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<div class="section" id="Python的“Itertools”">
<h1>Python的“Itertools”<a class="headerlink" href="#Python的“Itertools”" title="永久链接至标题">¶</a></h1>
<p>Python的<a class="reference external" href="https://docs.python.org/3/library/itertools.html">itertools模组</a>提供了一套快速的高效使用内存的创建迭代器的核心工具。我会将在本节快速地展示一些itertool的工具。大部分这些函数都会创建<a class="reference external" href="https://cn.pythonlikeyoumeanit.com/Module2_EssentialsOfPython/Generators_and_Comprehensions.html">生成器</a>，因此我们将会需要迭代这些生成器才能展示它们的使用方法。此模组极其有用——本文强烈建议你花一些时间去了解该模组的内容。</p>
<p>Python有三个属于itertools的内置函数 <code class="docutils literal notranslate"><span class="pre">range</span></code>，<code class="docutils literal notranslate"><span class="pre">enumerate</span></code>，<code class="docutils literal notranslate"><span class="pre">zip</span></code>，但因为它们极其有用，所以Python允许你在不导入它们的情况下直接就可以使用这些函数。熟练使用 <code class="docutils literal notranslate"><span class="pre">range</span></code>，<code class="docutils literal notranslate"><span class="pre">enumerate</span></code>，和 <code class="docutils literal notranslate"><span class="pre">zip</span></code> 是很重要的技能。</p>
<p><strong>range</strong></p>
<p>生成包含描述的“界限”（range）中的整数列表：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 将会生成 0.. 1.. 2.. ... 8.. 9</span>
<span class="o">>>></span> <span class="nb">range</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span>
<span class="nb">range</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">10</span><span class="p">)</span>
<span class="o">>>></span> <span class="nb">list</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">10</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="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="mi">6</span><span class="p">,</span> <span class="mi">7</span><span class="p">,</span> <span class="mi">8</span><span class="p">,</span> <span class="mi">9</span><span class="p">]</span>
<span class="c1"># 将会生成 0.. 3.. 6.. 9</span>
<span class="o">>>></span> <span class="nb">range</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">10</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>
<span class="nb">range</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">10</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>
<span class="o">>>></span> <span class="nb">list</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">10</span><span class="p">,</span> <span class="mi">3</span><span class="p">))</span>
<span class="p">[</span><span class="mi">0</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">6</span><span class="p">,</span> <span class="mi">9</span><span class="p">]</span>
</pre></div>
</div>
<p><strong>enumerate</strong></p>
<p>枚举可迭代物中的对象：每轮迭代提供一元组，其内容为迭代轮数（从0开始）以及对应的可迭代物成员。</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="c1"># 将会生成 (0, 'apple').. (1, 'banana').. (2, 'cat').. (3, 'dog')]</span>
<span class="o">>>></span> <span class="nb">enumerate</span><span class="p">([</span><span class="s2">"apple"</span><span class="p">,</span> <span class="s2">"banana"</span><span class="p">,</span> <span class="s2">"cat"</span><span class="p">,</span> <span class="s2">"dog"</span><span class="p">])</span>
<span class="o"><</span><span class="nb">enumerate</span> <span class="n">at</span> <span class="mh">0x23e3557b3f0</span><span class="o">></span>
<span class="o">>>></span> <span class="nb">list</span><span class="p">(</span><span class="nb">enumerate</span><span class="p">([</span><span class="s2">"apple"</span><span class="p">,</span> <span class="s2">"banana"</span><span class="p">,</span> <span class="s2">"cat"</span><span class="p">,</span> <span class="s2">"dog"</span><span class="p">]))</span>
<span class="p">[(</span><span class="mi">0</span><span class="p">,</span> <span class="s1">'apple'</span><span class="p">),</span> <span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="s1">'banana'</span><span class="p">),</span> <span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="s1">'cat'</span><span class="p">),</span> <span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="s1">'dog'</span><span class="p">)]</span>
</pre></div>
</div>
<p><strong>zip</strong></p>
<p>将多个可迭代物的对应成员拉链（zip）到元组中。这为凑对多个可迭代物中对象很有用。</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">names</span> <span class="o">=</span> <span class="p">[</span><span class="s2">"Angie"</span><span class="p">,</span> <span class="s2">"Brian"</span><span class="p">,</span> <span class="s2">"Cassie"</span><span class="p">,</span> <span class="s2">"David"</span><span class="p">]</span>
