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<div class="section" id="条件语">
<h1>条件语<a class="headerlink" href="#条件语" title="永久链接至标题">¶</a></h1>
<div class="admonition warning">
<p class="admonition-title fa fa-exclamation-circle"><strong>注</strong>：</p>
<p>在本资源各处会有一些阅读理解练习。这些题目旨在帮助读者活学活用文中的知识点。练习题的答案可以在本页底部找到。</p>
</div>
<p>在本节中，我们将会介绍 <code class="docutils literal notranslate"><span class="pre">if</span></code>，<code class="docutils literal notranslate"><span class="pre">else</span></code>，和 <code class="docutils literal notranslate"><span class="pre">elif</span></code> 语句。它们允许你编写仅仅在某些条件满足时才会执行的代码片段，或者在条件不满足时执行某些其它代码。比如说，以下代码在 <code class="docutils literal notranslate"><span class="pre">x</span></code> 是负数的时候返回 <code class="docutils literal notranslate"><span class="pre">x</span></code> 的平方，在 <code class="docutils literal notranslate"><span class="pre">x</span></code> 是正数时返回 <code class="docutils literal notranslate"><span class="pre">x</span></code> 的立方：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 简单的if-else区块</span>
<span class="k">if</span> <span class="n">x</span> <span class="o"><</span> <span class="mi">0</span><span class="p">:</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">x</span> <span class="o">**</span> <span class="mi">2</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">x</span> <span class="o">**</span> <span class="mi">3</span>
</pre></div>
</div>
<p>请通过“基本对象类型“一节来回忆布尔类型（代表真或否值）的一些基础特征。我们将会延续该节的讨论来介绍比较运算和成员检查，并讨论内置 <code class="docutils literal notranslate"><span class="pre">bool</span></code> 类的更多有用之处。</p>
<div class="section" id="比较运算">
<h2>比较运算<a class="headerlink" href="#比较运算" title="永久链接至标题">¶</a></h2>
<p>比较语句（comparison statement）最后一定会返回某个布尔对象：<code class="docutils literal notranslate"><span class="pre">True</span></code> 或 <code class="docutils literal notranslate"><span class="pre">False</span></code>。在Python中总共有八种比较运算：</p>
<table class="docutils align-default">
<colgroup>
<col style="width: 43%" />
<col style="width: 57%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>运算符</p></th>
<th class="head"><p>含义</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre"><</span></code></p></td>
<td><p>小于（不等于）</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre"><=</span></code></p></td>
<td><p>小于或等于</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">></span></code></p></td>
<td><p>大于（不等于）</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">>=</span></code></p></td>
<td><p>大于或等于</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">==</span></code></p></td>
<td><p>等于</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">!=</span></code></p></td>
<td><p>不等于</p></td>
</tr>
<tr class="row-even"><td><p><code class="docutils literal notranslate"><span class="pre">is</span></code></p></td>
<td><p>相同对象</p></td>
</tr>
<tr class="row-odd"><td><p><code class="docutils literal notranslate"><span class="pre">is</span> <span class="pre">not</span></code></p></td>
<td><p>不相同对象</p></td>
</tr>
</tbody>
</table>
<p>其中前六个运算符和数学中常见的相对应：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="mi">2</span> <span class="o"><</span> <span class="mi">3</span>
<span class="go">True</span>
</pre></div>
</div>
<p>注意 <code class="docutils literal notranslate"><span class="pre">=</span></code> 和 <code class="docutils literal notranslate"><span class="pre">==</span></code> 有着两个很不一样的含义。前者是赋值运算符，而后者是等值运算符：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="mi">3</span> <span class="c1"># 对变量 `x` 赋值 3</span>
<span class="gp">>>> </span><span class="n">x</span> <span class="o">==</span> <span class="mi">3</span> <span class="c1"># 检查 `x` 和 3 是否有着同样的值</span>
<span class="go">True</span>
</pre></div>
</div>
<p>Python允许你连接多个对比运算符来创建“复式”（compound）对比：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="mi">2</span> <span class="o"><</span> <span class="mi">3</span> <span class="o"><</span> <span class="mi">1</span> <span class="c1"># 等值于 (2 < 3) and (3 < 1)</span>
<span class="go">False</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">==</span></code> 运算符检查两个对象是否有着同样的值，而 <code class="docutils literal notranslate"><span class="pre">is</span></code> 运算符检查两个对象是否是<em>相同的</em>对象。比如说，创建两个有着一样内容的列表会得到两个<em>不同</em>的列表，但它们的“值”一样：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 演示 `==` 对比 `is`</span>
<span class="o">>>></span> <span class="n">x</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span>
