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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 <code class="file docutils literal notranslate"><span class="pre">/usr/bin/python</span></code> on
those machines where it is available; putting <code class="file docutils literal notranslate"><span class="pre">/usr/local/bin</span></code> in your
Unix shell’s search path makes it possible to start it by typing the command</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">python</span>
</pre></div>
</div>
<p>to the shell. 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., <code class="file docutils literal notranslate"><span class="pre">/usr/local/python</span></code> is a popular
alternative location.)</p>
<p>On Windows machines, the Python installation is usually placed in
<code class="file docutils literal notranslate"><span class="pre">C:\Python27</span></code>, 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-default notranslate"><div class="highlight"><pre><span></span><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">python27</span>
</pre></div>
</div>
<p>Typing an end-of-file character (<kbd class="kbd docutils literal notranslate">Control-D</kbd> on Unix, <kbd class="kbd docutils literal notranslate">Control-Z</kbd> 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: <code class="docutils literal notranslate"><span class="pre">quit()</span></code>.</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 <kbd class="kbd docutils literal notranslate">Control-P</kbd> 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"><span class="std std-ref">Interactive Input Editing and History Substitution</span></a> for an
introduction to the keys. If nothing appears to happen, or if <code class="docutils literal notranslate"><span class="pre">^P</span></code> 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 <code class="docutils literal notranslate"><span class="pre">python</span> <span class="pre">-c</span> <span class="pre">command</span> <span class="pre">[arg]</span> <span class="pre">...</span></code>,
which executes the statement(s) in <em>command</em>, analogous to the shell’s
<a class="reference internal" href="../using/cmdline.html#cmdoption-c"><code class="xref std std-option docutils literal notranslate"><span class="pre">-c</span></code></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
<code class="docutils literal notranslate"><span class="pre">python</span> <span class="pre">-m</span> <span class="pre">module</span> <span class="pre">[arg]</span> <span class="pre">...</span></code>, 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"><code class="xref std std-option docutils literal notranslate"><span class="pre">-i</span></code></a>
before the script.</p>
<p>All command-line options are described in <a class="reference internal" href="../using/cmdline.html#using-on-general"><span class="std std-ref">Command line and environment</span></a>.</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 <code class="docutils literal notranslate"><span class="pre">argv</span></code>
variable in the <code class="docutils literal notranslate"><span class="pre">sys</span></code> module. You can access this list by executing <code class="docutils literal notranslate"><span class="pre">import</span>
<span class="pre">sys</span></code>. The length of the list is at least one; when no script and no arguments
are given, <code class="docutils literal notranslate"><span class="pre">sys.argv[0]</span></code> is an empty string. When the script name is given as
<code class="docutils literal notranslate"><span class="pre">'-'</span></code> (meaning standard input), <code class="docutils literal notranslate"><span class="pre">sys.argv[0]</span></code> is set to <code class="docutils literal notranslate"><span class="pre">'-'</span></code>. When
<a class="reference internal" href="../using/cmdline.html#cmdoption-c"><code class="xref std std-option docutils literal notranslate"><span class="pre">-c</span></code></a> <em>command</em> is used, <code class="docutils literal notranslate"><span class="pre">sys.argv[0]</span></code> is set to <code class="docutils literal notranslate"><span class="pre">'-c'</span></code>. When
<a class="reference internal" href="../using/cmdline.html#cmdoption-m"><code class="xref std std-option docutils literal notranslate"><span class="pre">-m</span></code></a> <em>module</em> is used, <code class="docutils literal notranslate"><span class="pre">sys.argv[0]</span></code> is set to the full name of the
located module. Options found after <a class="reference internal" href="../using/cmdline.html#cmdoption-c"><code class="xref std std-option docutils literal notranslate"><span class="pre">-c</span></code></a> <em>command</em> or <a class="reference internal" href="../using/cmdline.html#cmdoption-m"><code class="xref std std-option docutils literal notranslate"><span class="pre">-m</span></code></a>
<em>module</em> are not consumed by the Python interpreter’s option processing but
left in <code class="docutils literal notranslate"><span class="pre">sys.argv</span></code> 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 (<code class="docutils literal notranslate"><span class="pre">>>></span></code>); for continuation lines it prompts
with the <em>secondary prompt</em>, by default three dots (<code class="docutils literal notranslate"><span class="pre">...</span></code>). The interpreter
prints a welcome message stating its version number and a copyright notice
before printing the first prompt:</p>
<div class="highlight-shell-session notranslate"><div class="highlight"><pre><span></span><span class="go">python</span>
<span class="go">Python 2.7 (#1, Feb 28 2010, 00:02:06)</span>
<span class="go">Type "help", "copyright", "credits" or "license" for more information.</span>
<span class="gp">></span>>>
</pre></div>
</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"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">if</span></code></a> statement:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><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="s2">"Be careful not to fall off!"</span>
<span class="gp">...</span>
<span class="go">Be careful not to fall off!</span>
</pre></div>
</div>
<p>For more on interactive mode, see <a class="reference internal" href="appendix.html#tut-interac"><span class="std std-ref">Interactive Mode</span></a>.</p>
</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="source-code-encoding">
<span id="tut-source-encoding"></span><h3>2.2.1. 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 ASCII.
To declare an encoding other than the default one, a special comment line
should be added as the <em>first</em> line of the file. The syntax is as follows:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># -*- coding: encoding -*-</span>
</pre></div>
</div>
<p>where <em>encoding</em> is one of the valid <a class="reference internal" href="../library/codecs.html#module-codecs" title="codecs: Encode and decode data and streams."><code class="xref py py-mod docutils literal notranslate"><span class="pre">codecs</span></code></a> supported by Python.</p>
<p>For example, to declare that Windows-1252 encoding is to be used, the first
line of your source code file should be:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># -*- coding: cp1252 -*-</span>
</pre></div>
</div>
<p>One exception to the <em>first line</em> rule is when the source code starts with a
<a class="reference internal" href="appendix.html#tut-scripts"><span class="std std-ref">UNIX “shebang” line</span></a>. In this case, the encoding
declaration should be added as the second line of the file. For example:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="ch">#!/usr/bin/env python</span>
<span class="c1"># -*- coding: cp1252 -*-</span>
</pre></div>
</div>
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<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="#source-code-encoding">2.2.1. Source Code Encoding</a></li>
</ul>
</li>
</ul>
</li>
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