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<div class="section" id="函数基础">
<h1>函数基础<a class="headerlink" href="#函数基础" 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>定义函数允许我们包装代码片段并描述进入和离开其中的信息。你可以在各种情况下重复使用这个“代码胶囊”。比如说，假设你想要数一个字符串中有多少元音，你可以定义以下的函数来达到该目的：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">count_vowels</span><span class="p">(</span><span class="n">in_string</span><span class="p">):</span>
<span class="sd">""" 返回 `in_string` 中有多少元音 """</span>
<span class="n">num_vowels</span> <span class="o">=</span> <span class="mi">0</span>
<span class="n">vowels</span> <span class="o">=</span> <span class="s2">"aeiouAEIOU"</span>
<span class="k">for</span> <span class="n">char</span> <span class="ow">in</span> <span class="n">in_string</span><span class="p">:</span>
<span class="k">if</span> <span class="n">char</span> <span class="ow">in</span> <span class="n">vowels</span><span class="p">:</span>
<span class="n">num_vowels</span> <span class="o">+=</span> <span class="mi">1</span> <span class="c1"># 等值于 num_vowels = num_vowels + 1</span>
<span class="k">return</span> <span class="n">num_vowels</span>
</pre></div>
</div>
<p>执行此代码会定义<em>函数</em> <code class="docutils literal notranslate"><span class="pre">count_vowels</span></code>。这个函数期待一个对象作为<em>输入参数</em>（input argument），由 <code class="docutils literal notranslate"><span class="pre">in_string</span></code> 代名，并会<em>返回</em>（return）该对象中元音的数量。使用 <code class="docutils literal notranslate"><span class="pre">count_vowels</span></code> 并向其输入一个对象的过程叫做<em>调用</em>（call）该函数：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">count_vowels</span><span class="p">(</span><span class="s2">"Hi my name is Ryan"</span><span class="p">)</span>
<span class="go">5</span>
</pre></div>
</div>
<p>最棒的一点在于你可以重复使用这个函数！</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">count_vowels</span><span class="p">(</span><span class="s2">"Apple"</span><span class="p">)</span>
<span class="go">2</span>
<span class="gp">>>> </span><span class="n">count_vowels</span><span class="p">(</span><span class="s2">"envelope"</span><span class="p">)</span>
<span class="go">4</span>
</pre></div>
</div>
<p>在本节中，我们将会了解定义和调用Python函数的语法。</p>
<div class="admonition note">
<p class="admonition-title fa fa-exclamation-circle"><strong>定义</strong>：</p>
<p>Python<strong>函数</strong>（function）是一个包装代码的对象。<em>调用</em>（call）函数将会执行包装的代码并<em>返回</em>（return）一个对象。你可以定义函数使其接受<em>参数</em>（argument），也就是输入进包装代码的对象。</p>
</div>
<div class="section" id="def-语句">
<h2><code class="docutils literal notranslate"><span class="pre">def</span></code> 语句<a class="headerlink" href="#def-语句" title="永久链接至标题">¶</a></h2>
<p>类似于 <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">for</span></code>，Python保留了 <code class="docutils literal notranslate"><span class="pre">def</span></code> 语句来表达函数（以及我们会在之后讨论的其它几个构造体）的定义。以下是Python函数定义的一般形式：</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="o"><</span><span class="n">function</span> <span class="n">name</span><span class="o">></span><span class="p">(</span><span class="o"><</span><span class="n">function</span> <span class="n">signature</span><span class="o">></span><span class="p">):</span>
<span class="sd">""" 说明字符串 """</span>
<span class="o"><</span><span class="n">encapsulated</span> <span class="n">code</span><span class="o">></span>
<span class="k">return</span> <span class="o"><</span><span class="nb">object</span><span class="o">></span>
</pre></div>
</div>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre"><function</span> <span class="pre">name></span></code> 是任何合法的变量名，在这之后<em>必须</em>跟随一对括号和一个冒号。</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre"><function</span> <span class="pre">signature></span></code> 描述函数的输入参数。如果函数不接受任何变量，你可以留白这一部分（你仍然需要提供括号，但是仅仅括号并不会包装任何参数）。</p></li>
<li><p>说明字符串（documentation string）（一般叫做“docstring”）可以长达多行并解释函数的目的。它是可选的。</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre"><encapsulated</span> <span class="pre">code></span></code> 包含正常的Python代码，其由相对于 <code class="docutils literal notranslate"><span class="pre">def</span></code> 语句的缩进限定。</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">return</span></code> 当函数中包装的代码遇到这个标示后，函数会返回标示后跟随的对象并立刻终止函数的执行。</p></li>
</ul>
<p>Python同样保留了 <code class="docutils literal notranslate"><span class="pre">return</span></code> 语句来表达函数的终止；如果遇到了 <code class="docutils literal notranslate"><span class="pre">return</span></code>，那程序将立即终止函数的执行并返回 <code class="docutils literal notranslate"><span class="pre">return</span></code> 右边的对象。</p>
<p>请注意，与if语句和for循环一样，<code class="docutils literal notranslate"><span class="pre">def</span></code> 语句必须由冒号结尾，且函数的主体代码必须<a class="reference external" href="https://cn.pythonlikeyoumeanit.com/Module2_EssentialsOfPython/Introduction.html#Python使用空格来限定区域">由空格限定</a>：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 错误的缩进</span>
<span class="k">def</span> <span class="nf">bad_func1</span><span class="p">():</span>
<span class="n">x</span> <span class="o">=</span> <span class="mi">1</span>
<span class="k">return</span> <span class="n">x</span> <span class="o">+</span> <span class="mi">2</span>
</pre></div>
</div>
<hr class="docutils" />
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 错误的缩进</span>
<span class="k">def</span> <span class="nf">bad_func2</span><span class="p">():</span>
<span class="n">x</span> <span class="o">=</span> <span class="mi">1</span>
<span class="k">return</span> <span class="n">x</span> <span class="o">+</span> <span class="mi">2</span>
</pre></div>
</div>
<hr class="docutils" />
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 漏了冒号</span>
<span class="k">def</span> <span class="nf">bad_func3</span><span class="p">()</span>
<span class="n">x</span> <span class="o">=</span> <span class="mi">1</span>
<span class="k">return</span> <span class="n">x</span> <span class="o">+</span> <span class="mi">2</span>
</pre></div>