<span class="o">>>></span> <span class="n">y</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span>
<span class="o">>>></span> <span class="n">x</span> <span class="o">==</span> <span class="n">y</span>
<span class="kc">True</span>
<span class="c1"># `x` 和 `y` 引用两个等值但不同的列表</span>
<span class="o">>>></span> <span class="n">x</span> <span class="ow">is</span> <span class="n">y</span>
<span class="kc">False</span>
</pre></div>
</div>
<p>因此 <code class="docutils literal notranslate"><span class="pre">is</span></code> 运算符最常用来检查一个变量是否引用 <code class="docutils literal notranslate"><span class="pre">None</span></code> 对象，或两个布尔对象中的一个：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="kc">None</span>
<span class="gp">>>> </span><span class="n">x</span> <span class="ow">is</span> <span class="kc">None</span>
<span class="go">True</span>
<span class="go"># (2 < 0) 返回对象 `False`</span>
<span class="go"># 因此这会成为：`False is False`</span>
<span class="gp">>>> </span><span class="p">(</span><span class="mi">2</span> <span class="o"><</span> <span class="mi">0</span><span class="p">)</span> <span class="ow">is</span> <span class="kc">False</span>
<span class="go">True</span>
</pre></div>
</div>
<p>使用 <code class="docutils literal notranslate"><span class="pre">is</span> <span class="pre">not</span></code> 来检查两个对象是否不是同一个对象：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="mi">1</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span>
<span class="go">True</span>
</pre></div>
</div>
</div>
<div class="section" id="bool-和非布尔类对象的真值">
<h2><code class="docutils literal notranslate"><span class="pre">bool</span></code> 和非布尔类对象的真值<a class="headerlink" href="#bool-和非布尔类对象的真值" title="永久链接至标题">¶</a></h2>
<p>请回忆两个布尔对象 <code class="docutils literal notranslate"><span class="pre">True</span></code> 和 <code class="docutils literal notranslate"><span class="pre">False</span></code> 严格来讲即属于 <code class="docutils literal notranslate"><span class="pre">bool</span></code> 也属于 <code class="docutils literal notranslate"><span class="pre">int</span></code> 类型，并分别有着值 <code class="docutils literal notranslate"><span class="pre">1</span></code> 和 <code class="docutils literal notranslate"><span class="pre">0</span></code> 这些知识点：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">isinstance</span><span class="p">(</span><span class="kc">True</span><span class="p">,</span> <span class="nb">int</span><span class="p">)</span>
<span class="go">True</span>
<span class="gp">>>> </span><span class="nb">int</span><span class="p">(</span><span class="kc">True</span><span class="p">)</span>
<span class="go">1</span>
<span class="gp">>>> </span><span class="nb">isinstance</span><span class="p">(</span><span class="kc">False</span><span class="p">,</span> <span class="nb">int</span><span class="p">)</span>
<span class="go">True</span>
<span class="gp">>>> </span><span class="nb">int</span><span class="p">(</span><span class="kc">False</span><span class="p">)</span>
<span class="go">0</span>
<span class="gp">>>> </span><span class="mi">3</span><span class="o">*</span><span class="kc">True</span> <span class="o">-</span> <span class="kc">False</span>
<span class="go">3</span>
<span class="gp">>>> </span><span class="kc">True</span> <span class="o">/</span> <span class="kc">False</span>
<span class="go">---------------------------------------------------------------------------</span>
<span class="go">ZeroDivisionError Traceback (most recent call last)</span>
<span class="go"><ipython-input-4-f8487d9d0863> in <module>()</span>
<span class="go">----> 1 True / False</span>
<span class="go">ZeroDivisionError: division by zero</span>
</pre></div>
</div>
<p>类似的，Python也会对非布尔对象提供相应的布尔值。比如说，数字0对应着 <code class="docutils literal notranslate"><span class="pre">False</span></code>，而任何非0的数字则对应 <code class="docutils literal notranslate"><span class="pre">True</span></code>。内置对象的布尔值可以通过Python内置的命令 <code class="docutils literal notranslate"><span class="pre">bool</span></code> 来得出：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 使用 `bool` 来查看非布尔类对象</span>
<span class="c1"># 的布尔值为 `True` 或 `False`</span>
<span class="o">>>></span> <span class="nb">bool</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="kc">False</span>
</pre></div>
</div>
<p>任何非0的Python整数对应 <code class="docutils literal notranslate"><span class="pre">True</span></code>：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 非0值最后计算为 `True`</span>
