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<div class="tutorial-container" style="max-width: 960px; margin: 80px auto 40px; padding: 0 20px;"><div class="tutorial-sidebar"><h3>目录</h3><a href="0.html">1. 简介</a><a href="1.html">2. Python历史</a><a href="2.html">3. 安装Python</a><a href="3.html">3.1. Python解释器</a><a href="4.html">4. 第一个Python程序</a><a href="5.html">4.1. 使用文本编辑器</a><a href="6.html">4.2. 输入和输出</a><a href="7.html">5. Python基础</a><a href="8.html">5.1. 数据类型和变量</a><a href="9.html">5.2. 字符串和编码</a><a href="10.html">5.3. 使用list和tuple</a><a href="11.html">5.4. 条件判断</a><a href="12.html">5.5. 模式匹配</a><a href="13.html">5.6. 循环</a><a href="14.html">5.7. 使用dict和set</a><a href="15.html">6. 函数</a><a href="16.html">6.1. 调用函数</a><a href="17.html">6.2. 定义函数</a><a href="18.html">6.3. 函数的参数</a><a href="19.html">6.4. 递归函数</a><a href="20.html">7. 高级特性</a><a href="21.html">7.1. 切片</a><a href="22.html">7.2. 迭代</a><a href="23.html">7.3. 列表生成式</a><a href="24.html">7.4. 生成器</a><a href="25.html">7.5. 迭代器</a><a href="26.html">8. 函数式编程</a><a href="27.html">8.1. 高阶函数</a><a href="28.html">8.1.1. map/reduce</a><a href="29.html">8.1.2. filter</a><a href="30.html">8.1.3. sorted</a><a href="31.html">8.2. 返回函数</a><a href="32.html">8.3. 匿名函数</a><a href="33.html">8.4. 装饰器</a><a href="34.html">8.5. 偏函数</a><a href="35.html">9. 模块</a><a href="36.html">9.1. 使用模块</a><a href="37.html">9.2. 安装第三方模块</a><a href="38.html">10. 面向对象编程</a><a href="39.html">10.1. 类和实例</a><a href="40.html">10.2. 访问限制</a><a href="41.html">10.3. 继承和多态</a><a href="42.html">10.4. 获取对象信息</a><a href="43.html">10.5. 实例属性和类属性</a><a href="44.html">11. 面向对象高级编程</a><a href="45.html">11.1. 使用__slots__</a><a href="46.html">11.2. 使用@property</a><a href="47.html">11.3. 多重继承</a><a href="48.html">11.4. 定制类</a><a href="49.html">11.5. 使用枚举类</a><a href="50.html">11.6. 使用元类</a><a href="51.html">12. 错误、调试和测试</a><a href="52.html">12.1. 错误处理</a><a href="53.html">12.2. 调试</a><a href="54.html">12.3. 单元测试</a><a href="55.html">12.4. 文档测试</a><a href="56.html">13. IO编程</a><a href="57.html">13.1. 文件读写</a><a href="58.html">13.2. StringIO和BytesIO</a><a href="59.html">13.3. 操作文件和目录</a><a href="60.html">13.4. 序列化</a><a href="61.html">14. 进程和线程</a><a href="62.html">14.1. 多进程</a><a href="63.html">14.2. 多线程</a><a href="64.html">14.3. ThreadLocal</a><a href="65.html">14.4. 进程 vs. 线程</a><a href="66.html">14.5. 分布式进程</a><a href="67.html">15. 正则表达式</a><a href="68.html">16. 常用内建模块</a><a href="69.html">16.1. datetime</a><a href="70.html">16.2. collections</a><a href="71.html">16.3. argparse</a><a href="72.html">16.4. base64</a><a href="73.html">16.5. struct</a><a href="74.html">16.6. hashlib</a><a href="75.html">16.7. hmac</a><a href="76.html">16.8. itertools</a><a href="77.html">16.9. contextlib</a><a href="78.html">16.10. urllib</a><a href="79.html">16.11. XML</a><a href="80.html">16.12. HTMLParser</a><a href="81.html">16.13. venv</a><a href="82.html">17. 常用第三方模块</a><a href="83.html">17.1. Pillow</a><a href="84.html">17.2. requests</a><a href="85.html">17.3. chardet</a><a href="86.html">17.4. psutil</a><a href="87.html">18. 图形界面</a><a href="88.html">18.1. 海龟绘图</a><a href="89.html">19. 网络编程</a><a href="90.html">19.1. TCP/IP简介</a><a href="91.html">19.2. TCP编程</a><a href="92.html">19.3. UDP编程</a><a href="93.html">20. 电子邮件</a><a href="94.html">20.1. SMTP发送邮件</a><a href="95.html">20.2. POP3收取邮件</a><a href="96.html">21. 访问数据库</a><a href="97.html">21.1. 使用SQLite</a><a href="98.html">21.2. 使用MySQL</a><a href="99.html">21.3. 使用SQLAlchemy</a><a href="100.html">22. Web开发</a><a href="101.html">22.1. HTTP协议简介</a><a href="102.html">22.2. HTML简介</a><a href="103.html">22.3. WSGI接口</a><a href="104.html">22.4. 使用Web框架</a><a href="105.html">22.5. 使用模板</a><a href="106.html">23. 异步IO</a><a href="107.html">23.1. 协程</a><a href="108.html">23.2. 使用asyncio</a><a href="109.html">23.3. 使用aiohttp</a><a href="110.html">24. FAQ</a><a href="111.html">25. 期末总结</a></div><div class="tutorial-content"><h1>7.2. 迭代</h1><div class="tutorial-body"><p>在Python中，迭代是通过<code>for ... in</code>来完成的，而很多语言比如C语言，迭代<code>list</code>是通过下标完成的，比如C代码：</p>