<span class="gp">>>> </span><span class="n">exam_1_scores</span> <span class="o">=</span> <span class="p">[</span><span class="mi">90</span><span class="p">,</span> <span class="mi">82</span><span class="p">,</span> <span class="mi">79</span><span class="p">,</span> <span class="mi">87</span><span class="p">]</span>
<span class="gp">>>> </span><span class="n">exam_2_scores</span> <span class="o">=</span> <span class="p">[</span><span class="mi">95</span><span class="p">,</span> <span class="mi">84</span><span class="p">,</span> <span class="mi">72</span><span class="p">,</span> <span class="mi">91</span><span class="p">]</span>
<span class="go"># 将会生成 ('Angie', 90, 95).. ('Brian', 82, 84).. ('Cassie', 79, 72).. ('David', 87, 91)]</span>
<span class="gp">>>> </span><span class="nb">zip</span><span class="p">(</span><span class="n">names</span><span class="p">,</span> <span class="n">exam_1_scores</span><span class="p">,</span> <span class="n">exam_2_scores</span><span class="p">)</span>
<span class="go"><zip at 0x20de1082608></span>
<span class="gp">>>> </span><span class="nb">list</span><span class="p">(</span><span class="nb">zip</span><span class="p">(</span><span class="n">names</span><span class="p">,</span> <span class="n">exam_1_scores</span><span class="p">,</span> <span class="n">exam_2_scores</span><span class="p">))</span>
<span class="go">[('Angie', 90, 95), ('Brian', 82, 84), ('Cassie', 79, 72), ('David', 87, 91)]</span>
</pre></div>
</div>
<hr class="docutils" />
<p>以下是一些 <code class="docutils literal notranslate"><span class="pre">itertools</span></code> 模组中有用的函数：</p>
<p><strong>itertools.chain</strong></p>
<p>将多个可迭代物背靠背连接在一起，返回单个可迭代物：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">itertools</span> <span class="kn">import</span> <span class="n">chain</span>
<span class="gp">>>> </span><span class="n">gen_1</span> <span class="o">=</span> <span class="nb">range</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span> <span class="c1"># 0.. 2.. 4</span>
<span class="gp">>>> </span><span class="n">gen_2</span> <span class="o">=</span> <span class="p">(</span><span class="n">i</span><span class="o">**</span><span class="mi">2</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span> <span class="mi">6</span><span class="p">))</span> <span class="c1"># 9.. 16.. 25</span>
<span class="gp">>>> </span><span class="n">iter_3</span> <span class="o">=</span> <span class="p">[</span><span class="s2">"moo"</span><span class="p">,</span> <span class="s2">"cow"</span><span class="p">]</span>
<span class="gp">>>> </span><span class="n">iter_4</span> <span class="o">=</span> <span class="s2">"him"</span>
<span class="go"># 将会生成：0.. 2.. 4.. 9.. 16.. 25.. 'moo'.. 'cow'.. 'h'.. 'i'.. 'm'</span>
<span class="gp">>>> </span><span class="n">chain</span><span class="p">(</span><span class="n">gen_1</span><span class="p">,</span> <span class="n">gen_2</span><span class="p">,</span> <span class="n">iter_3</span><span class="p">,</span> <span class="n">iter_4</span><span class="p">)</span>
<span class="go"><itertools.chain at 0x20de109ec18></span>
</pre></div>
</div>
<p><strong>itertools.combinations</strong> 生成所有长度为n的可迭代物成员“组合”：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">itertools</span> <span class="kn">import</span> <span class="n">combinations</span>
<span class="go"># 将会生成：(0, 1, 2).. (0, 1, 3).. (0, 2, 3).. (1, 2, 3)</span>
<span class="gp">>>> </span><span class="n">combinations</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="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">],</span> <span class="mi">3</span><span class="p">)</span> <span class="c1"># 从 [0, 1, 2, 3] 的数字生成所有长度为3的组合</span>
<span class="go"><itertools.combinations at 0x20de10a7728></span>
</pre></div>
</div>
<div class="admonition note">
<p class="admonition-title fa fa-exclamation-circle"><strong>阅读理解：Itertools第一部分</strong></p>
<p>使用 <code class="docutils literal notranslate"><span class="pre">itertools.combinations</span></code> 函数来找出从列表 <code class="docutils literal notranslate"><span class="pre">["apples",</span> <span class="pre">"bananas",</span> <span class="pre">"pears",</span> <span class="pre">"pears",</span> <span class="pre">"oranges"]</span></code> 随机选择两个成员会提供“apples”和“pears”组合的几率。</p>
