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<div class="section" id="using-the-python-interpreter">
<span id="tut-using"></span><h1>2. Using the Python Interpreter<a class="headerlink" href="#using-the-python-interpreter" title="Permalink to this headline">¶</a></h1>
<div class="section" id="invoking-the-interpreter">
<span id="tut-invoking"></span><h2>2.1. Invoking the Interpreter<a class="headerlink" href="#invoking-the-interpreter" title="Permalink to this headline">¶</a></h2>
<p>The Python interpreter is usually installed as <tt class="file docutils literal"><span class="pre">/usr/bin/python33.2</span></tt>
on those machines where it is available; putting <tt class="file docutils literal"><span class="pre">/usr/local/bin</span></tt> in your
Unix shell’s search path makes it possible to start it by typing the command</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="n">python3</span><span class="o">.</span><span class="mi">2</span>
</pre></div>
</div>
<p>to the shell. <a class="footnote-reference" href="#id3" id="id1">[1]</a> Since the choice of the directory where the interpreter lives
is an installation option, other places are possible; check with your local
Python guru or system administrator. (E.g., <tt class="file docutils literal"><span class="pre">/usr/local/python</span></tt> is a
popular alternative location.)</p>
<p>On Windows machines, the Python installation is usually placed in
<tt class="file docutils literal"><span class="pre">C:\Python32</span></tt>, though you can change this when you’re running the
installer. To add this directory to your path, you can type the following
command into the command prompt in a DOS box:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="nb">set</span> <span class="n">path</span><span class="o">=%</span><span class="n">path</span><span class="o">%</span><span class="p">;</span><span class="n">C</span><span class="p">:</span>\<span class="n">python32</span>
</pre></div>
</div>
<p>Typing an end-of-file character (<tt class="kbd docutils literal"><span class="pre">Control-D</span></tt> on Unix, <tt class="kbd docutils literal"><span class="pre">Control-Z</span></tt> on
Windows) at the primary prompt causes the interpreter to exit with a zero exit
status. If that doesn’t work, you can exit the interpreter by typing the
following command: <tt class="docutils literal"><span class="pre">quit()</span></tt>.</p>
<p>The interpreter’s line-editing features usually aren’t very sophisticated. On
Unix, whoever installed the interpreter may have enabled support for the GNU
readline library, which adds more elaborate interactive editing and history
features. Perhaps the quickest check to see whether command line editing is
supported is typing Control-P to the first Python prompt you get. If it beeps,
you have command line editing; see Appendix <a class="reference internal" href="interactive.html#tut-interacting"><em>Interactive Input Editing and History Substitution</em></a> for an
introduction to the keys. If nothing appears to happen, or if <tt class="docutils literal"><span class="pre">^P</span></tt> is echoed,
command line editing isn’t available; you’ll only be able to use backspace to
remove characters from the current line.</p>
<p>The interpreter operates somewhat like the Unix shell: when called with standard
input connected to a tty device, it reads and executes commands interactively;
when called with a file name argument or with a file as standard input, it reads
and executes a <em>script</em> from that file.</p>
<p>A second way of starting the interpreter is <tt class="docutils literal"><span class="pre">python</span> <span class="pre">-c</span> <span class="pre">command</span> <span class="pre">[arg]</span> <span class="pre">...</span></tt>,
which executes the statement(s) in <em>command</em>, analogous to the shell’s
<a class="reference internal" href="../using/cmdline.html#cmdoption-c"><em class="xref std std-option">-c</em></a> option. Since Python statements often contain spaces or other
characters that are special to the shell, it is usually advised to quote
<em>command</em> in its entirety with single quotes.</p>
<p>Some Python modules are also useful as scripts. These can be invoked using
<tt class="docutils literal"><span class="pre">python</span> <span class="pre">-m</span> <span class="pre">module</span> <span class="pre">[arg]</span> <span class="pre">...</span></tt>, which executes the source file for <em>module</em> as
if you had spelled out its full name on the command line.</p>
<p>When a script file is used, it is sometimes useful to be able to run the script
and enter interactive mode afterwards. This can be done by passing <a class="reference internal" href="../using/cmdline.html#cmdoption-i"><em class="xref std std-option">-i</em></a>