<pre class="hljs"><code class="language-python"><span class="hljs-keyword">for</span> (i=<span class="hljs-number">0</span>; i<length; i++) {

n = <span class="hljs-built_in">list</span>[i];

}

</code></pre>

<p>可以看出，Python的<code>for</code>循环抽象程度要高于C的<code>for</code>循环，因为Python的<code>for</code>循环不仅可以用在<code>list</code>或<code>tuple</code>上，还可以作用在其他可迭代对象上。</p>

<p><code>list</code>这种数据类型虽然有下标，但很多其他数据类型是没有下标的，但是，只要是可迭代对象，无论有无下标，都可以迭代，比如<code>dict</code>就可以迭代：</p>

<pre class="hljs"><code class="language-plain">>>> d = {'a': 1, 'b': 2, 'c': 3}

>>> for key in d:

... print(key)

...

</code></pre>

<p>因为<code>dict</code>的存储不是按照<code>list</code>的方式顺序排列，所以，迭代出的结果顺序很可能不一样。</p>

<p>默认情况下，<code>dict</code>迭代的是key。如果要迭代value，可以用<code>for value in d.values()</code>，如果要同时迭代key和value，可以用<code>for k, v in d.items()</code>。</p>

<p>由于字符串也是可迭代对象，因此，也可以作用于<code>for</code>循环：</p>

<pre class="hljs"><code class="language-plain">>>> for ch in 'ABC':

... print(ch)

...

</code></pre>

<p>所以，当我们使用<code>for</code>循环时，只要作用于一个可迭代对象，<code>for</code>循环就可以正常运行，而我们不太关心该对象究竟是<code>list</code>还是其他数据类型。</p>

<p>那么，如何判断一个对象是可迭代对象呢？方法是通过<code>collections.abc</code>模块的<code>Iterable</code>类型判断：</p>

<pre class="hljs"><code class="language-plain">>>> from collections.abc import Iterable

>>> isinstance('abc', Iterable) # str是否可迭代

True

>>> isinstance([1,2,3], Iterable) # list是否可迭代

True

>>> isinstance(123, Iterable) # 整数是否可迭代

False

</code></pre>

<p>最后一个小问题，如果要对<code>list</code>实现类似Java那样的下标循环怎么办？Python内置的<code>enumerate</code>函数可以把一个<code>list</code>变成索引-元素对，这样就可以在<code>for</code>循环中同时迭代索引和元素本身：</p>

<pre class="hljs"><code class="language-plain">>>> for i, value in enumerate(['A', 'B', 'C']):

... print(i, value)

...

0 A

1 B

2 C

</code></pre>

<p>上面的<code>for</code>循环里，同时引用了两个变量，在Python里是很常见的，比如下面的代码：</p>

<pre class="hljs"><code class="language-plain">>>> for x, y in [(1, 1), (2, 4), (3, 9)]:

... print(x, y)

...

1 1

2 4

3 9

</code></pre>

<p>请使用迭代查找一个list中最小和最大值，并返回一个tuple：</p>

<pre class="hljs"><code class="language-python"><span class="hljs-keyword">def</span> <span class="hljs-title function_">findMinAndMax</span>(<span class="hljs-params">L</span>):

<span class="hljs-keyword">return</span> (<span class="hljs-literal">None</span>, <span class="hljs-literal">None</span>)

<span class="hljs-keyword">if</span> findMinAndMax([]) != (<span class="hljs-literal">None</span>, <span class="hljs-literal">None</span>):

<span class="hljs-built_in">print</span>(<span class="hljs-string">'测试失败!'</span>)

<span class="hljs-keyword">elif</span> findMinAndMax([<span class="hljs-number">7</span>]) != (<span class="hljs-number">7</span>, <span class="hljs-number">7</span>):

<span class="hljs-built_in">print</span>(<span class="hljs-string">'测试失败!'</span>)

<span class="hljs-keyword">elif</span> findMinAndMax([<span class="hljs-number">7</span>, <span class="hljs-number">1</span>]) != (<span class="hljs-number">1</span>, <span class="hljs-number">7</span>):

<span class="hljs-built_in">print</span>(<span class="hljs-string">'测试失败!'</span>)

<span class="hljs-keyword">elif</span> findMinAndMax([<span class="hljs-number">7</span>, <span class="hljs-number">1</span>, <span class="hljs-number">3</span>, <span class="hljs-number">9</span>, <span class="hljs-number">5</span>]) != (<span class="hljs-number">1</span>, <span class="hljs-number">9</span>):

<span class="hljs-built_in">print</span>(<span class="hljs-string">'测试失败!'</span>)

<span class="hljs-keyword">else</span>:

<span class="hljs-built_in">print</span>(<span class="hljs-string">'测试成功!'</span>)

</code></pre>

<p>任何可迭代对象都可以作用于<code>for</code>循环，包括我们自定义的数据类型，只要符合迭代条件，就可以使用<code>for</code>循环。</p>

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