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<div class="section" id="built-in-constants">
<h1>3. Built-in Constants<a class="headerlink" href="#built-in-constants" title="Permalink to this headline">¶</a></h1>
<p>A small number of constants live in the built-in namespace. They are:</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last"><a title="None" class="reference internal" href="#None"><tt class="xref xref docutils literal"><span class="pre">None</span></tt></a>, <a title="False" class="reference internal" href="#False"><tt class="xref xref docutils literal"><span class="pre">False</span></tt></a>, <a title="True" class="reference internal" href="#True"><tt class="xref xref docutils literal"><span class="pre">True</span></tt></a> and <a title="__debug__" class="reference internal" href="#__debug__"><tt class="xref docutils literal"><span class="pre">__debug__</span></tt></a> cannot be
reassigned (assignments to them raise <a title="exceptions.SyntaxError" class="reference external" href="exceptions.html#exceptions.SyntaxError"><tt class="xref docutils literal"><span class="pre">SyntaxError</span></tt></a>), so they can be
considered “true” constants.</p>
</div>
<dl class="data">
<dt id="False">
<tt class="descname">False</tt><a class="headerlink" href="#False" title="Permalink to this definition">¶</a></dt>
<dd>The false value of the <a title="bool" class="reference external" href="functions.html#bool"><tt class="xref docutils literal"><span class="pre">bool</span></tt></a> type. Assignments to <tt class="xref docutils literal"><span class="pre">False</span></tt>
are illegal and raise a <a title="exceptions.SyntaxError" class="reference external" href="exceptions.html#exceptions.SyntaxError"><tt class="xref docutils literal"><span class="pre">SyntaxError</span></tt></a>.</dd></dl>
<dl class="data">
<dt id="True">
<tt class="descname">True</tt><a class="headerlink" href="#True" title="Permalink to this definition">¶</a></dt>
<dd>The true value of the <a title="bool" class="reference external" href="functions.html#bool"><tt class="xref docutils literal"><span class="pre">bool</span></tt></a> type. Assignments to <tt class="xref docutils literal"><span class="pre">True</span></tt>
are illegal and raise a <a title="exceptions.SyntaxError" class="reference external" href="exceptions.html#exceptions.SyntaxError"><tt class="xref docutils literal"><span class="pre">SyntaxError</span></tt></a>.</dd></dl>
<dl class="data">
<dt id="None">
<tt class="descname">None</tt><a class="headerlink" href="#None" title="Permalink to this definition">¶</a></dt>
<dd>The sole value of <tt class="xref docutils literal"><span class="pre">types.NoneType</span></tt>. <tt class="xref docutils literal"><span class="pre">None</span></tt> is frequently used to
represent the absence of a value, as when default arguments are not passed to a
function. Assignments to <tt class="xref docutils literal"><span class="pre">None</span></tt> are illegal and raise a <a title="exceptions.SyntaxError" class="reference external" href="exceptions.html#exceptions.SyntaxError"><tt class="xref docutils literal"><span class="pre">SyntaxError</span></tt></a>.</dd></dl>
<dl class="data">
<dt id="NotImplemented">
<tt class="descname">NotImplemented</tt><a class="headerlink" href="#NotImplemented" title="Permalink to this definition">¶</a></dt>
<dd>Special value which can be returned by the “rich comparison” special methods
(<a title="object.__eq__" class="reference external" href="../reference/datamodel.html#object.__eq__"><tt class="xref docutils literal"><span class="pre">__eq__()</span></tt></a>, <a title="object.__lt__" class="reference external" href="../reference/datamodel.html#object.__lt__"><tt class="xref docutils literal"><span class="pre">__lt__()</span></tt></a>, and friends), to indicate that the comparison
is not implemented with respect to the other type.</dd></dl>
<dl class="data">
<dt id="Ellipsis">
<tt class="descname">Ellipsis</tt><a class="headerlink" href="#Ellipsis" title="Permalink to this definition">¶</a></dt>
<dd><p>The same as <tt class="docutils literal"><span class="pre">...</span></tt>. Special value used mostly in conjunction with extended
slicing syntax for user-defined container data types, as in</p>
<div class="highlight-python3"><div class="highlight"><pre><span class="o">..</span> <span class="n">XXX</span> <span class="n">Someone</span> <span class="n">who</span> <span class="n">understands</span> <span class="n">extended</span> <span class="n">slicing</span> <span class="n">should</span> <span class="n">fill</span> <span class="ow">in</span> <span class="n">here</span><span class="o">.</span>
</pre></div>
</div>
</dd></dl>
<dl class="data">
<dt id="__debug__">
<tt class="descname">__debug__</tt><a class="headerlink" href="#__debug__" title="Permalink to this definition">¶</a></dt>
<dd>This constant is true if Python was not started with an <a class="reference external" href="../using/cmdline.html#cmdoption-O"><em class="xref">-O</em></a> option.