before the script. (This does not work if the script is read from standard
input, for the same reason as explained in the previous paragraph.)</p>
<div class="section" id="argument-passing">
<span id="tut-argpassing"></span><h3>2.1.1. Argument Passing<a class="headerlink" href="#argument-passing" title="Permalink to this headline">¶</a></h3>
<p>When known to the interpreter, the script name and additional arguments
thereafter are turned into a list of strings and assigned to the <tt class="docutils literal"><span class="pre">argv</span></tt>
variable in the <tt class="docutils literal"><span class="pre">sys</span></tt> module. You can access this list by executing <tt class="docutils literal"><span class="pre">import</span>
<span class="pre">sys</span></tt>. The length of the list is at least one; when no script and no arguments
are given, <tt class="docutils literal"><span class="pre">sys.argv[0]</span></tt> is an empty string. When the script name is given as
<tt class="docutils literal"><span class="pre">'-'</span></tt> (meaning standard input), <tt class="docutils literal"><span class="pre">sys.argv[0]</span></tt> is set to <tt class="docutils literal"><span class="pre">'-'</span></tt>. When
<a class="reference internal" href="../using/cmdline.html#cmdoption-c"><em class="xref std std-option">-c</em></a> <em>command</em> is used, <tt class="docutils literal"><span class="pre">sys.argv[0]</span></tt> is set to <tt class="docutils literal"><span class="pre">'-c'</span></tt>. When
<a class="reference internal" href="../using/cmdline.html#cmdoption-m"><em class="xref std std-option">-m</em></a> <em>module</em> is used, <tt class="docutils literal"><span class="pre">sys.argv[0]</span></tt> is set to the full name of the
located module. Options found after <a class="reference internal" href="../using/cmdline.html#cmdoption-c"><em class="xref std std-option">-c</em></a> <em>command</em> or <a class="reference internal" href="../using/cmdline.html#cmdoption-m"><em class="xref std std-option">-m</em></a>
<em>module</em> are not consumed by the Python interpreter’s option processing but
left in <tt class="docutils literal"><span class="pre">sys.argv</span></tt> for the command or module to handle.</p>
</div>
<div class="section" id="interactive-mode">
<span id="tut-interactive"></span><h3>2.1.2. Interactive Mode<a class="headerlink" href="#interactive-mode" title="Permalink to this headline">¶</a></h3>
<p>When commands are read from a tty, the interpreter is said to be in <em>interactive
mode</em>. In this mode it prompts for the next command with the <em>primary prompt</em>,
usually three greater-than signs (<tt class="docutils literal"><span class="pre">>>></span></tt>); for continuation lines it prompts
with the <em>secondary prompt</em>, by default three dots (<tt class="docutils literal"><span class="pre">...</span></tt>). The interpreter
prints a welcome message stating its version number and a copyright notice
before printing the first prompt:</p>
<div class="highlight-python3"><pre>$ python3.2
Python 3.2 (py3k, Sep 12 2007, 12:21:02)
[GCC 3.4.6 20060404 (Red Hat 3.4.6-8)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>></pre>
</div>
<p>Continuation lines are needed when entering a multi-line construct. As an
example, take a look at this <a class="reference internal" href="../reference/compound_stmts.html#if"><tt class="xref std std-keyword docutils literal"><span class="pre">if</span></tt></a> statement:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">the_world_is_flat</span> <span class="o">=</span> <span class="mi">1</span>
<span class="gp">>>> </span><span class="k">if</span> <span class="n">the_world_is_flat</span><span class="p">:</span>
<span class="gp">... </span> <span class="nb">print</span><span class="p">(</span><span class="s">"Be careful not to fall off!"</span><span class="p">)</span>
<span class="gp">...</span>
<span class="go">Be careful not to fall off!</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="the-interpreter-and-its-environment">
<span id="tut-interp"></span><h2>2.2. The Interpreter and Its Environment<a class="headerlink" href="#the-interpreter-and-its-environment" title="Permalink to this headline">¶</a></h2>
<div class="section" id="error-handling">
<span id="tut-error"></span><h3>2.2.1. Error Handling<a class="headerlink" href="#error-handling" title="Permalink to this headline">¶</a></h3>
<p>When an error occurs, the interpreter prints an error message and a stack trace.
In interactive mode, it then returns to the primary prompt; when input came from
a file, it exits with a nonzero exit status after printing the stack trace.