<span class="o">>>></span> <span class="nb">bool</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="kc">True</span>
</pre></div>
</div>
<p>以下的内置Python对象通过 <code class="docutils literal notranslate"><span class="pre">bool</span></code> 对应 <code class="docutils literal notranslate"><span class="pre">False</span></code>：</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">False</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">None</span></code></p></li>
<li><p>任何数字类的0:<code class="docutils literal notranslate"><span class="pre">0</span></code>, <code class="docutils literal notranslate"><span class="pre">0.0</span></code>, <code class="docutils literal notranslate"><span class="pre">0j</span></code></p></li>
<li><p>任何空序列，如空字符串或列表：<code class="docutils literal notranslate"><span class="pre">''</span></code>，<code class="docutils literal notranslate"><span class="pre">tuple()</span></code>，<code class="docutils literal notranslate"><span class="pre">[]</span></code>，<code class="docutils literal notranslate"><span class="pre">numpy.array([])</span></code></p></li>
<li><p>空词典（dictionary）或集（set）</p></li>
</ul>
<p>因此任何非0的数字或非空的序列或集合都会通过 <code class="docutils literal notranslate"><span class="pre">bool</span></code> 对应 <code class="docutils literal notranslate"><span class="pre">True</span></code>。</p>
<div class="admonition note">
<p class="admonition-title fa fa-exclamation-circle"><strong>经验</strong>：</p>
<p><code class="docutils literal notranslate"><span class="pre">bool</span></code> 函数允许你得到非布尔类对象对应的布尔值。比如说，<code class="docutils literal notranslate"><span class="pre">bool([])</span></code> 返回 <code class="docutils literal notranslate"><span class="pre">False</span></code>，而 <code class="docutils literal notranslate"><span class="pre">bool([1,</span> <span class="pre">2])</span></code> 返回 <code class="docutils literal notranslate"><span class="pre">True</span></code>。</p>
</div>
</div>
<div class="section" id="if，else，和-elif">
<h2><code class="docutils literal notranslate"><span class="pre">if</span></code>，<code class="docutils literal notranslate"><span class="pre">else</span></code>，和 <code class="docutils literal notranslate"><span class="pre">elif</span></code><a class="headerlink" href="#if，else，和-elif" title="永久链接至标题">¶</a></h2>
<p>我们现在将介绍简单而强大的 <code class="docutils literal notranslate"><span class="pre">if</span></code>，<code class="docutils literal notranslate"><span class="pre">else</span></code>，和 <code class="docutils literal notranslate"><span class="pre">elif</span></code> 条件语句（conditional statement）。这将允许我们在代码中创建简单的分枝。比如说，假设你在为某电子游戏编写代码，并想要根据你角色的血量（整数）来更新ta的状态，下面的代码就是一个例子：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">num_health</span> <span class="o">></span> <span class="mi">80</span><span class="p">:</span>
<span class="n">status</span> <span class="o">=</span> <span class="s2">"good"</span>
<span class="k">elif</span> <span class="n">num_health</span> <span class="o">></span> <span class="mi">50</span><span class="p">:</span>
<span class="n">status</span> <span class="o">=</span> <span class="s2">"okay"</span>
<span class="k">elif</span> <span class="n">num_health</span> <span class="o">></span> <span class="mi">0</span><span class="p">:</span>
<span class="n">status</span> <span class="o">=</span> <span class="s2">"danger"</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">status</span> <span class="o">=</span> <span class="s2">"dead"</span>
</pre></div>
</div>
<p>每个 <code class="docutils literal notranslate"><span class="pre">if</span></code>，<code class="docutils literal notranslate"><span class="pre">elif</span></code>，和 <code class="docutils literal notranslate"><span class="pre">else</span></code> 语句必须由冒号结尾，且其中所有语句都必须由<a class="reference external" href="https://cn.pythonlikeyoumeanit.com/Module2_EssentialsOfPython/Introduction.html#Python使用空格来限定区域">空格限定</a>。</p>
<p>以下的伪代码演示条件语句的通用模版：</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="o"><</span><span class="n">expression_1</span><span class="o">></span><span class="p">:</span>
<span class="n">本缩进区域的代码只有在以下条件满足时才会被执行</span><span class="o">...</span>
<span class="o">-</span> <span class="nb">bool</span><span class="p">(</span><span class="o"><</span><span class="n">expression_1</span><span class="o">></span><span class="p">)</span> <span class="n">为</span> <span class="kc">True</span>