</div>
<hr class="docutils" />
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 漏了括号</span>
<span class="k">def</span> <span class="nf">bad_func4</span><span class="p">:</span>
<span class="n">x</span> <span class="o">=</span> <span class="mi">1</span>
<span class="k">return</span> <span class="n">x</span> <span class="o">+</span> <span class="mi">2</span>
</pre></div>
</div>
<hr class="docutils" />
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 这个没问题</span>
<span class="k">def</span> <span class="nf">ok_func</span><span class="p">():</span>
<span class="n">x</span> <span class="o">=</span> <span class="mi">1</span>
<span class="k">return</span> <span class="n">x</span> <span class="o">+</span> <span class="mi">2</span>
</pre></div>
</div>
<div class="admonition note">
<p class="admonition-title fa fa-exclamation-circle"><strong>阅读理解：编写简单函数</strong></p>
<p>编写一个名为 <code class="docutils literal notranslate"><span class="pre">count_even</span></code> 的函数。它应该接受一个名为 <code class="docutils literal notranslate"><span class="pre">numbers</span></code> 的整数可迭代物作为参数。该函数应该返回列表中偶数的数量。注意包含一个合理的docstring。</p>
</div>
</div>
<div class="section" id="return-语句">
<h2><code class="docutils literal notranslate"><span class="pre">return</span></code> 语句<a class="headerlink" href="#return-语句" title="永久链接至标题">¶</a></h2>
<p>一般来讲，<code class="docutils literal notranslate"><span class="pre">return</span></code> 语句可以返回任何Python对象。同时，你可以使用一个<strong>空的</strong> <code class="docutils literal notranslate"><span class="pre">return</span></code> 语句或在函数中完全不使用<strong>返回</strong>（return）语句。在这两种情况下，<em>函数都会返回 ``None`` 对象</em>。</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 本函数返回 `None`</span>
<span class="c1"># 一个“空”的返回语句</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">():</span>
<span class="n">x</span> <span class="o">=</span> <span class="mi">1</span>
<span class="k">return</span>
</pre></div>
</div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 本函数返回 `None`</span>
<span class="c1"># 完全没有用返回语句</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">():</span>
<span class="n">x</span> <span class="o">=</span> <span class="mi">1</span>
</pre></div>
</div>
<p>所有Python函数都会返回<em>某个对象</em>。就算是内置的 <code class="docutils literal notranslate"><span class="pre">print</span></code> 函数在打印后也会返回 <code class="docutils literal notranslate"><span class="pre">None</span></code>！</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># `print` 函数返回 `None`</span>
<span class="o">>>></span> <span class="n">x</span> <span class="o">=</span> <span class="nb">print</span><span class="p">(</span><span class="s2">"hi"</span><span class="p">)</span>
<span class="n">hi</span>
<span class="o">>>></span> <span class="n">x</span> <span class="ow">is</span> <span class="kc">None</span>
<span class="kc">True</span>
</pre></div>
</div>
<div class="admonition warning">
<p class="admonition-title fa fa-exclamation-circle"><strong>警告</strong>！</p>
<p>请注意不要<em>错误地</em>漏掉或使用空的返回语句。你将依然能够调用你的函数，但是它在任何情况下都会返回 <code class="docutils literal notranslate"><span class="pre">None</span></code>！</p>
</div>
<p>函数并不一定要有除了返回语句之外的代码。比如说，我们可以使用 <code class="docutils literal notranslate"><span class="pre">sum</span></code> 和生成器理解（见本模组前一节）来简化我们的 <code class="docutils literal notranslate"><span class="pre">count_vowels</span></code> 函数：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 你可以直接描述函数返回的对象</span>
<span class="k">def</span> <span class="nf">count_vowels</span><span class="p">(</span><span class="n">in_string</span><span class="p">):</span>
<span class="sd">""" 返回 `in_string` 中元音的数量 """</span>
<span class="k">return</span> <span class="nb">sum</span><span class="p">(</span><span class="mi">1</span> <span class="k">for</span> <span class="n">char</span> <span class="ow">in</span> <span class="n">in_string</span> <span class="k">if</span> <span class="n">char</span> <span class="ow">in</span> <span class="s2">"aeiouAEIOU"</span><span class="p">)</span>
</pre></div>
</div>
<div class="section" id="多个-return-语句">
<h3>多个 <code class="docutils literal notranslate"><span class="pre">return</span></code> 语句<a class="headerlink" href="#多个-return-语句" title="永久链接至标题">¶</a></h3>
<p>你可以在一个函数内使用不止一个 <code class="docutils literal notranslate"><span class="pre">return</span></code> 语句。在处理边缘情况或优化代码时这可能会有用。假设你想要你的函数通过<a class="reference external" href="https://en.wikipedia.org/wiki/Taylor_series#Exponential_function">泰勒级数</a>（Taylor series）来模糊计算 <span class="math notranslate nohighlight">\(e^{x}\)</span>，当 <span class="math notranslate nohighlight">\(x = 0\)</span> 时函数应该立刻返回 <code class="docutils literal notranslate"><span class="pre">1.0</span></code>：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">compute_exp</span><span class="p">(</span><span class="n">x</span><span class="p">):</span>
<span class="sd">""" 使用泰勒级数来计算 e^x """</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="k">return</span> <span class="mf">1.0</span>
<span class="kn">from</span> <span class="nn">math</span> <span class="kn">import</span> <span class="n">factorial</span>
<span class="k">return</span> <span class="nb">sum</span><span class="p">(</span><span class="n">x</span><span class="o">**</span><span class="n">n</span> <span class="o">/</span> <span class="n">factorial</span><span class="p">(</span><span class="n">n</span><span class="p">)</span> <span class="k">for</span> <span class="n">n</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">100</span><span class="p">))</span>
</pre></div>
</div>
<p>如果 <code class="docutils literal notranslate"><span class="pre">x==0</span></code> 为 <code class="docutils literal notranslate"><span class="pre">True</span></code>，那么程序将执行第一个 <code class="docutils literal notranslate"><span class="pre">return</span></code> 语句，返回 <code class="docutils literal notranslate"><span class="pre">1.0</span></code> 并立刻“终止”该函数，且不执行之后的代码。</p>