(Exceptions handled by an <a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal"><span class="pre">except</span></tt></a> clause in a <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a> statement
are not errors in this context.) Some errors are unconditionally fatal and
cause an exit with a nonzero exit; this applies to internal inconsistencies and
some cases of running out of memory. All error messages are written to the
standard error stream; normal output from executed commands is written to
standard output.</p>
<p>Typing the interrupt character (usually Control-C or DEL) to the primary or
secondary prompt cancels the input and returns to the primary prompt. <a class="footnote-reference" href="#id4" id="id2">[2]</a>
Typing an interrupt while a command is executing raises the
<a class="reference internal" href="../library/exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><tt class="xref py py-exc docutils literal"><span class="pre">KeyboardInterrupt</span></tt></a> exception, which may be handled by a <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a>
statement.</p>
</div>
<div class="section" id="executable-python-scripts">
<span id="tut-scripts"></span><h3>2.2.2. Executable Python Scripts<a class="headerlink" href="#executable-python-scripts" title="Permalink to this headline">¶</a></h3>
<p>On BSD’ish Unix systems, Python scripts can be made directly executable, like
shell scripts, by putting the line</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="c">#! /usr/bin/env python3.2</span>
</pre></div>
</div>
<p>(assuming that the interpreter is on the user’s <span class="target" id="index-0"></span><tt class="xref std std-envvar docutils literal"><span class="pre">PATH</span></tt>) at the beginning
of the script and giving the file an executable mode. The <tt class="docutils literal"><span class="pre">#!</span></tt> must be the
first two characters of the file. On some platforms, this first line must end
with a Unix-style line ending (<tt class="docutils literal"><span class="pre">'\n'</span></tt>), not a Windows (<tt class="docutils literal"><span class="pre">'\r\n'</span></tt>) line
ending. Note that the hash, or pound, character, <tt class="docutils literal"><span class="pre">'#'</span></tt>, is used to start a
comment in Python.</p>
<p>The script can be given an executable mode, or permission, using the
<strong class="program">chmod</strong> command:</p>
<div class="highlight-python3"><pre>$ chmod +x myscript.py</pre>
</div>
<p>On Windows systems, there is no notion of an “executable mode”. The Python
installer automatically associates <tt class="docutils literal"><span class="pre">.py</span></tt> files with <tt class="docutils literal"><span class="pre">python.exe</span></tt> so that
a double-click on a Python file will run it as a script. The extension can
also be <tt class="docutils literal"><span class="pre">.pyw</span></tt>, in that case, the console window that normally appears is
suppressed.</p>
</div>
<div class="section" id="source-code-encoding">
<span id="tut-source-encoding"></span><h3>2.2.3. Source Code Encoding<a class="headerlink" href="#source-code-encoding" title="Permalink to this headline">¶</a></h3>
<p>By default, Python source files are treated as encoded in UTF-8. In that
encoding, characters of most languages in the world can be used simultaneously
in string literals, identifiers and comments — although the standard library
only uses ASCII characters for identifiers, a convention that any portable code
should follow. To display all these characters properly, your editor must
recognize that the file is UTF-8, and it must use a font that supports all the
characters in the file.</p>
<p>It is also possible to specify a different encoding for source files. In order
to do this, put one more special comment line right after the <tt class="docutils literal"><span class="pre">#!</span></tt> line to
define the source file encoding:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="c"># -*- coding: encoding -*-</span>
</pre></div>
</div>
<p>With that declaration, everything in the source file will be treated as having
the encoding <em>encoding</em> instead of UTF-8. The list of possible encodings can be
found in the Python Library Reference, in the section on <a class="reference internal" href="../library/codecs.html#module-codecs" title="codecs: Encode and decode data and streams."><tt class="xref py py-mod docutils literal"><span class="pre">codecs</span></tt></a>.</p>
<p>For example, if your editor of choice does not support UTF-8 encoded files and
insists on using some other encoding, say Windows-1252, you can write:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="c"># -*- coding: cp-1252 -*-</span>
</pre></div>
</div>
<p>and still use all characters in the Windows-1252 character set in the source
files. The special encoding comment must be in the <em>first or second</em> line
within the file.</p>
</div>
<div class="section" id="the-interactive-startup-file">
<span id="tut-startup"></span><h3>2.2.4. The Interactive Startup File<a class="headerlink" href="#the-interactive-startup-file" title="Permalink to this headline">¶</a></h3>
<p>When you use Python interactively, it is frequently handy to have some standard
commands executed every time the interpreter is started. You can do this by
setting an environment variable named <span class="target" id="index-1"></span><a class="reference internal" href="../using/cmdline.html#envvar-PYTHONSTARTUP"><tt class="xref std std-envvar docutils literal"><span class="pre">PYTHONSTARTUP</span></tt></a> to the name of a
file containing your start-up commands. This is similar to the <tt class="file docutils literal"><span class="pre">.profile</span></tt>
feature of the Unix shells.</p>
<p>This file is only read in interactive sessions, not when Python reads commands
from a script, and not when <tt class="file docutils literal"><span class="pre">/dev/tty</span></tt> is given as the explicit source of
commands (which otherwise behaves like an interactive session). It is executed
in the same namespace where interactive commands are executed, so that objects
that it defines or imports can be used without qualification in the interactive
session. You can also change the prompts <tt class="docutils literal"><span class="pre">sys.ps1</span></tt> and <tt class="docutils literal"><span class="pre">sys.ps2</span></tt> in this
file.</p>
<p>If you want to read an additional start-up file from the current directory, you
can program this in the global start-up file using code like <tt class="docutils literal"><span class="pre">if</span>
<span class="pre">os.path.isfile('.pythonrc.py'):</span> <span class="pre">exec(open('.pythonrc.py').read())</span></tt>.