<span class="k">elif</span> <span class="o"><</span><span class="n">expression_2</span><span class="o">></span><span class="p">:</span>
<span class="n">本缩进区域的代码只有在以下条件满足时才会被执行</span><span class="o">...</span>
<span class="o">-</span> <span class="nb">bool</span><span class="p">(</span><span class="o"><</span><span class="n">expression_1</span><span class="o">></span><span class="p">)</span> <span class="n">之前为</span> <span class="kc">False</span>
<span class="o">-</span> <span class="nb">bool</span><span class="p">(</span><span class="o"><</span><span class="n">expression_2</span><span class="o">></span><span class="p">)</span> <span class="n">为</span> <span class="kc">True</span>
<span class="o">...</span>
<span class="o">...</span>
<span class="k">elif</span> <span class="o"><</span><span class="n">expression_n</span><span class="o">></span><span class="p">:</span>
<span class="n">本缩进区域的代码只有在以下条件满足时才会被执行</span><span class="o">...</span>
<span class="o">-</span> <span class="nb">bool</span><span class="p">(</span><span class="o"><</span><span class="n">expression_1</span><span class="o">></span><span class="p">)</span> <span class="n">之前为</span> <span class="kc">False</span>
<span class="o">-</span> <span class="nb">bool</span><span class="p">(</span><span class="o"><</span><span class="n">expression_2</span><span class="o">></span><span class="p">)</span> <span class="n">之前为</span> <span class="kc">False</span>
<span class="o">...</span>
<span class="o">...</span>
<span class="o">-</span> <span class="nb">bool</span><span class="p">(</span><span class="o"><</span><span class="n">expression_n</span><span class="o">-</span><span class="mi">1</span><span class="o">></span><span class="p">)</span> <span class="n">之前为</span> <span class="kc">False</span>
<span class="o">-</span> <span class="nb">bool</span><span class="p">(</span><span class="o"><</span><span class="n">expression_n</span><span class="o">></span><span class="p">)</span> <span class="n">为</span> <span class="kc">True</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">本缩进区域的代码只有在</span>
<span class="n">之前所有表达式为</span> <span class="kc">False</span> <span class="n">时</span>
<span class="n">才会被执行</span><span class="o">...</span>
</pre></div>
</div>
<p>实际运用时它可以看起来是这样：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">x</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">]</span>
<span class="k">if</span> <span class="mi">3</span> <span class="o"><</span> <span class="nb">len</span><span class="p">(</span><span class="n">x</span><span class="p">):</span>
<span class="c1"># bool(3 < 2) 返回 False，所以此区块的代码将被跳过</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"`x` has more than three items in it"</span><span class="p">)</span>
<span class="k">elif</span> <span class="nb">len</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o">==</span> <span class="mi">2</span>
<span class="c1"># bool(len(x) == 2) 返回 True，此区块的代码被执行</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"`x` has two items in it"</span><span class="p">)</span>
<span class="k">elif</span> <span class="nb">len</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o">==</span> <span class="mi">1</span>
<span class="c1"># 此语句根本没有被检查</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"`x` has one items in it"</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="c1"># 此语句根本没有被检查</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"`x` is an empty list"</span><span class="p">)</span>
<span class="s2">"`x` has two items in it"</span>
</pre></div>
</div>
<p>在最简洁的格式下，一个条件语句只需要一个 <code class="docutils literal notranslate"><span class="pre">if</span></code> 句段。<code class="docutils literal notranslate"><span class="pre">else</span></code> 和 <code class="docutils literal notranslate"><span class="pre">elif</span></code> 句段只能在一个 <code class="docutils literal notranslate"><span class="pre">if</span></code> 句段之后。</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 一个只有“if”句段的条件语句</span>
<span class="n">x</span> <span class="o">=</span> <span class="o">-</span><span class="mi">1</span>
<span class="k">if</span> <span class="n">x</span> <span class="o"><</span> <span class="mi">0</span><span class="p">:</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">x</span> <span class="o">**</span> <span class="mi">2</span>
<span class="c1"># `x` 现在为 1</span>
</pre></div>
