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<section id="built-in-constants">
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<span id="built-in-consts"></span><h1>内置常量<a class="headerlink" href="#built-in-constants" title="永久链接至标题">¶</a></h1>
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<p>有少数的常量存在于内置命名空间中。 它们是：</p>
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<dl class="data">
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<dt id="False">
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<code class="sig-name descname">False</code><a class="headerlink" href="#False" title="永久链接至目标">¶</a></dt>
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<dd><p><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> 类型的假值。 给 <code class="docutils literal notranslate"><span class="pre">False</span></code> 赋值是非法的并会引发 <a class="reference internal" href="exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a>。</p>
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</dd></dl>
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<dl class="data">
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<dt id="True">
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<code class="sig-name descname">True</code><a class="headerlink" href="#True" title="永久链接至目标">¶</a></dt>
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<dd><p><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> 类型的真值。 给 <code class="docutils literal notranslate"><span class="pre">True</span></code> 赋值是非法的并会引发 <a class="reference internal" href="exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a>。</p>
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</dd></dl>
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<dl class="data">
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<dt id="None">
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<code class="sig-name descname">None</code><a class="headerlink" href="#None" title="永久链接至目标">¶</a></dt>
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<dd><p><code class="docutils literal notranslate"><span class="pre">NoneType</span></code> 类型的唯一值。 <code class="docutils literal notranslate"><span class="pre">None</span></code> 经常用于表示缺少值，当因为默认参数未传递给函数时。 给 <code class="docutils literal notranslate"><span class="pre">None</span></code> 赋值是非法的并会引发 <a class="reference internal" href="exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a>。</p>
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<dt id="NotImplemented">
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<code class="sig-name descname">NotImplemented</code><a class="headerlink" href="#NotImplemented" title="永久链接至目标">¶</a></dt>
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<dd><p>二进制特殊方法应返回的特殊值（例如，<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>、<code class="xref py py-meth docutils literal notranslate"><span class="pre">__add</span> <span class="pre">__()</span></code>、<a class="reference internal" href="../reference/datamodel.html#object.__rsub__" title="object.__rsub__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__rsub__()</span></code></a> 等）表示操作没有针对其他类型实现；为了相同的目的，可以通过就地二进制特殊方法（例如，<code class="xref py py-meth docutils literal notranslate"><span class="pre">__imul</span> <span class="pre">__()</span></code>、<code class="xref py py-meth docutils literal notranslate"><span class="pre">__</span> <span class="pre">rightnd__()</span></code> 等）返回。 它的逻辑值为真。</p>
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<div class="admonition note">
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<p class="admonition-title">注解</p>
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<p>当二进制（或就地）方法返回``NotImplemented``时，解释器将尝试对另一种类型（或其他一些回滚操作，取决于运算符）的反射操作。 如果所有尝试都返回``NotImplemented``，则解释器将引发适当的异常。 错误返回的``NotImplemented``将导致误导性错误消息或返回到Python代码中的``NotImplemented``值。</p>
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<p>参见 <a class="reference internal" href="numbers.html#implementing-the-arithmetic-operations"><span class="std std-ref">实现算术运算</span></a> 为例。</p>
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</div>
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<div class="admonition note">
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<p class="admonition-title">注解</p>
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<p><code class="docutils literal notranslate"><span class="pre">NotImplementedError</span></code> 和 <code class="docutils literal notranslate"><span class="pre">NotImplemented</span></code> 不可互换，即使它们有相似的名称和用途。 有关何时使用它的详细信息，请参阅 <a class="reference internal" href="exceptions.html#NotImplementedError" title="NotImplementedError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">NotImplementedError</span></code></a>。</p>
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</div>
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</dd></dl>
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<span class="target" id="index-0"></span><dl class="data">
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<dt id="Ellipsis">
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<code class="sig-name descname">Ellipsis</code><a class="headerlink" href="#Ellipsis" title="永久链接至目标">¶</a></dt>
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<dd><p>与省略号文字字面 “<code class="docutils literal notranslate"><span class="pre">...</span></code>” 相同。 特殊值主要与用户定义的容器数据类型的扩展切片语法结合使用。</p>