<p>如上所述，就算下方有额外的代码，<code class="docutils literal notranslate"><span class="pre">return</span></code> 语句也会导致函数立刻终止且不执行之后的代码。<em>在一次函数调用中，程序不可能遇到多个 ``return`` 语句</em>。因此，如果你想要返回多个对象，那么你的函数必须返回单个包含这些项目的容器，如列表会元组。</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 函数返回多个项目</span>
<span class="k">def</span> <span class="nf">bad_f</span><span class="p">(</span><span class="n">x</span><span class="p">):</span> <span class="c1"># 译者注：函数名字大意为：不好的函数</span>
<span class="sd">""" 返回 x**2 和 x**3"""</span>
<span class="k">return</span> <span class="n">x</span><span class="o">**</span><span class="mi">2</span>
<span class="c1"># 此代码永远都不会被执行！</span>
<span class="k">return</span> <span class="n">x</span><span class="o">**</span><span class="mi">3</span>
<span class="k">def</span> <span class="nf">good_f</span><span class="p">(</span><span class="n">x</span><span class="p">):</span> <span class="c1"># 译者注：函数名字大意为：好的函数</span>
<span class="sd">""" return x**2 and x**3"""</span>
<span class="k">return</span> <span class="p">(</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="mi">3</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">bad_f</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="go">4</span>
<span class="gp">>>> </span><span class="n">good_f</span><span class="p">(</span><span class="mi">2</span><span class="p">)</span>
<span class="go">(4, 8)</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="单行函数">
<h2>单行函数<a class="headerlink" href="#单行函数" title="永久链接至标题">¶</a></h2>
<p>你可以在一行内定义只有一个返回语句的函数：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">add_2</span><span class="p">(</span><span class="n">x</span><span class="p">):</span>
<span class="k">return</span> <span class="n">x</span> <span class="o">+</span> <span class="mi">2</span>
</pre></div>
</div>
<p>可以被重写为：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">add_2</span><span class="p">(</span><span class="n">x</span><span class="p">):</span> <span class="k">return</span> <span class="n">x</span> <span class="o">+</span> <span class="mi">2</span>
</pre></div>
</div>
<p>尽量只在函数极其简单到不需要docstring就能理解时使用此功能。切记不要滥用。</p>
</div>
<div class="section" id="参数">
<h2>参数<a class="headerlink" href="#参数" title="永久链接至标题">¶</a></h2>
<p>你可以在函数签名（function signature）部分通过一序列由逗号隔开的变量名来描述函数的<em>位置</em>参数（positional argument）。比如说，以下代码为函数 <code class="docutils literal notranslate"><span class="pre">is_bounded</span></code> 提供了 <code class="docutils literal notranslate"><span class="pre">x</span></code>，<code class="docutils literal notranslate"><span class="pre">lower</span></code>，和 <code class="docutils literal notranslate"><span class="pre">upper</span></code> 三个参数：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">is_bounded</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">lower</span><span class="p">,</span> <span class="n">upper</span><span class="p">):</span>
<span class="k">return</span> <span class="n">lower</span> <span class="o"><=</span> <span class="n">x</span> <span class="o"><=</span> <span class="n">upper</span>
</pre></div>
</div>
<p>你可以用几种不同的方法来向函数提供参数：</p>
<div class="section" id="通过位置提供参数">
<h3>通过位置提供参数<a class="headerlink" href="#通过位置提供参数" title="永久链接至标题">¶</a></h3>
<p>输入到 <code class="docutils literal notranslate"><span class="pre">is_bounded</span></code> 中的对象会根据它们的位置来赋值函数的输入变量。也就是说，<code class="docutils literal notranslate"><span class="pre">is_bounded(3,</span> <span class="pre">2,</span> <span class="pre">4)</span></code> 会赋值 <code class="docutils literal notranslate"><span class="pre">x=3</span></code>，<code class="docutils literal notranslate"><span class="pre">lower=2</span></code>，和 <code class="docutils literal notranslate"><span class="pre">upper=4</span></code>，其根据函数输入参数的位置顺序来对应：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 计算：2 <= 3 <= 4</span>
<span class="c1"># 通过位置来提供输入</span>
<span class="o">>>></span> <span class="n">is_bounded</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">4</span><span class="p">)</span>
<span class="kc">True</span>
</pre></div>
</div>
<p>向函数输入太少或太多参数会导致 <code class="docutils literal notranslate"><span class="pre">TypeError</span></code>。</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 输入太少：报错</span>
<span class="n">is_bounded</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span>
<span class="c1"># 输入太多：报错</span>
<span class="n">is_bounded</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="mi">4</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="通过关键词提供参数">
<h3>通过关键词提供参数<a class="headerlink" href="#通过关键词提供参数" title="永久链接至标题">¶</a></h3>
<p>在顺序不重要的时候，你也可以在给函数输入参数时提供它们的关键词（也就是名字）来对应对象和参数。这会帮助你编写易读和灵活的代码：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 计算：2 <= 3 <= 4</span>
<span class="c1"># 通过参数名字来提供输入对象</span>
<span class="o">>>></span> <span class="n">is_bounded</span><span class="p">(</span><span class="n">lower</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="mi">3</span><span class="p">,</span> <span class="n">upper</span><span class="o">=</span><span class="mi">4</span><span class="p">)</span>
<span class="kc">True</span>
</pre></div>
</div>
<p>你可以混合关键词参数和位置参数，但位置参数应该在前：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 计算：2 <= 3 <= 4</span>
<span class="c1"># `x` 是位置参数</span>
<span class="c1"># `lower` 和 `upper` 关键词参数</span>
<span class="o">>>></span> <span class="n">is_bounded</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="n">upper</span><span class="o">=</span><span class="mi">4</span><span class="p">,</span> <span class="n">lower</span><span class="o">=</span><span class="mi">2</span><span class="p">)</span>
<span class="kc">True</span>
</pre></div>
</div>
<p>请注意，如果你提供了一个关键词参数，那么它之后所有的参数都应该是关键词参数：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 你不可以在关键词参数后使用位置参数</span>
<span class="o">>>></span> <span class="n">is_bounded</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="n">lower</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="mi">4</span><span class="p">)</span>
