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<div class="section" id="built-in-constants">
<span id="built-in-consts"></span><h1>4. 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>
<dl class="data">
<dt id="False">
<code class="descname">False</code><a class="headerlink" href="#False" title="Permalink to this definition">¶</a></dt>
<dd><p>The false value of the <a class="reference internal" href="functions.html#bool" title="bool"><code class="xref py py-class docutils literal notranslate"><span class="pre">bool</span></code></a> type.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 2.3.</span></p>
</div>
</dd></dl>
<dl class="data">
<dt id="True">
<code class="descname">True</code><a class="headerlink" href="#True" title="Permalink to this definition">¶</a></dt>
<dd><p>The true value of the <a class="reference internal" href="functions.html#bool" title="bool"><code class="xref py py-class docutils literal notranslate"><span class="pre">bool</span></code></a> type.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 2.3.</span></p>
</div>
</dd></dl>
<dl class="data">
<dt id="None">
<code class="descname">None</code><a class="headerlink" href="#None" title="Permalink to this definition">¶</a></dt>
<dd><p>The sole value of <a class="reference internal" href="types.html#types.NoneType" title="types.NoneType"><code class="xref py py-attr docutils literal notranslate"><span class="pre">types.NoneType</span></code></a>. <code class="docutils literal notranslate"><span class="pre">None</span></code> is frequently used to
represent the absence of a value, as when default arguments are not passed to a
function.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 2.4: </span>Assignments to <code class="docutils literal notranslate"><span class="pre">None</span></code> are illegal and raise a <a class="reference internal" href="exceptions.html#exceptions.SyntaxError" title="exceptions.SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a>.</p>
</div>
</dd></dl>
<dl class="data">
<dt id="NotImplemented">
<code class="descname">NotImplemented</code><a class="headerlink" href="#NotImplemented" title="Permalink to this definition">¶</a></dt>
<dd><p>Special value which can be returned by the “rich comparison” special methods
(<a class="reference internal" href="../reference/datamodel.html#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__eq__()</span></code></a>, <a class="reference internal" href="../reference/datamodel.html#object.__lt__" title="object.__lt__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__lt__()</span></code></a>, and friends), to indicate that the comparison
is not implemented with respect to the other type.</p>
</dd></dl>
<dl class="data">
<dt id="Ellipsis">
<code class="descname">Ellipsis</code><a class="headerlink" href="#Ellipsis" title="Permalink to this definition">¶</a></dt>
<dd><p>Special value used in conjunction with extended slicing syntax.</p>
</dd></dl>
<dl class="data">
<dt id="__debug__">
<code class="descname">__debug__</code><a class="headerlink" href="#__debug__" title="Permalink to this definition">¶</a></dt>
<dd><p>This constant is true if Python was not started with an <a class="reference internal" href="../using/cmdline.html#cmdoption-o"><code class="xref std std-option docutils literal notranslate"><span class="pre">-O</span></code></a> option.
See also the <a class="reference internal" href="../reference/simple_stmts.html#assert"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">assert</span></code></a> statement.</p>
</dd></dl>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>The names <a class="reference internal" href="#None" title="None"><code class="xref py py-data docutils literal notranslate"><span class="pre">None</span></code></a> and <a class="reference internal" href="#__debug__" title="__debug__"><code class="xref py py-data docutils literal notranslate"><span class="pre">__debug__</span></code></a> cannot be reassigned
(assignments to them, even as an attribute name, raise <a class="reference internal" href="exceptions.html#exceptions.SyntaxError" title="exceptions.SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a>),
so they can be considered “true” constants.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 2.7: </span>Assignments to <code class="docutils literal notranslate"><span class="pre">__debug__</span></code> as an attribute became illegal.</p>
</div>
</div>
<div class="section" id="constants-added-by-the-site-module">
<h2>4.1. Constants added by the <a class="reference internal" href="site.html#module-site" title="site: Module responsible for site-specific configuration."><code class="xref py py-mod docutils literal notranslate"><span class="pre">site</span></code></a> module<a class="headerlink" href="#constants-added-by-the-site-module" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference internal" href="site.html#module-site" title="site: Module responsible for site-specific configuration."><code class="xref py py-mod docutils literal notranslate"><span class="pre">site</span></code></a> module (which is imported automatically during startup, except
if the <a class="reference internal" href="../using/cmdline.html#id2"><code class="xref std std-option docutils literal notranslate"><span class="pre">-S</span></code></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">
<code class="descname">quit</code><span class="sig-paren">(</span><span class="optional">[</span><em>code=None</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#quit" title="Permalink to this definition">¶</a></dt>
<dt id="exit">
<code class="descname">exit</code><span class="sig-paren">(</span><span class="optional">[</span><em>code=None</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#exit" title="Permalink to this definition">¶</a></dt>
<dd><p>Objects that when printed, print a message like “Use quit() or Ctrl-D
(i.e. EOF) to exit”, and when called, raise <a class="reference internal" href="exceptions.html#exceptions.SystemExit" title="exceptions.SystemExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SystemExit</span></code></a> with the
specified exit code.</p>
</dd></dl>
<dl class="data">
<dt id="copyright">
<code class="descname">copyright</code><a class="headerlink" href="#copyright" title="Permalink to this definition">¶</a></dt>
<dt id="credits">
<code class="descname">credits</code><a class="headerlink" href="#credits" title="Permalink to this definition">¶</a></dt>
<dd><p>Objects that when printed or called, print the text of copyright or
credits, respectively.</p>
</dd></dl>
<dl class="data">
<dt id="license">
<code class="descname">license</code><a class="headerlink" href="#license" title="Permalink to this definition">¶</a></dt>
<dd><p>Object that when printed, prints the message “Type license() to see the
full license text”, and when called, displays the full license text in a
pager-like fashion (one screen at a time).</p>
</dd></dl>
</div>
</div>
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<h3><a href="../contents.html">Table of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">4. Built-in Constants</a><ul>
<li><a class="reference internal" href="#constants-added-by-the-site-module">4.1. Constants added by the <code class="xref py py-mod docutils literal notranslate"><span class="pre">site</span></code> module</a></li>
</ul>
</li>
</ul>
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