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</dd></dl>
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<dl class="data">
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<dt id="__debug__">
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<code class="sig-name descname">__debug__</code><a class="headerlink" href="#__debug__" title="永久链接至目标">¶</a></dt>
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<dd><p>如果 Python 没有以 <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> 选项启动，则此常量为真值。 另请参见 <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> 语句。</p>
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</dd></dl>
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<div class="admonition note">
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<p class="admonition-title">注解</p>
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<p>变量名 <a class="reference internal" href="#None" title="None"><code class="xref py py-data docutils literal notranslate"><span class="pre">None</span></code></a>，<a class="reference internal" href="#False" title="False"><code class="xref py py-data docutils literal notranslate"><span class="pre">False</span></code></a>，<a class="reference internal" href="#True" title="True"><code class="xref py py-data docutils literal notranslate"><span class="pre">True</span></code></a> 和 <code class="xref py py-data docutils literal notranslate"><span class="pre">__</span> <span class="pre">debug__</span></code> 无法重新赋值（赋值给它们，即使是属性名，将引发 <a class="reference internal" href="exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SyntaxError</span></code></a> ），所以它们可以被认为是“真正的”常数。</p>
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</div>
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<section id="constants-added-by-the-site-module">
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<h2>由 <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> 模块添加的常量<a class="headerlink" href="#constants-added-by-the-site-module" title="永久链接至标题">¶</a></h2>
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<p><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> 模块（在启动期间自动导入，除非给出 <a class="reference internal" href="../using/cmdline.html#id3"><code class="xref std std-option docutils literal notranslate"><span class="pre">-S</span></code></a> 命令行选项）将几个常量添加到内置命名空间。 它们对交互式解释器 shell 很有用，并且不应在程序中使用。</p>
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<dl class="data">
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<dt id="quit">
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<code class="sig-name descname">quit</code><span class="sig-paren">(</span><em class="sig-param">code=None</em><span class="sig-paren">)</span><a class="headerlink" href="#quit" title="永久链接至目标">¶</a></dt>
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<dt id="exit">
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<code class="sig-name descname">exit</code><span class="sig-paren">(</span><em class="sig-param">code=None</em><span class="sig-paren">)</span><a class="headerlink" href="#exit" title="永久链接至目标">¶</a></dt>
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<dd><p>当打印此对象时，会打印出一条消息，例如“Use quit() or Ctrl-D (i.e. EOF) to exit”，当调用此对象时，将使用指定的退出代码来引发 <a class="reference internal" href="exceptions.html#SystemExit" title="SystemExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SystemExit</span></code></a>。</p>
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</dd></dl>
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<dl class="data">
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<dt id="copyright">
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<code class="sig-name descname">copyright</code><a class="headerlink" href="#copyright" title="永久链接至目标">¶</a></dt>
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<dt id="credits">
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<code class="sig-name descname">credits</code><a class="headerlink" href="#credits" title="永久链接至目标">¶</a></dt>
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<dd><p>打印或调用的对象分别打印版权或作者的文本。</p>
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</dd></dl>
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<dl class="data">
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<dt id="license">
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<code class="sig-name descname">license</code><a class="headerlink" href="#license" title="永久链接至目标">¶</a></dt>
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<dd><p>当打印此对象时，会打印出一条消息“Type license() to see the full license text”，当调用此对象时，将以分页形式显示完整的许可证文本（每次显示一屏）。</p>
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</dd></dl>
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</section>
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<h3><a href="../contents.html">目录</a></h3>
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<li><a class="reference internal" href="#">内置常量</a><ul>
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<li><a class="reference internal" href="#constants-added-by-the-site-module">由 <code class="xref py py-mod docutils literal notranslate"><span class="pre">site</span></code> 模块添加的常量</a></li>
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