</div>
<p>类似的，条件语句可以有一个 <code class="docutils literal notranslate"><span class="pre">if</span></code> 和一个 <code class="docutils literal notranslate"><span class="pre">else</span></code> 而没有任何 <code class="docutils literal notranslate"><span class="pre">elif</span></code>：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 一个有一个“if”句段和一个“else”句段</span>
<span class="c1"># 的条件语句</span>
<span class="n">x</span> <span class="o">=</span> <span class="mi">4</span>
<span class="k">if</span> <span class="n">x</span> <span class="o">></span> <span class="mi">2</span><span class="p">:</span>
<span class="n">x</span> <span class="o">=</span> <span class="o">-</span><span class="mi">2</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">x</span> <span class="o">+</span> <span class="mi">1</span>
<span class="c1"># `x` 现在为 -2</span>
</pre></div>
</div>
<p>条件语句也可以有一个 <code class="docutils literal notranslate"><span class="pre">if</span></code> 和一个 <code class="docutils literal notranslate"><span class="pre">elif</span></code> 而没有 <code class="docutils literal notranslate"><span class="pre">else</span></code>：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 一个有一个“if”句段和一个“elif”句段</span>
<span class="c1"># 的条件语句</span>
<span class="n">x</span> <span class="o">=</span> <span class="s1">'abc'</span>
<span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o"><</span> <span class="mi">9</span><span class="p">:</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">x</span> <span class="o">*</span> <span class="mi">3</span>
<span class="k">elif</span> <span class="nb">len</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="o">></span> <span class="mi">40</span><span class="p">:</span>
<span class="n">x</span> <span class="o">=</span> <span class="s1">'cba'</span>
<span class="c1"># `x` 现在为 'abcabcabc'</span>
</pre></div>
</div>
<p>请注意在一个if-elif-else语句中只有其中一段可以被执行；要么“if”区块的代码被执行，要么一个“elif”区块的代码被执行，要么“else”区块的代码被执行。但是，背靠背的if语句是完全独立于对方的。因此它们的代码区块可能在单次运行中都被执行（假设它们对应的条件语句都最后为 <code class="docutils literal notranslate"><span class="pre">True</span></code>）。</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 连续的if语句是互相独立的</span>
<span class="n">x</span> <span class="o">=</span> <span class="mi">5</span>
<span class="n">y</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">if</span> <span class="n">x</span> <span class="o"><</span> <span class="mi">10</span><span class="p">:</span>
<span class="n">y</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="k">if</span> <span class="n">x</span> <span class="o"><</span> <span class="mi">20</span><span class="p">:</span>
<span class="n">y</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="c1"># y 现在为 2</span>
</pre></div>
</div>
<div class="admonition note">
<p class="admonition-title fa fa-exclamation-circle"><strong>阅读理解：条件语句</strong></p>
<ol class="arabic simple">
<li><p>设 <code class="docutils literal notranslate"><span class="pre">my_list</span></code> 为一个列表。根据以下代码：</p></li>
</ol>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">first_item</span> <span class="o">=</span> <span class="kc">None</span>
<span class="k">if</span> <span class="n">my_list</span><span class="p">:</span>
<span class="n">first_item</span> <span class="o">=</span> <span class="n">my_list</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
</pre></div>
</div>
<p>如果 <code class="docutils literal notranslate"><span class="pre">my_list</span></code> 为 <code class="docutils literal notranslate"><span class="pre">[]</span></code>，会发生什么？这是否会产生一个 <code class="docutils literal notranslate"><span class="pre">IndexError</span></code>？<code class="docutils literal notranslate"><span class="pre">first_item</span></code> 会是什么？</p>
<ol class="arabic simple" start="2">
<li><p>设变量 <code class="docutils literal notranslate"><span class="pre">my_file</span></code> 为一个存储着文件名的字符串，其中英文句号 <code class="docutils literal notranslate"><span class="pre">.</span></code> 代表着文件名的结尾和文件类型的开始。编写代码来获取 <code class="docutils literal notranslate"><span class="pre">my_file</span></code> 的文件名。</p></li>
</ol>
<p><code class="docutils literal notranslate"><span class="pre">my_file</span></code> 中最多只会有一个句号。妥善处理 <code class="docutils literal notranslate"><span class="pre">my_file</span></code> <em>没有</em>文件类型的情况。</p>