<span class="ne">SyntaxError</span><span class="p">:</span> <span class="n">positional</span> <span class="n">argument</span> <span class="n">follows</span> <span class="n">keyword</span> <span class="n">argument</span>
</pre></div>
</div>
</div>
<div class="section" id="有默认值的参数">
<h3>有默认值的参数<a class="headerlink" href="#有默认值的参数" title="永久链接至标题">¶</a></h3>
<p>你可以提供参数的默认值。如果用户在调用函数时没有提供此参数，那么函数将会使用定义的默认值。请回忆我们的 <code class="docutils literal notranslate"><span class="pre">count_vowels</span></code> 函数。假设我们想要提供将“y”算为一个元音的选择。因为我们知道人们一般不会将“y”视为元音，所以我们可以默认将“y”除外：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">count_vowels</span><span class="p">(</span><span class="n">in_string</span><span class="p">,</span> <span class="n">include_y</span><span class="o">=</span><span class="kc">False</span><span class="p">):</span>
<span class="sd">""" 返回 `in_string` 中元音的数量"""</span>
<span class="n">vowels</span> <span class="o">=</span> <span class="s2">"aeiouAEIOU"</span>
<span class="k">if</span> <span class="n">include_y</span><span class="p">:</span>
<span class="n">vowels</span> <span class="o">+=</span> <span class="s2">"yY"</span> <span class="c1"># 将 "y" 加到元音列表中</span>
<span class="k">return</span> <span class="nb">sum</span><span class="p">(</span><span class="mi">1</span> <span class="k">for</span> <span class="n">char</span> <span class="ow">in</span> <span class="n">in_string</span> <span class="k">if</span> <span class="n">char</span> <span class="ow">in</span> <span class="n">vowels</span><span class="p">)</span>
</pre></div>
</div>
<p>现在，如果在调用 <code class="docutils literal notranslate"><span class="pre">count_vowels</span></code> 时只提供了 <code class="docutils literal notranslate"><span class="pre">in_string</span></code>，那么 <code class="docutils literal notranslate"><span class="pre">include_y</span></code> 会使用默认值 <code class="docutils literal notranslate"><span class="pre">False</span></code>：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 使用默认值：不将y算为元音</span>
<span class="o">>>></span> <span class="n">count_vowels</span><span class="p">(</span><span class="s2">"Happy"</span><span class="p">)</span>
<span class="mi">1</span>
</pre></div>
</div>
<p>我们可以提供默认值之外的输入：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 不使用不认值：将y算为元音</span>
<span class="o">>>></span> <span class="n">count_vowels</span><span class="p">(</span><span class="s2">"Happy"</span><span class="p">,</span> <span class="kc">True</span><span class="p">)</span>
<span class="mi">2</span>
<span class="c1"># 你依然可以通过名字来描述输入参数</span>
<span class="o">>>></span> <span class="n">count_vowels</span><span class="p">(</span><span class="n">include_y</span><span class="o">=</span><span class="kc">True</span><span class="p">,</span> <span class="n">in_string</span><span class="o">=</span><span class="s2">"Happy"</span><span class="p">)</span>
<span class="mi">2</span>
</pre></div>
</div>
<p>在函数签名中，有默认值的输入参数必须在所有位置参数之后：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 这没问题</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">z</span><span class="p">,</span> <span class="n">count</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="n">upper</span><span class="o">=</span><span class="mi">2</span><span class="p">):</span>
<span class="k">return</span> <span class="kc">None</span>
</pre></div>
</div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 这会导致语法错误</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">count</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="n">upper</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">z</span><span class="p">):</span>
<span class="k">return</span> <span class="kc">None</span>
</pre></div>
</div>
<div class="admonition note">
<p class="admonition-title fa fa-exclamation-circle"><strong>阅读理解：函数和参数</strong></p>
<p>编写一个函数 <code class="docutils literal notranslate"><span class="pre">max_or_min</span></code>。它接受两个位置参数 <code class="docutils literal notranslate"><span class="pre">x</span></code> 和 <code class="docutils literal notranslate"><span class="pre">y</span></code>（将会被赋值为数字）以及一个 <code class="docutils literal notranslate"><span class="pre">mode</span></code> 变量，其默认值为 <code class="docutils literal notranslate"><span class="pre">"max"</span></code>。</p>
<p>此函数应该根据 <code class="docutils literal notranslate"><span class="pre">mode</span></code> 返回 <code class="docutils literal notranslate"><span class="pre">min(x,</span> <span class="pre">y)</span></code> 或 <code class="docutils literal notranslate"><span class="pre">max(x,</span> <span class="pre">y)</span></code>。如果 <code class="docutils literal notranslate"><span class="pre">mode</span></code> 既不是 <code class="docutils literal notranslate"><span class="pre">"max"</span></code> 也不是 <code class="docutils literal notranslate"><span class="pre">"min"</span></code> 的时候让函数返回 <code class="docutils literal notranslate"><span class="pre">None</span></code>。</p>
<p>包含一个描述性的docstring。</p>
</div>
</div>
<div class="section" id="支持任意多的位置参数">
<h3>支持任意多的位置参数<a class="headerlink" href="#支持任意多的位置参数" title="永久链接至标题">¶</a></h3>
<p>Python提供了定义可以接受任意多位置参数的函数的语法。使用 <code class="docutils literal notranslate"><span class="pre">def</span> <span class="pre">f(*<var_name>)</span></code> 语法来定义这类输入。</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># * 符号表明调用 `f` 时可以向 `args`</span>
<span class="c1"># 输入任意多的参数。</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">):</span>
<span class="c1"># 所有向 `f` 输入的参数都会被“打包”成一个元组</span>
<span class="c1"># 并赋值给变量 `args`。</span>
<span class="c1"># `f()` 会赋值 `args = tuple()`</span>
<span class="c1"># `f(x, y, ...)` 会赋值 `args = (x, y, ...)`</span>
<span class="k">return</span> <span class="n">args</span>
</pre></div>
</div>