If you want to use the startup file in a script, you must do this explicitly
in the script:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">os</span>
<span class="n">filename</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">environ</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s">'PYTHONSTARTUP'</span><span class="p">)</span>
<span class="k">if</span> <span class="n">filename</span> <span class="ow">and</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">isfile</span><span class="p">(</span><span class="n">filename</span><span class="p">):</span>
<span class="n">exec</span><span class="p">(</span><span class="nb">open</span><span class="p">(</span><span class="n">filename</span><span class="p">)</span><span class="o">.</span><span class="n">read</span><span class="p">())</span>
</pre></div>
</div>
</div>
<div class="section" id="the-customization-modules">
<span id="tut-customize"></span><h3>2.2.5. The Customization Modules<a class="headerlink" href="#the-customization-modules" title="Permalink to this headline">¶</a></h3>
<p>Python provides two hooks to let you customize it: <tt class="xref py py-mod docutils literal"><span class="pre">sitecustomize</span></tt> and
<tt class="xref py py-mod docutils literal"><span class="pre">usercustomize</span></tt>. To see how it works, you need first to find the location
of your user site-packages directory. Start Python and run this code:</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">site</span>
<span class="gp">>>> </span><span class="n">site</span><span class="o">.</span><span class="n">getusersitepackages</span><span class="p">()</span>
<span class="go">'/home/user/.local/lib/python3.2/site-packages'</span>
</pre></div>
</div>
<p>Now you can create a file named <tt class="file docutils literal"><span class="pre">usercustomize.py</span></tt> in that directory and
put anything you want in it. It will affect every invocation of Python, unless
it is started with the <a class="reference internal" href="../using/cmdline.html#cmdoption-s"><em class="xref std std-option">-s</em></a> option to disable the automatic import.</p>
<p><tt class="xref py py-mod docutils literal"><span class="pre">sitecustomize</span></tt> works in the same way, but is typically created by an
administrator of the computer in the global site-packages directory, and is
imported before <tt class="xref py py-mod docutils literal"><span class="pre">usercustomize</span></tt>. See the documentation of the <a class="reference internal" href="../library/site.html#module-site" title="site: Module responsible for site-specific configuration."><tt class="xref py py-mod docutils literal"><span class="pre">site</span></tt></a>
module for more details.</p>
<p class="rubric">Footnotes</p>
<table class="docutils footnote" frame="void" id="id3" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>On Unix, the Python 3.x interpreter is by default not installed with the
executable named <tt class="docutils literal"><span class="pre">python</span></tt>, so that it does not conflict with a
simultaneously installed Python 2.x executable.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id4" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id2">[2]</a></td><td>A problem with the GNU Readline package may prevent this.</td></tr>
</tbody>
</table>
</div>
</div>
</div>
</div>
</div>
</div>
<div class="sphinxsidebar">
<div class="sphinxsidebarwrapper">
<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">2. Using the Python Interpreter</a><ul>
<li><a class="reference internal" href="#invoking-the-interpreter">2.1. Invoking the Interpreter</a><ul>
<li><a class="reference internal" href="#argument-passing">2.1.1. Argument Passing</a></li>
<li><a class="reference internal" href="#interactive-mode">2.1.2. Interactive Mode</a></li>
</ul>
</li>
<li><a class="reference internal" href="#the-interpreter-and-its-environment">2.2. The Interpreter and Its Environment</a><ul>
<li><a class="reference internal" href="#error-handling">2.2.1. Error Handling</a></li>
<li><a class="reference internal" href="#executable-python-scripts">2.2.2. Executable Python Scripts</a></li>
<li><a class="reference internal" href="#source-code-encoding">2.2.3. Source Code Encoding</a></li>
<li><a class="reference internal" href="#the-interactive-startup-file">2.2.4. The Interactive Startup File</a></li>
<li><a class="reference internal" href="#the-customization-modules">2.2.5. The Customization Modules</a></li>
</ul>
</li>
</ul>
</li>
</ul>
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distrib
>
Mandriva
>
2010.1
>
x86_64
>
media
>
contrib-backports
>
by-pkgid
>
3ba3bd1608c672ba2129b098a48e9e4d
>
files
>
894