</div>
<div class="admonition note">
<p class="admonition-title fa fa-exclamation-circle"><strong>阅读理解：Itertools第二部分</strong></p>
<p>设列表 <code class="docutils literal notranslate"><span class="pre">x_vals</span> <span class="pre">=</span> <span class="pre">[0.1,</span> <span class="pre">0.3,</span> <span class="pre">0.6,</span> <span class="pre">0.9]</span></code>，创建一个生成器 <code class="docutils literal notranslate"><span class="pre">y_gen</span></code> 来生成 <span class="math notranslate nohighlight">\(y = x^2\)</span> 在每一个 <span class="math notranslate nohighlight">\(x\)</span> 对应的的y值。然后，使用 <code class="docutils literal notranslate"><span class="pre">zip</span></code> 来创建一个包含 <span class="math notranslate nohighlight">\((x, y)\)</span> 元组的列表。</p>
</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#typesseq-range">range</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/library/functions.html#enumerate">enumerate</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/library/functions.html#zip">zip</a></p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/library/itertools.html">itertools</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>Itertools第一部分：解</strong></p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">itertools</span> <span class="kn">import</span> <span class="n">combinations</span>
<span class="gp">>>> </span><span class="n">ls</span> <span class="o">=</span> <span class="p">[</span><span class="s2">"apples"</span><span class="p">,</span> <span class="s2">"bananas"</span><span class="p">,</span> <span class="s2">"pears"</span><span class="p">,</span> <span class="s2">"pears"</span><span class="p">,</span> <span class="s2">"oranges"</span><span class="p">]</span>
<span class="gp">>>> </span><span class="n">comb_ls</span> <span class="o">=</span> <span class="nb">list</span><span class="p">(</span><span class="n">combinations</span><span class="p">(</span><span class="n">ls</span><span class="p">,</span> <span class="mi">2</span><span class="p">))</span>
<span class="gp">>>> </span><span class="n">comb_ls</span><span class="o">.</span><span class="n">count</span><span class="p">((</span><span class="s2">"apples"</span><span class="p">,</span> <span class="s2">"pears"</span><span class="p">))</span> <span class="o">/</span> <span class="nb">len</span><span class="p">(</span><span class="n">comb_ls</span><span class="p">)</span>
<span class="go">0.2</span>
</pre></div>
</div>
<p><strong>Itertools第二部分：解</strong></p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">x_vals</span> <span class="o">=</span> <span class="p">[</span><span class="mf">0.1</span><span class="p">,</span> <span class="mf">0.3</span><span class="p">,</span> <span class="mf">0.6</span><span class="p">,</span> <span class="mf">0.9</span><span class="p">]</span>
<span class="gp">>>> </span><span class="n">y_gen</span> <span class="o">=</span> <span class="p">(</span><span class="n">x</span><span class="o">**</span><span class="mi">2</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">x_vals</span><span class="p">)</span>
<span class="gp">>>> </span><span class="nb">list</span><span class="p">(</span><span class="nb">zip</span><span class="p">(</span><span class="n">x_vals</span><span class="p">,</span> <span class="n">y_gen</span><span class="p">))</span>
<span class="go">[(0.1, 0.01), (0.3, 0.09), (0.6, 0.36), (0.9, 0.81)]</span>
</pre></div>
</div>
<p>在这个情况下，使用 <code class="docutils literal notranslate"><span class="pre">zip</span></code> 其实有点没必要的复杂化了。我们完全可以跳过创建 <code class="docutils literal notranslate"><span class="pre">y_gen</span></code> 并使用列表理解：</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">x_vals</span> <span class="o">=</span> <span class="p">[</span><span class="mf">0.1</span><span class="p">,</span> <span class="mf">0.3</span><span class="p">,</span> <span class="mf">0.6</span><span class="p">,</span> <span class="mf">0.9</span><span class="p">]</span>
<span class="gp">>>> </span><span class="p">[(</span><span class="n">x</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="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">x_vals</span><span class="p">]</span>
<span class="go">[(0.1, 0.01), (0.3, 0.09), (0.6, 0.36), (0.9, 0.81)]</span>
</pre></div>
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