<p>因为Python不能提前知道 <code class="docutils literal notranslate"><span class="pre">f</span></code> 会收到多少个参数，因此它所有的输入参数都会被<em>打包成一个元组</em>并赋值给变量 <code class="docutils literal notranslate"><span class="pre">args</span></code>：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 向 `f` 输入0个参数</span>
<span class="o">>>></span> <span class="n">f</span><span class="p">()</span>
<span class="p">()</span>
<span class="c1"># 向 `f` 输入1个参数</span>
<span class="o">>>></span> <span class="n">f</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="p">(</span><span class="mi">1</span><span class="p">,)</span>
<span class="c1"># 向 `f` 输入3个参数</span>
<span class="o">>>></span> <span class="n">f</span><span class="p">((</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">),</span> <span class="kc">True</span><span class="p">,</span> <span class="s2">"cow"</span><span class="p">)</span>
<span class="p">((</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">),</span> <span class="kc">True</span><span class="p">,</span> <span class="s2">"cow"</span><span class="p">)</span>
</pre></div>
</div>
<p>你可以将此语法与位置参数和默认参数混合使用。任何在被打包的变量后的变量<em>必须提供参数名</em>：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="o">*</span><span class="n">seq</span><span class="p">,</span> <span class="n">y</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"x is: "</span><span class="p">,</span> <span class="n">x</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"seq is: "</span><span class="p">,</span> <span class="n">seq</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"y is: "</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span>
<span class="k">return</span> <span class="kc">None</span>
</pre></div>
</div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">f</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="mi">4</span><span class="p">,</span> <span class="n">y</span><span class="o">=</span><span class="mi">5</span><span class="p">)</span> <span class="c1"># 必须提供 `y` 的名字</span>
</pre></div>
</div>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>x 是： 1
seq 是： (2, 3, 4)
y 是： 5
</pre></div>
</div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">f</span><span class="p">(</span><span class="s2">"cat"</span><span class="p">,</span> <span class="n">y</span><span class="o">=</span><span class="s2">"dog"</span><span class="p">)</span> <span class="c1"># 没有输入任何额外的位置参数</span>
</pre></div>
</div>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>x 是： "cat"
seq 是： ()
y 是： "dog"
</pre></div>
</div>
<div class="admonition note">
<p class="admonition-title fa fa-exclamation-circle"><strong>阅读理解：任意多的参数</strong></p>
<p>编写一个名为 <code class="docutils literal notranslate"><span class="pre">mean</span></code> 的函数，其接受任意多的数字参数并计算所有数字的平均值。如，<code class="docutils literal notranslate"><span class="pre">mean(1,</span> <span class="pre">2,</span> <span class="pre">3)</span></code> 应返回 <span class="math notranslate nohighlight">\(\frac{1 + 2 + 3}{3} = 2.0\)</span></p>
<p>如果没有收到任何输入，函数应返回 <code class="docutils literal notranslate"><span class="pre">0.</span></code>。记得测试你的函数并编写一个docstring。</p>
</div>
<p>如我们所见，在函数定义的签名中使用 <code class="docutils literal notranslate"><span class="pre">*</span></code> 意味着<em>将任意多的参数打包成一个元组</em>。同时，在调用函数时 <code class="docutils literal notranslate"><span class="pre">*</span></code> 也意味着<em>解包可迭代物</em>并将其作为位置参数输入到函数中：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 在调用函数时使用 `*` 来解包可迭代物，并将其</span>
<span class="c1"># 成员作为位置参数输入到函数中</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">z</span><span class="p">):</span>
<span class="k">return</span> <span class="n">x</span> <span class="o">+</span> <span class="n">y</span> <span class="o">+</span> <span class="n">z</span>
<span class="o">>>></span> <span class="n">f</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="mi">6</span>
<span class="c1"># `*` 意味着：解包 `[1, 2, 3]` 的内容并将每个</span>
<span class="c1"># 物件分别输入为x，y，和z</span>
<span class="o">>>></span> <span class="n">f</span><span class="p">(</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="c1"># 等值于：f(1, 2, 3)</span>
<span class="mi">6</span>
</pre></div>
</div>
<p>在以下范例中，我们使用 <code class="docutils literal notranslate"><span class="pre">*</span></code> 来：</p>
<ol class="arabic simple">
<li><p>定义一个接受任意多参数并将其打包为元组的函数</p></li>
<li><p>通过解包可迭代物来调用函数并输入任意多的参数</p></li>
</ol>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">number_of_args</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">):</span>
<span class="k">return</span> <span class="nb">len</span><span class="p">(</span><span class="n">args</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">number_of_args</span><span class="p">(</span><span class="kc">None</span><span class="p">,</span> <span class="kc">None</span><span class="p">,</span> <span class="kc">None</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span>
<span class="go">4</span>
<span class="gp">>>> </span><span class="n">some_list</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="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">]</span>
<span class="go"># 将列表本身作为唯一的参数输入</span>
<span class="gp">>>> </span><span class="n">number_of_args</span><span class="p">(</span><span class="n">some_list</span><span class="p">)</span>
<span class="go">1</span>
<span class="go"># 将列表的5个成员解包并作为多个参数输入进函数</span>
<span class="gp">>>> </span><span class="n">number_of_args</span><span class="p">(</span><span class="o">*</span><span class="n">some_list</span><span class="p">)</span>
<span class="go">5</span>
</pre></div>
</div>
</div>
<div class="section" id="支持任意多的关键词参数">
<h3>支持任意多的关键词参数<a class="headerlink" href="#支持任意多的关键词参数" title="永久链接至标题">¶</a></h3>
<p>我们可以使用 <code class="docutils literal notranslate"><span class="pre">def</span> <span class="pre">f(**<var_name>)</span></code> 语法来定义一个接受任意多的<em>关键词</em>（keyword）参数的函数。</p>
<p>注意单个星号 <code class="docutils literal notranslate"><span class="pre">*</span></code> 用来代表任意多的<em>位置</em>参数，而 <code class="docutils literal notranslate"><span class="pre">**</span></code> 代表着任意多的<em>关键词</em>参数。</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># ** 符号意味着在调用 `f` 时可以向 `args` 输入</span>