<p>也就是说：</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">"code.py"</span></code> <span class="math notranslate nohighlight">\(\rightarrow\)</span> <code class="docutils literal notranslate"><span class="pre">"code"</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">"doc2.pdf"</span></code> <span class="math notranslate nohighlight">\(\rightarrow\)</span> <code class="docutils literal notranslate"><span class="pre">"doc2"</span></code></p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">"hello_world"</span></code> <span class="math notranslate nohighlight">\(\rightarrow\)</span> <code class="docutils literal notranslate"><span class="pre">"hello_world"</span></code></p></li>
</ul>
</div>
<div class="section" id="单行的if-else语句">
<h3>单行的if-else语句<a class="headerlink" href="#单行的if-else语句" title="永久链接至标题">¶</a></h3>
<p>Python支持一个单行版本的if-else语句，其功能比较有限。以下代码：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">num</span> <span class="o">>=</span> <span class="mi">0</span><span class="p">:</span>
<span class="n">sign</span> <span class="o">=</span> <span class="s2">"positive"</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">sign</span> <span class="o">=</span> <span class="s2">"negative"</span>
</pre></div>
</div>
<p>可以被简写成：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">sign</span> <span class="o">=</span> <span class="s2">"positive"</span> <span class="k">if</span> <span class="n">num</span> <span class="o">>=</span><span class="mi">0</span> <span class="k">else</span> <span class="s2">"negative"</span>
</pre></div>
</div>
<p>这代表了单行（inline）if-else语句的内在语法：</p>
<div class="admonition note">
<p class="admonition-title fa fa-exclamation-circle"><strong>单行if-else语句</strong>：</p>
<p>表达式 <code class="docutils literal notranslate"><span class="pre">A</span> <span class="pre">if</span> <span class="pre"><condition></span> <span class="pre">else</span> <span class="pre">B</span></code> 当 <code class="docutils literal notranslate"><span class="pre">bool(<condition>)</span></code> 为 <code class="docutils literal notranslate"><span class="pre">True</span></code> 时返回 <code class="docutils literal notranslate"><span class="pre">A</span></code>，不然该表达式会返回 <code class="docutils literal notranslate"><span class="pre">B</span></code>。</p>
</div>
<p>和完整的“if-elif-else”表达式对比，这个语法是极其受限制的——它不允许“elif”语句，也不允许if/else之间有多行的代码块。</p>
<p>单行if-else语句可以在任何地方使用，不仅仅是在赋值语句的右侧。这有时很方便：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 在不同情况下使用单行if-else语句</span>
<span class="o">>>></span> <span class="n">x</span> <span class="o">=</span> <span class="mi">2</span>
<span class="c1"># 如 `x` 并不是负数，这将储存 1</span>
<span class="c1"># 如 `x` 为负数，这将储存 0</span>
<span class="o">>>></span> <span class="n">my_list</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span> <span class="k">if</span> <span class="n">x</span> <span class="o">>=</span> <span class="mi">0</span> <span class="k">else</span> <span class="mi">0</span><span class="p">]</span>
<span class="o">>>></span> <span class="n">my_list</span>
<span class="p">[</span><span class="mi">1</span><span class="p">]</span>
<span class="o">>>></span> <span class="s2">"a"</span> <span class="k">if</span> <span class="n">x</span> <span class="o">==</span> <span class="mi">1</span> <span class="k">else</span> <span class="s2">"b"</span>
<span class="s1">'b'</span>
</pre></div>
</div>
<p>我们将在讨论列表理解语句（list comprehension statement）时注意到这个语法很有用的地方，但请注意谨慎使用这一语法。比如说，不要在数学表达式中使用单行if-else语句，因为最后的代码令人抓狂：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 永远不要这么做，永远！</span>
<span class="mi">2</span> <span class="o">-</span> <span class="mi">3</span> <span class="k">if</span> <span class="n">x</span> <span class="o"><</span> <span class="mi">1</span> <span class="k">else</span> <span class="mi">1</span> <span class="o">+</span> <span class="mi">6</span><span class="o">*</span><span class="mi">2</span> <span class="k">if</span> <span class="n">x</span> <span class="o">>=</span> <span class="mi">0</span> <span class="k">else</span> <span class="mi">9</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="短路逻辑表达式">
<h2>短路逻辑表达式<a class="headerlink" href="#短路逻辑表达式" title="永久链接至标题">¶</a></h2>
<p>在对条件语句有了更深刻的理解之后，我们将扩展我们对Python逻辑表达式的研究来讨论短路（short-circuiting）。在Python中，一个逻辑表达式会从左到右运算，并在表达式结果确定的那一步就返回，且<em>不会运算表达式剩下的部分</em>。也就是说，表达式可以<em>短路</em>。</p>
