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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>10.2. 访问限制</h1><div class="tutorial-body"><p>但是，从前面Student类的定义来看，外部代码还是可以自由地修改一个实例的<code>name</code>、<code>score</code>属性：</p>
<pre class="hljs"><code class="language-plain">>>> bart = Student('Bart Simpson', 59)
>>> bart.score
59
>>> bart.score = 99
>>> bart.score
99
</code></pre>
<p>如果要让内部属性不被外部访问，可以把属性的名称前加上两个下划线<code>__</code>，在Python中，实例的变量名如果以<code>__</code>开头，就变成了一个私有变量（private），只有内部可以访问，外部不能访问，所以，我们把Student类改一改：</p>
<pre class="hljs"><code class="language-python"><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, score</span>):
<span class="hljs-variable language_">self</span>.__name = name
<span class="hljs-variable language_">self</span>.__score = score
<span class="hljs-keyword">def</span> <span class="hljs-title function_">print_score</span>(<span class="hljs-params">self</span>):
<span class="hljs-built_in">print</span>(<span class="hljs-string">'%s: %s'</span> % (<span class="hljs-variable language_">self</span>.__name, <span class="hljs-variable language_">self</span>.__score))
</code></pre>
<p>改完后，对于外部代码来说，没什么变动，但是已经无法从外部访问<code>实例变量.__name</code>和<code>实例变量.__score</code>了：</p>
<pre class="hljs"><code class="language-plain">>>> bart = Student('Bart Simpson', 59)
>>> bart.__name
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'Student' object has no attribute '__name'
</code></pre>
<p>这样就确保了外部代码不能随意修改对象内部的状态，这样通过访问限制的保护，代码更加健壮。</p>
<p>但是如果外部代码要获取name和score怎么办？可以给Student类增加<code>get_name</code>和<code>get_score</code>这样的方法：</p>
<pre class="hljs"><code class="language-python"><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_">get_name</span>(<span class="hljs-params">self</span>):
<span class="hljs-keyword">return</span> <span class="hljs-variable language_">self</span>.__name
<span class="hljs-keyword">def</span> <span class="hljs-title function_">get_score</span>(<span class="hljs-params">self</span>):
<span class="hljs-keyword">return</span> <span class="hljs-variable language_">self</span>.__score
</code></pre>
<p>如果又要允许外部代码修改score怎么办？可以再给Student类增加<code>set_score</code>方法：</p>
<pre class="hljs"><code class="language-python"><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_">set_score</span>(<span class="hljs-params">self, score</span>):
<span class="hljs-variable language_">self</span>.__score = score
</code></pre>
<p>你也许会问，原先那种直接通过<code>bart.score = 99</code>也可以修改啊，为什么要定义一个方法大费周折？因为在方法中，可以对参数做检查，避免传入无效的参数：</p>
<pre class="hljs"><code class="language-python"><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_">set_score</span>(<span class="hljs-params">self, score</span>):
<span class="hljs-keyword">if</span> <span class="hljs-number">0</span> <= score <= <span class="hljs-number">100</span>:
<span class="hljs-variable language_">self</span>.__score = score
<span class="hljs-keyword">else</span>:
<span class="hljs-keyword">raise</span> ValueError(<span class="hljs-string">'bad score'</span>)
</code></pre>
<p>需要注意的是，在Python中，变量名类似<code>__xxx__</code>的，也就是以双下划线开头，并且以双下划线结尾的，是特殊变量，特殊变量是可以直接访问的，不是private变量，所以，不能用<code>__name__</code>、<code>__score__</code>这样的变量名。</p>
<p>有些时候，你会看到以一个下划线开头的实例变量名，比如<code>_name</code>，这样的实例变量外部是可以访问的，但是，按照约定俗成的规定，当你看到这样的变量时，意思就是，“虽然我可以被访问，但是，请把我视为私有变量，不要随意访问”。</p>
<p>双下划线开头的实例变量是不是一定不能从外部访问呢？其实也不是。不能直接访问<code>__name</code>是因为Python解释器对外把<code>__name</code>变量改成了<code>_Student__name</code>，所以，仍然可以通过<code>_Student__name</code>来访问<code>__name</code>变量：</p>
<pre class="hljs"><code class="language-plain">>>> bart._Student__name
'Bart Simpson'
</code></pre>
<p>但是强烈建议你不要这么干，因为不同版本的Python解释器可能会把<code>__name</code>改成不同的变量名。</p>
<p>总的来说就是，Python本身没有任何机制阻止你干坏事，一切全靠自觉。</p>
<p>最后注意下面的这种<em>错误写法</em>：</p>
<pre class="hljs"><code class="language-plain">>>> bart = Student('Bart Simpson', 59)
>>> bart.get_name()
'Bart Simpson'
>>> bart.__name = 'New Name' # 设置__name变量！
>>> bart.__name
'New Name'
</code></pre>
<p>表面上看，外部代码“成功”地设置了<code>__name</code>变量，但实际上这个<code>__name</code>变量和class内部的<code>__name</code>变量<em>不是</em>一个变量！内部的<code>__name</code>变量已经被Python解释器自动改成了<code>_Student__name</code>，而外部代码给<code>bart</code>新增了一个<code>__name</code>变量。不信试试：</p>
<pre class="hljs"><code class="language-plain">>>> bart.get_name() # get_name()内部返回self.__name
'Bart Simpson'
</code></pre>
<h3>练习</h3>
<p>请把下面的<code>Student</code>对象的<code>gender</code>字段对外隐藏起来，用<code>get_gender()</code>和<code>set_gender()</code>代替，并检查参数有效性：</p>
<pre class="hljs"><code class="language-python"><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>, <span class="hljs-string">'male'</span>)
<span class="hljs-keyword">if</span> bart.get_gender() != <span class="hljs-string">'male'</span>:
<span class="hljs-built_in">print</span>(<span class="hljs-string">'测试失败!'</span>)
<span class="hljs-keyword">else</span>:
bart.set_gender(<span class="hljs-string">'female'</span>)
<span class="hljs-keyword">if</span> bart.get_gender() != <span class="hljs-string">'female'</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>
<h3>参考源码</h3>
<p><a href="protected_student.py">protected_student.py</a></p>
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继承和多态
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</a></div><div style="display: flex; justify-content: space-between; margin-top: 32px; padding-top: 20px; border-top: 1px solid var(--border, #e5e7eb);"><a href="39.html" style="color: var(--primary, #3b82f6); text-decoration: none; font-weight: 500;">← 10.1. 类和实例</a><a href="41.html" style="color: var(--primary, #3b82f6); text-decoration: none; font-weight: 500;">10.3. 继承和多态 →</a></div><p style="margin-top: 16px; font-size: 13px; color: var(--text-secondary, #999);">原文：<a href="https://liaoxuefeng.com/books/python/oop/access/index.html" style="color: var(--primary, #3b82f6);">廖雪峰 - 10.2. 访问限制</a></p></div></div>
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