<span class="c1"># 任意多的关键词参数。</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="o">**</span><span class="n">args</span><span class="p">):</span>
<span class="c1"># 所有输入到 `f` 的关键词参数都会被“打包”成一个词典</span>
<span class="c1"># 并赋值到变量 `args` 中</span>
<span class="c1"># `f()` 会赋值 `args = {}`（一个空词典）</span>
<span class="c1"># `f(x=1, y=2, ...)` 会赋值 `args = {"x":1, "y":2, ...}`</span>
<span class="k">return</span> <span class="n">args</span>
</pre></div>
</div>
<p>因为Python无法预先知道 <code class="docutils literal notranslate"><span class="pre">f</span></code> 会收到多少个关键词参数，所以所有收到的关键词参数都会被打包成一个<em>词典</em>（dictionary）。词典允许你通过关键词（将其转化成字符串）查询并设置关键词对应的值。这个词典被赋值到变量 <code class="docutils literal notranslate"><span class="pre">args</span></code> 上。我们会在<a class="reference external" href="https://cn.pythonlikeyoumeanit.com/Module2_EssentialsOfPython/DataStructures_II_Dictionaries.html">后面一节</a>专门讨论词典。</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">f</span><span class="p">()</span> <span class="c1"># 向 `f` 输入0个参数</span>
<span class="go">{}</span>
<span class="gp">>>> </span><span class="n">f</span><span class="p">(</span><span class="n">x</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span> <span class="c1"># 向 `f` 输入1个参数</span>
<span class="go">{'x': 1}</span>
<span class="gp">>>> </span><span class="n">f</span><span class="p">(</span><span class="n">x</span><span class="o">=</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">),</span> <span class="n">val</span><span class="o">=</span><span class="kc">True</span><span class="p">,</span> <span class="n">moo</span><span class="o">=</span><span class="s2">"cow"</span><span class="p">)</span> <span class="c1"># 向 `f` 输入3个参数</span>
<span class="go">{'moo': 'cow', 'val': True, 'x': (0, 1)}</span>
</pre></div>
</div>
<p>这一语法可以和位置参数和默认参数混合使用。在函数定义签名中，<code class="docutils literal notranslate"><span class="pre">**</span></code> 后不能有任何额外的参数：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"x 是："</span><span class="p">,</span> <span class="n">x</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"y 是："</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"kwargs 是："</span><span class="p">,</span> <span class="n">kwargs</span><span class="p">)</span>
<span class="k">return</span> <span class="kc">None</span>
<span class="c1"># 译者注：kwargs是keyword arguments的缩写，也就是关键词参数。</span>
<span class="c1"># 译者注：此命名方式是传统，所以请尽量遵循</span>
</pre></div>
</div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 向 `f` 输入任意的关键词参数</span>
<span class="o">>>></span> <span class="n">f</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="n">y</span><span class="o">=</span><span class="mi">9</span><span class="p">,</span> <span class="n">z</span><span class="o">=</span><span class="mi">3</span><span class="p">,</span> <span class="n">k</span><span class="o">=</span><span class="s2">"hi"</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>x 是： 1
y 是： 9
kwargs 是： {'z': 3, 'k': 'hi'}
</pre></div>
</div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 没有输入任何额外的关键词参数</span>
<span class="o">>>></span> <span class="n">f</span><span class="p">(</span><span class="s2">"cat"</span><span class="p">,</span> <span class="n">y</span><span class="o">=</span><span class="s2">"dog"</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">x</span> <span class="ow">is</span><span class="p">:</span> <span class="n">cat</span>
<span class="n">y</span> <span class="ow">is</span><span class="p">:</span> <span class="n">dog</span>
<span class="n">kwargs</span> <span class="ow">is</span><span class="p">:</span> <span class="p">{}</span>
</pre></div>
</div>
<p>以下函数接受任意多的位置参数<em>和</em>任意多的关键词参数：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 接受任意多的位置和关键词参数</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="o">*</span><span class="n">x</span><span class="p">,</span> <span class="o">**</span><span class="n">y</span><span class="p">):</span>
<span class="c1"># 所有位置参数都打包成元组 `x`</span>
<span class="c1"># 所有关键词参数都打包成词典 `y`</span>
<span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">y</span><span class="p">)</span>
<span class="k">return</span> <span class="kc">None</span>
<span class="o">>>></span> <span class="n">f</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="n">hi</span><span class="o">=-</span><span class="mi">1</span><span class="p">,</span> <span class="n">bye</span><span class="o">=-</span><span class="mi">2</span><span class="p">,</span> <span class="n">sigh</span><span class="o">=-</span><span class="mi">3</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></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="p">{</span><span class="s1">'hi'</span><span class="p">:</span> <span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="s1">'bye'</span><span class="p">:</span> <span class="o">-</span><span class="mi">2</span><span class="p">,</span> <span class="s1">'sigh'</span><span class="p">:</span> <span class="o">-</span><span class="mi">3</span><span class="p">}</span>
</pre></div>
</div>
<p>如上所见，在函数定义的签名中 <code class="docutils literal notranslate"><span class="pre">**</span></code> 意味着<em>将任意多的关键词参数打包成一个词典</em>。同时，在调用函数时 <code class="docutils literal notranslate"><span class="pre">**</span></code> 意味着<em>解包词典</em>并将其的键值对（key-value pair）作为函数关键词参数输入：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 在调用函数时使用 `**` 来解包词典并将其成员作为</span>
<span class="c1"># 关键词参数输入到函数中</span>
<span class="k">def</span> <span class="nf">f</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">z</span><span class="p">):</span>
<span class="k">return</span> <span class="mi">0</span><span class="o">*</span><span class="n">x</span> <span class="o">+</span> <span class="mi">1</span><span class="o">*</span><span class="n">y</span> <span class="o">+</span> <span class="mi">2</span><span class="o">*</span><span class="n">z</span>
