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<div class="section" id="appendix">
<span id="tut-appendix"></span><h1>16. Appendix<a class="headerlink" href="#appendix" title="Permalink to this headline">¶</a></h1>
<div class="section" id="interactive-mode">
<span id="tut-interac"></span><h2>16.1. Interactive Mode<a class="headerlink" href="#interactive-mode" title="Permalink to this headline">¶</a></h2>
<div class="section" id="error-handling">
<span id="tut-error"></span><h3>16.1.1. 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"><code class="xref std std-keyword docutils literal"><span class="pre">except</span></code></a> clause in a <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal"><span class="pre">try</span></code></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 <code class="kbd docutils literal"><span class="pre">Control-C</span></code> or <code class="kbd docutils literal"><span class="pre">Delete</span></code>) 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#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal"><span class="pre">KeyboardInterrupt</span></code></a> exception, which may be handled by a <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal"><span class="pre">try</span></code></a>
statement.</p>
</div>
<div class="section" id="executable-python-scripts">
<span id="tut-scripts"></span><h3>16.1.2. 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></span><span class="ch">#!/usr/bin/env python3.5</span>
</pre></div>
</div>
<p>(assuming that the interpreter is on the user’s <span class="target" id="index-0"></span><code class="xref std std-envvar docutils literal"><span class="pre">PATH</span></code>) at the beginning
of the script and giving the file an executable mode. The <code class="docutils literal"><span class="pre">#!</span></code> must be the
first two characters of the file. On some platforms, this first line must end
with a Unix-style line ending (<code class="docutils literal"><span class="pre">'\n'</span></code>), not a Windows (<code class="docutils literal"><span class="pre">'\r\n'</span></code>) line
ending. Note that the hash, or pound, character, <code class="docutils literal"><span class="pre">'#'</span></code>, 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-bash"><div class="highlight"><pre><span></span>$ chmod +x myscript.py
</pre></div>
</div>
<p>On Windows systems, there is no notion of an “executable mode”. The Python
installer automatically associates <code class="docutils literal"><span class="pre">.py</span></code> files with <code class="docutils literal"><span class="pre">python.exe</span></code> so that
a double-click on a Python file will run it as a script. The extension can
also be <code class="docutils literal"><span class="pre">.pyw</span></code>, in that case, the console window that normally appears is
suppressed.</p>
</div>
<div class="section" id="the-interactive-startup-file">
<span id="tut-startup"></span><h3>16.1.3. 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"><code class="xref std std-envvar docutils literal"><span class="pre">PYTHONSTARTUP</span></code></a> to the name of a
file containing your start-up commands. This is similar to the <code class="file docutils literal"><span class="pre">.profile</span></code>
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 <code class="file docutils literal"><span class="pre">/dev/tty</span></code> 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 <code class="docutils literal"><span class="pre">sys.ps1</span></code> and <code class="docutils literal"><span class="pre">sys.ps2</span></code> 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 <code 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></code>.
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></span><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="s1">'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="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="n">filename</span><span class="p">)</span> <span class="k">as</span> <span class="n">fobj</span><span class="p">:</span>
<span class="n">startup_file</span> <span class="o">=</span> <span class="n">fobj</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="n">exec</span><span class="p">(</span><span class="n">startup_file</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="the-customization-modules">
<span id="tut-customize"></span><h3>16.1.4. 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: <code class="xref py py-mod docutils literal"><span class="pre">sitecustomize</span></code> and
<code class="xref py py-mod docutils literal"><span class="pre">usercustomize</span></code>. 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></span><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/python3.5/site-packages'</span>
</pre></div>
</div>
<p>Now you can create a file named <code class="file docutils literal"><span class="pre">usercustomize.py</span></code> 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"><code class="xref std std-option docutils literal"><span class="pre">-s</span></code></a> option to disable the automatic import.</p>
<p><code class="xref py py-mod docutils literal"><span class="pre">sitecustomize</span></code> 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 <code class="xref py py-mod docutils literal"><span class="pre">usercustomize</span></code>. See the documentation of the <a class="reference internal" href="../library/site.html#module-site" title="site: Module responsible for site-specific configuration."><code class="xref py py-mod docutils literal"><span class="pre">site</span></code></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>
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<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">16. Appendix</a><ul>
<li><a class="reference internal" href="#interactive-mode">16.1. Interactive Mode</a><ul>
<li><a class="reference internal" href="#error-handling">16.1.1. Error Handling</a></li>
<li><a class="reference internal" href="#executable-python-scripts">16.1.2. Executable Python Scripts</a></li>
<li><a class="reference internal" href="#the-interactive-startup-file">16.1.3. The Interactive Startup File</a></li>
<li><a class="reference internal" href="#the-customization-modules">16.1.4. The Customization Modules</a></li>
</ul>
</li>
</ul>
</li>
</ul>
<h4>Previous topic</h4>
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title="previous chapter">15. Floating Point Arithmetic: Issues and Limitations</a></p>
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