uTools-Manuals/docs/python/constants.html

<div class="body" role="main"><div class="section" id="built-in-constants"><h1><span class="yiyi-st" id="yiyi-9">3.</span><span class="yiyi-st" id="yiyi-10">内建常量 </span></h1><p><span class="yiyi-st" id="yiyi-11">内置的命名空间中存在少数几个常量。</span><span class="yiyi-st" id="yiyi-12">它们是：</span></p><dl class="data"><dt id="False"><span class="yiyi-st" id="yiyi-13"><code class="descname">False</code> </span></dt><dd><p><span class="yiyi-st" id="yiyi-14"><a class="reference internal" href="functions.html#bool" title="bool"><code class="xref py py-class docutils literal"><span class="pre">bool</span></code></a>类型的假值。</span><span class="yiyi-st" id="yiyi-15">对 <code class="docutils literal"><span class="pre">False</span></code> 的赋值是非法的将会导致 <a class="reference internal" href="exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal"><span class="pre">SyntaxError</span></code></a>。</span></p></dd></dl><dl class="data"><dt id="True"><span class="yiyi-st" id="yiyi-16"><code class="descname">True</code> </span></dt><dd><p><span class="yiyi-st" id="yiyi-17"><a class="reference internal" href="functions.html#bool" title="bool"><code class="xref py py-class docutils literal"><span class="pre">bool</span></code></a>类型的真值。</span><span class="yiyi-st" id="yiyi-18">对 <code class="docutils literal"><span class="pre">True</span></code> 的赋值是非法的将会导致<a class="reference internal" href="exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal"><span class="pre">SyntaxError</span></code></a>。</span></p></dd></dl><dl class="data"><dt id="None"><span class="yiyi-st" id="yiyi-19"><code class="descname">None</code> </span></dt><dd><p><span class="yiyi-st" id="yiyi-20"><code class="docutils literal"><span class="pre">NoneType</span></code> 类型的唯一值。</span><span class="yiyi-st" id="yiyi-21"><code class="docutils literal"><span class="pre">None</span></code> 常用来表示缺省值，当函数的参数并没有被传入时作为默认参数传入。</span><span class="yiyi-st" id="yiyi-22">对 <code class="docutils literal"><span class="pre">None</span></code> 赋值是非法的将会导致 <a class="reference internal" href="exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal"><span class="pre">SyntaxError</span></code></a>。</span></p></dd></dl><dl class="data"><dt id="NotImplemented"><span class="yiyi-st" id="yiyi-23"><code class="descname">NotImplemented</code></span></dt><dd><p><span class="yiyi-st" id="yiyi-24">应该由特殊的二元方法返回的特殊值（例如</span><span class="yiyi-st" id="yiyi-25"><a class="reference internal" href="../reference/datamodel.html#object.__eq__" title="object.__eq__"><code class="xref py py-meth docutils literal"><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"><span class="pre">__lt__()</span></code></a>, <a class="reference internal" href="../reference/datamodel.html#object.__add__" title="object.__add__"><code class="xref py py-meth docutils literal"><span class="pre">__add__()</span></code></a>, <a class="reference internal" href="../reference/datamodel.html#object.__rsub__" title="object.__rsub__"><code class="xref py py-meth docutils literal"><span class="pre">__rsub__()</span></code></a>等）</span><span class="yiyi-st" id="yiyi-26">以指示没有相对于另一类型实现所述操作；可以通过就地二进制特殊方法（例如，</span><span class="yiyi-st" id="yiyi-27"><a class="reference internal" href="../reference/datamodel.html#object.__imul__" title="object.__imul__"><code class="xref py py-meth docutils literal"><span class="pre">__imul__()</span></code></a>，<a class="reference internal" href="../reference/datamodel.html#object.__iand__" title="object.__iand__"><code class="xref py py-meth docutils literal"><span class="pre">__iand__()</span></code></a>等）</span><span class="yiyi-st" id="yiyi-28">为同一目的。</span><span class="yiyi-st" id="yiyi-29">它的真值是True。</span></p></dd></dl><div class="admonition note"><p class="first admonition-title"><span class="yiyi-st" id="yiyi-30">注意</span></p><p><span class="yiyi-st" id="yiyi-31">当返回<code class="docutils literal"><span class="pre">NotImplemented</span></code>时，解释器将根据运算符尝试其他类型的反射操作或其他一些返回。