<span class="o">>>></span> <span class="n">f</span><span class="p">(</span><span class="n">z</span><span class="o">=</span><span class="mi">10</span><span class="p">,</span> <span class="n">x</span><span class="o">=</span><span class="mi">9</span><span class="p">,</span> <span class="n">y</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
<span class="mi">21</span>
<span class="o">>>></span> <span class="n">args</span> <span class="o">=</span> <span class="p">{</span><span class="s2">"x"</span><span class="p">:</span> <span class="mi">9</span><span class="p">,</span> <span class="s2">"y"</span><span class="p">:</span> <span class="mi">1</span><span class="p">,</span> <span class="s2">"z"</span><span class="p">:</span> <span class="mi">10</span><span class="p">}</span>
<span class="o">>>></span> <span class="n">f</span><span class="p">(</span><span class="o">**</span><span class="n">args</span><span class="p">)</span> <span class="c1"># 等值于：f(x=9, y=1, z=10)</span>
<span class="mi">21</span>
</pre></div>
</div>
<p>在以下范例中，我们用 <code class="docutils literal notranslate"><span class="pre">**</span></code> 来：</p>
<ol class="arabic simple">
<li><p>定义一个接受任意多关键词参数的函数并将其打包成词典。</p></li>
<li><p>调用函数并通过解包词典向其输入任意多的关键词参数。</p></li>
</ol>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">print_kwargs</span><span class="p">(</span><span class="o">**</span><span class="n">args</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="n">args</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">print_kwargs</span><span class="p">(</span><span class="n">a</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="n">b</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">c</span><span class="o">=</span><span class="mi">3</span><span class="p">,</span> <span class="n">d</span><span class="o">=</span><span class="mi">4</span><span class="p">)</span>
<span class="go">{'a': 1, 'b': 2, 'c': 3, 'd': 4}</span>
<span class="gp">>>> </span><span class="n">some_dict</span> <span class="o">=</span> <span class="p">{</span><span class="s2">"hi"</span><span class="p">:</span><span class="mi">1</span><span class="p">,</span> <span class="s2">"bye"</span><span class="p">:</span><span class="mi">2</span><span class="p">}</span>
<span class="go"># 解包词典的键值对并将其作为关键词参数输入到函数中</span>
<span class="gp">>>> </span><span class="n">print_kwargs</span><span class="p">(</span><span class="n">a</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">umbrella</span><span class="o">=</span><span class="kc">True</span><span class="p">,</span> <span class="o">**</span><span class="n">some_dict</span><span class="p">)</span>
<span class="go">{'a': 2, 'umbrella': True, 'hi': 1, 'bye': 2}</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="函数也是对象">
<h2>函数也是对象<a class="headerlink" href="#函数也是对象" title="永久链接至标题">¶</a></h2>
<p>在定义之后，函数和任何其它Python对象，如列表或字符串或整数，的行为差不多。你可以将函数赋值到变量上：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">var</span> <span class="o">=</span> <span class="n">count_vowels</span> <span class="c1"># `var` 现在引用函数 `count_vowels`</span>
<span class="gp">>>> </span><span class="n">var</span><span class="p">(</span><span class="s2">"Hello"</span><span class="p">)</span> <span class="c1"># 你现在可以“调用” `var`</span>
<span class="go">2</span>
</pre></div>
</div>
<p>你可以将函数存储到一个列表中：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="n">my_list</span> <span class="o">=</span> <span class="p">[</span><span class="n">count_vowels</span><span class="p">,</span> <span class="nb">print</span><span class="p">]</span>
<span class="k">for</span> <span class="n">func</span> <span class="ow">in</span> <span class="n">my_list</span><span class="p">:</span>
<span class="n">func</span><span class="p">(</span><span class="s2">"hello"</span><span class="p">)</span>
<span class="c1"># 迭代0：调用 `count_vowels("hello")`</span>
<span class="c1"># 迭代1：调用 `print("hello")`</span>
</pre></div>
</div>
<p>你也可以在代码任何地方调用函数，且它的返回值会在原地返回：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">count_vowels</span><span class="p">(</span><span class="s2">"pillow"</span><span class="p">)</span> <span class="o">></span> <span class="mi">1</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"that's a lot of vowels!"</span><span class="p">)</span>
</pre></div>
</div>
<p>当然，这在列表理解表达式中也成立：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">sum</span><span class="p">(</span><span class="n">count_vowels</span><span class="p">(</span><span class="n">word</span><span class="p">,</span> <span class="n">include_y</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span> <span class="k">for</span> <span class="n">word</span> <span class="ow">in</span> <span class="p">[</span><span class="s2">"hi"</span><span class="p">,</span> <span class="s2">"bye"</span><span class="p">,</span> <span class="s2">"guy"</span><span class="p">,</span> <span class="s2">"sigh"</span><span class="p">])</span>
<span class="go">6</span>
</pre></div>
</div>
<p>“打印”一个函数并不会揭露些什么。这仅仅打印此函数对象在内存中的地址：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span><span class="p">(</span><span class="n">count_vowels</span><span class="p">)</span>
<span class="go"><function count_vowels at 0x000002A32898C6A8></span>
</pre></div>
</div>
</div>
<div class="section" id="官方说明文档链接">
<h2>官方说明文档链接<a class="headerlink" href="#官方说明文档链接" 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/stdtypes.html#functions">“函数”的定义</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/tutorial/controlflow.html#defining-functions">定义函数</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/tutorial/controlflow.html#default-argument-values">定义参数值</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/tutorial/controlflow.html#keyword-arguments">关键词参数</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/tutorial/controlflow.html#arbitrary-argument-lists">提供任意多的参数</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/tutorial/controlflow.html#unpacking-argument-lists">解包参数</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/tutorial/controlflow.html#documentation-strings">docstring</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/tutorial/controlflow.html#function-annotations">函数注解</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>编写简单函数：解</strong></p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">count_even</span><span class="p">(</span><span class="n">numbers</span><span class="p">):</span>