<p>比如说，<code class="docutils literal notranslate"><span class="pre">and</span></code> 运算仅仅会在它两边的表达式都为 <code class="docutils literal notranslate"><span class="pre">True</span></code> 时才会返回 <code class="docutils literal notranslate"><span class="pre">True</span></code>。因此，表达式 <code class="docutils literal notranslate"><span class="pre">False</span> <span class="pre">and</span> <span class="pre"><anything></span></code> 一定会返回 <code class="docutils literal notranslate"><span class="pre">False</span></code>；同时，这个表达式会返回 <code class="docutils literal notranslate"><span class="pre">False</span></code> 而<em>不运算</em> <code class="docutils literal notranslate"><span class="pre">bool(<anything>)</span></code>。</p>
<p>为了演示这一行为，请见下面的例子：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 演示短路逻辑表达式</span>
<span class="o">>>></span> <span class="kc">False</span> <span class="ow">and</span> <span class="mi">1</span><span class="o">/</span><span class="mi">0</span> <span class="c1"># 运算 `1/0` 会导致错误</span>
<span class="kc">False</span>
</pre></div>
</div>
<p>根据上文的讨论，<code class="docutils literal notranslate"><span class="pre">False</span> <span class="pre">and</span></code> 会短路这个表达式，让它不运算 <code class="docutils literal notranslate"><span class="pre">bool(1/0)</span></code> 就返回。换转参数顺序会明显地揭示这一点。</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 表达式是从左到右运算的</span>
<span class="o">>>></span> <span class="mi">1</span><span class="o">/</span><span class="mi">0</span> <span class="ow">and</span> <span class="kc">False</span>
<span class="o">---------------------------------------------------------------------------</span>
<span class="ne">ZeroDivisionError</span> <span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">)</span>
<span class="o"><</span><span class="n">ipython</span><span class="o">-</span><span class="nb">input</span><span class="o">-</span><span class="mi">1</span><span class="o">-</span><span class="mi">3471672109</span><span class="n">ee</span><span class="o">></span> <span class="ow">in</span> <span class="o"><</span><span class="n">module</span><span class="o">></span><span class="p">()</span>
<span class="o">----></span> <span class="mi">1</span> <span class="mi">1</span><span class="o">/</span><span class="mi">0</span> <span class="ow">and</span> <span class="kc">False</span>
<span class="ne">ZeroDivisionError</span><span class="p">:</span> <span class="n">division</span> <span class="n">by</span> <span class="n">zero</span>
</pre></div>
</div>
<p>在实际操作中，你可以通过短路来浓缩你的代码。假设我们代码的一节在处理变量 <code class="docutils literal notranslate"><span class="pre">x</span></code>，其可能是一个<a class="reference external" href="https://cn.pythonlikeyoumeanit.com/Module2_EssentialsOfPython/Basic_Objects.html#数字类型">数字</a>或<a class="reference external" href="https://cn.pythonlikeyoumeanit.com/Module2_EssentialsOfPython/Basic_Objects.html#字符串">字符串</a>。假设我们当 <code class="docutils literal notranslate"><span class="pre">x</span></code> 是全大写的字符串时想要有特殊的处理方式。以下代码</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 这在 `x` 不是字符串时会报错</span>
<span class="k">if</span> <span class="n">x</span><span class="o">.</span><span class="n">isupper</span><span class="p">():</span>
<span class="c1"># 对此大写字符串进行操作</span>
</pre></div>
</div>
<p>是有问题的，因为 <code class="docutils literal notranslate"><span class="pre">isupper</span></code> 只应该在我们确定 <code class="docutils literal notranslate"><span class="pre">x</span></code> 是字符串之后才调用；如果 <code class="docutils literal notranslate"><span class="pre">x</span></code> 是一个数字，我们的代码会报错。我们可以重写代码</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 一个可用但难看的过滤非字符串对象的方法</span>
<span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="nb">str</span><span class="p">):</span>
<span class="k">if</span> <span class="n">x</span><span class="o">.</span><span class="n">isupper</span><span class="p">():</span>
<span class="c1"># 对此大写字符串进行操作</span>
</pre></div>
</div>
<p>但一个更加优雅和简短的处理多层次检测的方法是短路逻辑表达式。</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 利用短路来简写必要的检查</span>
<span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="nb">str</span><span class="p">)</span> <span class="ow">and</span> <span class="n">x</span><span class="o">.</span><span class="n">isupper</span><span class="p">():</span>
<span class="c1"># 对此大写字符串进行操作</span>
</pre></div>
</div>