</span><span class="yiyi-st" id="yiyi-32">如果所有尝试的操作返回<code class="docutils literal"><span class="pre">NotImplemented</span></code>，解释器将引发一个适当的异常。</span></p><p class="last"><span class="yiyi-st" id="yiyi-33">有关详细信息，请参见<a class="reference internal" href="numbers.html#implementing-the-arithmetic-operations"><span>Implementing the arithmetic operations</span></a>。</span></p></div><dl class="data"><dt id="Ellipsis"><span class="yiyi-st" id="yiyi-34"><code class="descname">Ellipsis</code></span></dt><dd><p><span class="yiyi-st" id="yiyi-35">与<code class="docutils literal"><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。</span></p></dd></dl><dl class="data"><dt id="__debug__"><span class="yiyi-st" id="yiyi-36"><code class="descname">__debug__</code></span></dt><dd><p><span class="yiyi-st" id="yiyi-37">如果Python未使用<a class="reference internal" href="../using/cmdline.html#cmdoption-O"><code class="xref std std-option docutils literal"><span class="pre">-O</span></code></a>选项启动，则此常数为true。</span><span class="yiyi-st" id="yiyi-38">另请参见<a class="reference internal" href="../reference/simple_stmts.html#assert"><code class="xref std std-keyword docutils literal"><span class="pre">assert</span></code></a>语句。</span></p></dd></dl><div class="admonition note"><p class="first admonition-title"><span class="yiyi-st" id="yiyi-39">注意</span></p><p class="last"><span class="yiyi-st" id="yiyi-40">名称<a class="reference internal" href="#None" title="None"><code class="xref py py-data docutils literal"><span class="pre">None</span></code></a>、<a class="reference internal" href="#False" title="False"><code class="xref py py-data docutils literal"><span class="pre">False</span></code></a>、<a class="reference internal" href="#True" title="True"><code class="xref py py-data docutils literal"><span class="pre">True</span></code></a>和<a class="reference internal" href="#__debug__" title="__debug__"><code class="xref py py-data docutils literal"><span class="pre">__debug__</span></code></a>不能重新赋值（给它们赋值，即使作为属性的名称，也会引发<a class="reference internal" href="exceptions.html#SyntaxError" title="SyntaxError"><code class="xref py py-exc docutils literal"><span class="pre">SyntaxError</span></code></a>），因此它们可以被认为是“真正的”常量。</span></p></div><div class="section" id="constants-added-by-the-site-module"><h2><span class="yiyi-st" id="yiyi-41">3.1.</span><span class="yiyi-st" id="yiyi-42">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"><span class="pre">site</span></code></a>模块添加的常量</span></h2><p><span class="yiyi-st" id="yiyi-43"><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"><span class="pre">site</span></code></a>模块（在启动期间自动导入，除非给出了<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>命令行选项）向内置命名空间添加了几个常量。</span><span class="yiyi-st" id="yiyi-44">它们对交互解释器的shell非常有帮助以并且不应该在程序中使用.</span></p><dl class="data"><dt id="quit"><span class="yiyi-st" id="yiyi-45"><code class="descname">quit</code><span class="sig-paren">(</span><em>code=None</em><span class="sig-paren">)</span></span></dt><dt id="exit"><span class="yiyi-st" id="yiyi-46"><code class="descname">exit</code><span class="sig-paren">(</span><em>code=None</em><span class="sig-paren">)</span></span></dt><dd><p><span class="yiyi-st" id="yiyi-47">打印时打印对象，打印一条消息，如“退出()”或Ctrl-D（即“</span><span class="yiyi-st" id="yiyi-48">EOF）退出，并且在调用时，使用指定的退出代码提升<a class="reference internal" href="exceptions.html#SystemExit" title="SystemExit"><code class="xref py py-exc docutils literal"><span class="pre">SystemExit</span></code></a>。</span></p></dd></dl><dl class="data"><dt id="copyright"><span class="yiyi-st" id="yiyi-49"><code class="descname">copyright</code></span></dt><dt id="license"><span class="yiyi-st" id="yiyi-50"><code class="descname">license</code></span></dt><dt id="credits"><span class="yiyi-st" id="yiyi-51"><code class="descname">credits</code></span></dt><dd><p><span class="yiyi-st" id="yiyi-52">当打印这些对象时，打印一条类似“Type license() to see the full license text”的信息，当调用它们时，则以分页的方式（一次一个屏幕）显示相应的文本。</span></p></dd></dl></div></div></div>