % len(qs))
httpobj.endheaders()
httpobj.send(qs)
# find out what the server said in response...
reply, msg, hdrs = httpobj.getreply()
if reply != 200:
 sys.stdout.write(httpobj.getfile().read())
</pre></div>
</div>
Note that in general for percent-encoded POST operations, query strings must be
quoted using <a class="reference internal" href="../library/urllib.html#urllib.urlencode" title="urllib.urlencode"><code class="xref py py-func docutils literal notranslate">urllib.urlencode()</code></a>. For example, to send
<code class="docutils literal notranslate">name=Guy Steele, Jr.</code>:
<div class="highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import urllib
&gt;&gt;&gt; urllib.urlencode({&#39;name&#39;: &#39;Guy Steele, Jr.&#39;})
&#39;name=Guy+Steele%2C+Jr.&#39;
</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>
You can find a collection of useful links on the <a class="reference external" href="https://wiki.python.org/moin/WebProgramming">Web Programming wiki page</a>.
</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>
Use the standard library module <a class="reference internal" href="../library/smtplib.html#module-smtplib" title="smtplib: SMTP protocol client (requires sockets)."><code class="xref py py-mod docutils literal notranslate">smtplib</code></a>.
Here’s a very simple interactive mail sender that uses it. This method will
work on any host that supports an SMTP listener.
<div class="highlight-default notranslate"><div class="highlight"><pre>import sys, smtplib

fromaddr = raw_input(&quot;From: &quot;)
toaddrs = raw_input(&quot;To: &quot;).split(&#39;,&#39;)
print &quot;Enter message, end with ^D:&quot;
msg = &#39;&#39;
while True:
 line = sys.stdin.readline()
 if not line:
 break
 msg += line

# The actual mail send
server = smtplib.SMTP(&#39;localhost&#39;)
server.sendmail(fromaddr, toaddrs, msg)
server.quit()
</pre></div>
</div>
A Unix-only alternative uses sendmail. The location of the sendmail program
varies between systems; sometimes it is <code class="docutils literal notranslate">/usr/lib/sendmail</code>, sometimes
<code class="docutils literal notranslate">/usr/sbin/sendmail</code>. The sendmail manual page will help you out. Here’s
some sample code:
<div class="highlight-default notranslate"><div class="highlight"><pre>import os

SENDMAIL = &quot;/usr/sbin/sendmail&quot; # sendmail location
p = os.popen(&quot;%s -t -i&quot; % SENDMAIL, &quot;w&quot;)
p.write(&quot;To: receiver@example.com\n&quot;)
p.write(&quot;Subject: test\n&quot;)
p.write(&quot;\n&quot;) # blank line separating headers from body
p.write(&quot;Some text\n&quot;)
p.write(&quot;some more text\n&quot;)
sts = p.close()
if sts != 0:
 print &quot;Sendmail exit status&quot;, sts
</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>
The select module is commonly used to help with asynchronous I/O on sockets.
To prevent the TCP connect from blocking, you can set the socket to non-blocking
mode. Then when you do the <code class="docutils literal notranslate">connect()</code>, you will either connect immediately
(unlikely) or get an exception that contains the error number as <code class="docutils literal notranslate">.errno</code>.
<code class="docutils literal notranslate">errno.EINPROGRESS</code> 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.
You can use the <code class="docutils literal notranslate">connect_ex()</code> method to avoid creating an exception. It will
just return the errno value. To poll, you can call <code class="docutils literal notranslate">connect_ex()</code> again later
– 0 or <code class="docutils literal notranslate">errno.EISCONN</code> indicate that you’re connected – or you can pass this
socket to select to check if it’s writable.
</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>
Yes.
Python 2.3 includes the <a class="reference internal" href="../library/bsddb.html#module-bsddb" title="bsddb: Interface to Berkeley DB database library"><code class="xref py py-mod docutils literal notranslate">bsddb</code></a> package which provides an interface to the
BerkeleyDB library. Interfaces to disk-based hashes such as <a class="reference internal" href="../library/dbm.html#module-dbm" title="dbm: The standard &quot;database&quot; interface, based on ndbm. (Unix)"><code class="xref py py-mod docutils literal notranslate">DBM</code></a>
and <a class="reference internal" href="../library/gdbm.html#module-gdbm" title="gdbm: GNU's reinterpretation of dbm. (Unix)"><code class="xref py py-mod docutils literal notranslate">GDBM</code></a> are also included with standard Python.
Support for most relational databases is available. See the
<a class="reference external" href="https://wiki.python.org/moin/DatabaseProgramming">DatabaseProgramming wiki page</a> for details.
</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>
The <a class="reference internal" href="../library/pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate">pickle</code></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."><code class="xref py py-mod docutils literal notranslate">shelve</code></a> library module uses pickle and (g)dbm to create persistent
mappings containing arbitrary Python objects. For better performance, you can
use the <a class="reference internal" href="../library/pickle.html#module-cPickle" title="cPickle: Faster version of pickle, but not subclassable."><code class="xref py py-mod docutils literal notranslate">cPickle</code></a> module.
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)."><code class="xref py py-mod docutils literal notranslate">marshal</code></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.
</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>
By default <a class="reference internal" href="../library/pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate">pickle</code></a> uses a relatively old and slow format for backward
compatibility. You can however specify other protocol versions that are
faster:
<div class="highlight-default notranslate"><div class="highlight"><pre>largeString = &#39;z&#39; * (100 * 1024)
myPickle = cPickle.dumps(largeString, protocol=1)
</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>
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 <code class="docutils literal notranslate">.close()</code> method of the database. The underlying library caches database
contents which need to be converted to on-disk form and written.
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.
</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>
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.
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.
You should move away from Berkeley DB version 1 files because the hash file code
contains known bugs that can corrupt your data.
</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>
The standard module <a class="reference internal" href="../library/random.html#module-random" title="random: Generate pseudo-random numbers with various common distributions."><code class="xref py py-mod docutils literal notranslate">random</code></a> implements a random number generator. Usage
is simple:
<div class="highlight-default notranslate"><div class="highlight"><pre>import random
random.random()
</pre></div>
</div>
This returns a random floating point number in the range [0, 1).
There are also many other specialized generators in this module, such as:
<ul class="simple">
<li><code class="docutils literal notranslate">randrange(a, b)</code> chooses an integer in the range [a, b).</li>
<li><code class="docutils literal notranslate">uniform(a, b)</code> chooses a floating point number in the range [a, b).</li>
<li><code class="docutils literal notranslate">normalvariate(mean, sdev)</code> samples the normal (Gaussian) distribution.</li>
</ul>
Some higher-level functions operate on sequences directly, such as:
<ul class="simple">
<li><code class="docutils literal notranslate">choice(S)</code> chooses random element from a given sequence</li>
<li><code class="docutils literal notranslate">shuffle(L)</code> shuffles a list in-place, i.e. permutes it randomly</li>
</ul>
There’s also a <code class="docutils literal notranslate">Random</code> class you can instantiate to create independent
multiple random number generators.
</div>
</div>
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<li><a class="reference internal" href="#common-tasks">Common tasks</a></li>
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