<p>请注意如果 <code class="docutils literal notranslate"><span class="pre">x</span></code> 不是字符串，那 <code class="docutils literal notranslate"><span class="pre">isinstance(x,</span> <span class="pre">str)</span></code> 会返回 <code class="docutils literal notranslate"><span class="pre">False</span></code>；因此 <code class="docutils literal notranslate"><span class="pre">isinstance(x,</span> <span class="pre">str)</span> <span class="pre">and</span> <span class="pre">x.isupper()</span></code> 会短路并直接返回 <code class="docutils literal notranslate"><span class="pre">False</span></code>，且不运算 <code class="docutils literal notranslate"><span class="pre">bool(x.isupper())</span></code>。这是这种检查任务的推荐结局方法。这代码对比等值的多层if语句更简短和可读。</p>
<div class="admonition note">
<p class="admonition-title fa fa-exclamation-circle"><strong>阅读理解：短路表达式</strong></p>
<p>回忆前文检查 <code class="docutils literal notranslate"><span class="pre">x</span></code> 是大写字符串的范例。以下代码的“bug“是什么？为什么它并没有正确地利用短路？</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 我做错了什么？</span>
<span class="k">if</span> <span class="n">x</span><span class="o">.</span><span class="n">isupper</span><span class="p">()</span> <span class="ow">and</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="nb">str</span><span class="p">):</span>
<span class="c1"># 对此大写字符串进行操作</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="官方说明文档连接">
<h2>官方说明文档连接<a class="headerlink" href="#官方说明文档连接" title="永久链接至标题">¶</a></h2>
<ul class="simple">
<li><p><a class="reference external" href="https://docs.python.org/3/library/functions.html#bool">布尔值</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/library/stdtypes.html#truth-value-testing">检查真值</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/library/stdtypes.html#boolean-operations-and-or-not">布尔操作符</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/library/stdtypes.html#comparisons">对比</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/reference/compound_stmts.html#the-if-statement">’if’语句</a></p></li>
</ul>
</div>
<div class="section" id="阅读理解答案">
<h2>阅读理解答案<a class="headerlink" href="#阅读理解答案" title="永久链接至标题">¶</a></h2>
<p><strong>条件语句：解</strong></p>
<ol class="arabic simple">
<li><p>如果 <code class="docutils literal notranslate"><span class="pre">my_list</span></code> 为 <code class="docutils literal notranslate"><span class="pre">[]</span></code>，那 <code class="docutils literal notranslate"><span class="pre">bool(my_list)</span></code> 会返回 <code class="docutils literal notranslate"><span class="pre">False</span></code>，而对应的代码块则会被跳过。因此 <code class="docutils literal notranslate"><span class="pre">first_item</span></code> 会是 <code class="docutils literal notranslate"><span class="pre">None</span></code>。</p></li>
<li><p>首先，检查 <code class="docutils literal notranslate"><span class="pre">my_file</span></code> 是否包含 <code class="docutils literal notranslate"><span class="pre">.</span></code>。如果是，找到它的索引并对字符串进行切片（切到该索引）。如果不是，<code class="docutils literal notranslate"><span class="pre">my_file</span></code> 就已经是文件名了。</p></li>
</ol>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">my_file</span> <span class="o">=</span> <span class="s2">"code.pdf"</span>
<span class="k">if</span> <span class="s2">"."</span> <span class="ow">in</span> <span class="n">my_file</span><span class="p">:</span>
<span class="n">dot_index</span> <span class="o">=</span> <span class="n">my_file</span><span class="o">.</span><span class="n">index</span><span class="p">(</span><span class="s2">"."</span><span class="p">)</span>
<span class="n">filename</span> <span class="o">=</span> <span class="n">my_file</span><span class="p">[:</span><span class="n">dot_index</span><span class="p">]</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">filename</span> <span class="o">=</span> <span class="n">my_file</span>
</pre></div>
</div>
<p><strong>短路表达式：解</strong></p>
<p>此代码</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 我做错了什么？</span>
<span class="k">if</span> <span class="n">x</span><span class="o">.</span><span class="n">isupper</span><span class="p">()</span> <span class="ow">and</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="nb">str</span><span class="p">):</span>
<span class="c1"># 对此大写字符串进行操作</span>
</pre></div>
</div>
<p>并没有考虑表达式从左到右运算的顺序。也就是说，在这个例子中 <code class="docutils literal notranslate"><span class="pre">bool(x.isupper())</span></code> 永远都会首先被运算，并且会在 <code class="docutils literal notranslate"><span class="pre">x</span></code> 不是字符串时报错。因此，之后的 <code class="docutils literal notranslate"><span class="pre">isinstance(x,</span> <span class="pre">str)</span></code> 语句完全没用。</p>
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