Assignments to <a title="__debug__" class="reference internal" href="#__debug__"><tt class="xref docutils literal"><span class="pre">__debug__</span></tt></a> are illegal and raise a <a title="exceptions.SyntaxError" class="reference external" href="exceptions.html#exceptions.SyntaxError"><tt class="xref docutils literal"><span class="pre">SyntaxError</span></tt></a>.
See also the <a class="reference external" href="../reference/simple_stmts.html#assert"><tt class="xref docutils literal"><span class="pre">assert</span></tt></a> statement.</dd></dl>
<div class="section" id="constants-added-by-the-site-module">
<h2>3.1. Constants added by the <a title="A standard way to reference site-specific modules." class="reference external" href="site.html#module-site"><tt class="xref docutils literal"><span class="pre">site</span></tt></a> module<a class="headerlink" href="#constants-added-by-the-site-module" title="Permalink to this headline">¶</a></h2>
<p>The <a title="A standard way to reference site-specific modules." class="reference external" href="site.html#module-site"><tt class="xref docutils literal"><span class="pre">site</span></tt></a> module (which is imported automatically during startup, except
if the <a class="reference external" href="../using/cmdline.html#cmdoption-S"><em class="xref">-S</em></a> command-line option is given) adds several constants to the
built-in namespace. They are useful for the interactive interpreter shell and
should not be used in programs.</p>
<dl class="data">
<dt id="quit">
<tt class="descname">quit</tt><big>(</big><em>code=None</em><big>)</big><a class="headerlink" href="#quit" title="Permalink to this definition">¶</a></dt>
<dt id="exit">
<tt class="descname">exit</tt><big>(</big><em>code=None</em><big>)</big><a class="headerlink" href="#exit" title="Permalink to this definition">¶</a></dt>
<dd>Objects that when printed, print a message like “Use quit() or Ctrl-D
(i.e. EOF) to exit”, and when called, raise <a title="exceptions.SystemExit" class="reference external" href="exceptions.html#exceptions.SystemExit"><tt class="xref docutils literal"><span class="pre">SystemExit</span></tt></a> with the
specified exit code, and when .</dd></dl>
<dl class="data">
<dt id="copyright">
<tt class="descname">copyright</tt><a class="headerlink" href="#copyright" title="Permalink to this definition">¶</a></dt>
<dt id="license">
<tt class="descname">license</tt><a class="headerlink" href="#license" title="Permalink to this definition">¶</a></dt>
<dt id="credits">
<tt class="descname">credits</tt><a class="headerlink" href="#credits" title="Permalink to this definition">¶</a></dt>
<dd>Objects that when printed, print a message like “Type license() to see the
full license text”, and when called, display the corresponding text in a
pager-like fashion (one screen at a time).</dd></dl>
</div>
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<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference external" href="">3. Built-in Constants</a><ul>
<li><a class="reference external" href="#constants-added-by-the-site-module">3.1. Constants added by the <tt class="docutils literal"><span class="pre">site</span></tt> module</a></li>
</ul>
</li>
</ul>
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