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Using the Python Interpreter — Python 2.7.5 documentation</title> <link rel="stylesheet" href="../_static/default.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript"> var DOCUMENTATION_OPTIONS = { URL_ROOT: '../', VERSION: '2.7.5', COLLAPSE_INDEX: false, FILE_SUFFIX: '.html', HAS_SOURCE: true }; </script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 2.7.5 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="top" title="Python 2.7.5 documentation" href="../index.html" /> <link rel="up" title="The Python Tutorial" href="index.html" /> <link rel="next" title="3. An Informal Introduction to Python" href="introduction.html" /> <link rel="prev" title="1. Whetting Your Appetite" href="appetite.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head> <body> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="introduction.html" title="3. An Informal Introduction to Python" accesskey="N">next</a> |</li> <li class="right" > <a href="appetite.html" title="1. Whetting Your Appetite" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="http://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.5 documentation</a> » </li> <li><a href="index.html" accesskey="U">The Python Tutorial</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <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/local/bin/python</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-python"><div class="highlight"><pre><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., <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:\Python27</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-python"><pre>set path=%path%;C:\python27</pre> </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.</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-python"><pre>python Python 2.7 (#1, Feb 28 2010, 00:02:06) 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-python"><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="k">print</span> <span class="s">"Be careful not to fall off!"</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="#id2" id="id1">[1]</a> Typing an interrupt while a command is executing raises the <a class="reference internal" href="../library/exceptions.html#exceptions.KeyboardInterrupt" title="exceptions.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-python"><div class="highlight"><pre><span class="c">#! /usr/bin/env python</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-python"><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>It is possible to use encodings different than ASCII in Python source files. The best way to do it is to 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-python"><pre># -*- coding: encoding -*-</pre> </div> <p>With that declaration, all characters in the source file will be treated as having the encoding <em>encoding</em>, and it will be possible to directly write Unicode string literals in the selected encoding. 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, to write Unicode literals including the Euro currency symbol, the ISO-8859-15 encoding can be used, with the Euro symbol having the ordinal value 164. This script will print the value 8364 (the Unicode codepoint corresponding to the Euro symbol) and then exit:</p> <div class="highlight-python"><div class="highlight"><pre><span class="c"># -*- coding: iso-8859-15 -*-</span> <span class="n">currency</span> <span class="o">=</span> <span class="s">u"€"</span> <span class="k">print</span> <span class="nb">ord</span><span class="p">(</span><span class="n">currency</span><span class="p">)</span> </pre></div> </div> <p>If your editor supports saving files as <tt class="docutils literal"><span class="pre">UTF-8</span></tt> with a UTF-8 <em>byte order mark</em> (aka BOM), you can use that instead of an encoding declaration. IDLE supports this capability if <tt class="docutils literal"><span class="pre">Options/General/Default</span> <span class="pre">Source</span> <span class="pre">Encoding/UTF-8</span></tt> is set. Notice that this signature is not understood in older Python releases (2.2 and earlier), and also not understood by the operating system for script files with <tt class="docutils literal"><span class="pre">#!</span></tt> lines (only used on Unix systems).</p> <p>By using UTF-8 (either through the signature or an encoding declaration), characters of most languages in the world can be used simultaneously in string literals and comments. Using non-ASCII characters in identifiers is not supported. 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> </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">execfile('.pythonrc.py')</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-python"><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="nb">execfile</span><span class="p">(</span><span class="n">filename</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-python"><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="id2" 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>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> <h4>Previous topic</h4> <p class="topless"><a href="appetite.html" title="previous chapter">1. Whetting Your Appetite</a></p> <h4>Next topic</h4> <p class="topless"><a href="introduction.html" title="next chapter">3. 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