<span class="sd">""" 返回可迭代物中偶数的数量 """</span>
<span class="n">total_even</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">for</span> <span class="n">num</span> <span class="ow">in</span> <span class="n">numbers</span><span class="p">:</span>
<span class="k">if</span> <span class="n">num</span> <span class="o">%</span> <span class="mi">2</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="n">total</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="k">return</span> <span class="n">total</span>
</pre></div>
</div>
<p>或通过使用生成器理解：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">count_even</span><span class="p">(</span><span class="n">numbers</span><span class="p">):</span>
<span class="sd">""" 返回可迭代物中偶数的数量 """</span>
<span class="k">return</span> <span class="nb">sum</span><span class="p">(</span><span class="mi">1</span> <span class="k">for</span> <span class="n">num</span> <span class="ow">in</span> <span class="n">numbers</span> <span class="k">if</span> <span class="n">num</span> <span class="o">%</span> <span class="mi">2</span> <span class="o">==</span> <span class="mi">0</span><span class="p">)</span>
</pre></div>
</div>
<p><strong>函数和参数：解</strong></p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">max_or_min</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">mode</span><span class="o">=</span><span class="s2">"max"</span><span class="p">):</span>
<span class="sd">""" 根据 `mode` 参数返回 `max(x,y)` 或 `min(x,y)`。</span>
<span class="sd"> Parameters</span>
<span class="sd"> ----------</span>
<span class="sd"> x : Number</span>
<span class="sd"> y : Number</span>
<span class="sd"> mode : str</span>
<span class="sd"> 'max' 或 'min'</span>
<span class="sd"> Returns</span>
<span class="sd"> -------</span>
<span class="sd"> 两值的最大或最小值。如果mode不合法那么会返回 `None`。"""</span>
<span class="k">if</span> <span class="n">mode</span> <span class="o">==</span> <span class="s2">"max"</span><span class="p">:</span>
<span class="k">return</span> <span class="nb">max</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span>
<span class="k">elif</span> <span class="n">mode</span> <span class="o">==</span> <span class="s2">"min"</span><span class="p">:</span>
<span class="k">return</span> <span class="nb">min</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">return</span> <span class="kc">None</span>
</pre></div>
</div>
<p>请注意你其实可以在 <code class="docutils literal notranslate"><span class="pre">mode</span></code> 输入不正确时让你的函数报错（raise an “exception”）。事实上，这才应该是这种情况下更加合理的函数行为。</p>
<p>这种解决方案如下：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">max_or_min</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">mode</span><span class="o">=</span><span class="s2">"max"</span><span class="p">):</span>
<span class="k">if</span> <span class="n">mode</span> <span class="o">==</span> <span class="s2">"max"</span><span class="p">:</span>
<span class="k">return</span> <span class="nb">max</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span>
<span class="k">elif</span> <span class="n">mode</span> <span class="o">==</span> <span class="s2">"min"</span><span class="p">:</span>
<span class="k">return</span> <span class="nb">min</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span><span class="s2">"`mode` was passed an invalid value: </span><span class="si">{}</span><span class="s2">"</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="n">mode</span><span class="p">))</span>
</pre></div>
</div>
<p><strong>任意多的参数：解</strong></p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">mean</span><span class="p">(</span><span class="o">*</span><span class="n">seq</span><span class="p">):</span>
<span class="sd">""" 返回函数参数的平均值 """</span>
<span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">seq</span><span class="p">)</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="k">return</span> <span class="mi">0</span>
<span class="n">total</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">for</span> <span class="n">num</span> <span class="ow">in</span> <span class="n">seq</span><span class="p">:</span>
<span class="n">total</span> <span class="o">+=</span> <span class="n">num</span>
<span class="k">return</span> <span class="n">total</span> <span class="o">/</span> <span class="nb">len</span><span class="p">(</span><span class="n">seq</span><span class="p">)</span>
</pre></div>
</div>
<p>或者我们可以利用以下两点来做一些骚操作：</p>
<ul class="simple">
<li><p>当 <code class="docutils literal notranslate"><span class="pre">seq</span></code> 为空时 <code class="docutils literal notranslate"><span class="pre">bool(seq)</span></code> 是 <code class="docutils literal notranslate"><span class="pre">False</span></code> 的事实</p></li>
<li><p>单行if-else语法</p></li>
</ul>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">mean</span><span class="p">(</span><span class="o">*</span><span class="n">seq</span><span class="p">):</span>
<span class="sd">""" 返回函数参数的平均值 """</span>
<span class="k">return</span> <span class="nb">sum</span><span class="p">(</span><span class="n">seq</span><span class="p">)</span> <span class="o">/</span> <span class="nb">len</span><span class="p">(</span><span class="n">seq</span><span class="p">)</span> <span class="k">if</span> <span class="n">seq</span> <span class="k">else</span> <span class="mi">0</span>
</pre></div>
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