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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>11.5. 使用枚举类</h1><div class="tutorial-body"><pre class="hljs"><code class="language-python">JAN = <span class="hljs-number">1</span>
FEB = <span class="hljs-number">2</span>
MAR = <span class="hljs-number">3</span>
...
NOV = <span class="hljs-number">11</span>
DEC = <span class="hljs-number">12</span>
</code></pre>
<p>好处是简单，缺点是类型是<code>int</code>，并且仍然是变量。</p>
<p>更好的方法是为这样的枚举类型定义一个class类型，然后，每个常量都是class的一个唯一实例。Python提供了<code>Enum</code>类来实现这个功能：</p>
<pre class="hljs"><code class="language-python"><span class="hljs-keyword">from</span> enum <span class="hljs-keyword">import</span> Enum
Month = Enum(<span class="hljs-string">'Month'</span>, (<span class="hljs-string">'Jan'</span>, <span class="hljs-string">'Feb'</span>, <span class="hljs-string">'Mar'</span>, <span class="hljs-string">'Apr'</span>, <span class="hljs-string">'May'</span>, <span class="hljs-string">'Jun'</span>, <span class="hljs-string">'Jul'</span>, <span class="hljs-string">'Aug'</span>, <span class="hljs-string">'Sep'</span>, <span class="hljs-string">'Oct'</span>, <span class="hljs-string">'Nov'</span>, <span class="hljs-string">'Dec'</span>))
</code></pre>
<p>这样我们就获得了<code>Month</code>类型的枚举类，可以直接使用<code>Month.Jan</code>来引用一个常量，或者枚举它的所有成员：</p>
<pre class="hljs"><code class="language-python"><span class="hljs-keyword">for</span> name, member <span class="hljs-keyword">in</span> Month.__members__.items():
<span class="hljs-built_in">print</span>(name, <span class="hljs-string">'=>'</span>, member, <span class="hljs-string">','</span>, member.value)
</code></pre>
<p><code>value</code>属性则是自动赋给成员的<code>int</code>常量，默认从<code>1</code>开始计数。</p>
<p>如果需要更精确地控制枚举类型，可以从<code>Enum</code>派生出自定义类：</p>
<pre class="hljs"><code class="language-python"><span class="hljs-keyword">from</span> enum <span class="hljs-keyword">import</span> Enum, unique
<span class="hljs-meta">@unique</span>
<span class="hljs-keyword">class</span> <span class="hljs-title class_">Weekday</span>(<span class="hljs-title class_ inherited__">Enum</span>):
Sun = <span class="hljs-number">0</span> <span class="hljs-comment"># Sun的value被设定为0</span>
Mon = <span class="hljs-number">1</span>
Tue = <span class="hljs-number">2</span>
Wed = <span class="hljs-number">3</span>
Thu = <span class="hljs-number">4</span>
Fri = <span class="hljs-number">5</span>
Sat = <span class="hljs-number">6</span>
</code></pre>
<p><code>@unique</code>装饰器可以帮助我们检查保证没有重复值。</p>
<p>访问这些枚举类型可以有若干种方法：</p>
<pre class="hljs"><code class="language-plain">>>> day1 = Weekday.Mon
>>> print(day1)
Weekday.Mon
>>> print(Weekday.Tue)
Weekday.Tue
>>> print(Weekday['Tue'])
Weekday.Tue
>>> print(Weekday.Tue.value)
2
>>> print(day1 == Weekday.Mon)
True
>>> print(day1 == Weekday.Tue)
False
>>> print(Weekday(1))
Weekday.Mon
>>> print(day1 == Weekday(1))
True
>>> Weekday(7)
Traceback (most recent call last):
...
ValueError: 7 is not a valid Weekday
>>> for name, member in Weekday.__members__.items():
... print(name, '=>', member)
...
Sun => Weekday.Sun
Mon => Weekday.Mon
Tue => Weekday.Tue
Wed => Weekday.Wed
Thu => Weekday.Thu
Fri => Weekday.Fri
Sat => Weekday.Sat
</code></pre>
<p>可见，既可以用成员名称引用枚举常量，又可以直接根据value的值获得枚举常量。</p>
<h3>练习</h3>
<p>把<code>Student</code>的<code>gender</code>属性改造为枚举类型，可以避免使用字符串：</p>
<pre class="hljs"><code class="language-python"><span class="hljs-keyword">from</span> enum <span class="hljs-keyword">import</span> Enum, unique
<span class="hljs-keyword">class</span> <span class="hljs-title class_">Gender</span>(<span class="hljs-title class_ inherited__">Enum</span>):
Male = <span class="hljs-number">0</span>
Female = <span class="hljs-number">1</span>
<span class="hljs-keyword">class</span> <span class="hljs-title class_">Student</span>(<span class="hljs-title class_ inherited__">object</span>):
<span class="hljs-keyword">def</span> <span class="hljs-title function_">__init__</span>(<span class="hljs-params">self, name, gender</span>):
<span class="hljs-variable language_">self</span>.name = name
<span class="hljs-variable language_">self</span>.gender = gender
<span class="hljs-comment"># 测试:</span>
bart = Student(<span class="hljs-string">'Bart'</span>, Gender.Male)
<span class="hljs-keyword">if</span> bart.gender == Gender.Male:
<span class="hljs-built_in">print</span>(<span class="hljs-string">'测试通过!'</span>)
<span class="hljs-keyword">else</span>:
<span class="hljs-built_in">print</span>(<span class="hljs-string">'测试失败!'</span>)
</code></pre>
<h3>小结</h3>
<p><code>Enum</code>可以把一组相关常量定义在一个class中，且class不可变，而且成员可以直接比较。</p>
<h3>参考源码</h3>
<p><a href="use_enum.py">use_enum.py</a></p>
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