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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>16.4. base64</h1><div class="tutorial-body"><p>用记事本打开<code>exe</code>、<code>jpg</code>、<code>pdf</code>这些文件时，我们都会看到一大堆乱码，因为二进制文件包含很多无法显示和打印的字符，所以，如果要让记事本这样的文本处理软件能处理二进制数据，就需要一个二进制到字符串的转换方法。Base64是一种最常见的二进制编码方法。</p>
<p>Base64的原理很简单，首先，准备一个包含64个字符的数组：</p>
<pre class="hljs"><code class="language-python">[<span class="hljs-string">'A'</span>, <span class="hljs-string">'B'</span>, <span class="hljs-string">'C'</span>, ... <span class="hljs-string">'a'</span>, <span class="hljs-string">'b'</span>, <span class="hljs-string">'c'</span>, ... <span class="hljs-string">'0'</span>, <span class="hljs-string">'1'</span>, ... <span class="hljs-string">'+'</span>, <span class="hljs-string">'/'</span>]
</code></pre>
<p>然后，对二进制数据进行处理，每3个字节一组，一共是<code>3x8=24</code>bit，划为4组，每组正好6个bit：</p>
<pre class="ascii"><code>┌───────────────┬───────────────┬───────────────┐
│ b1 │ b2 │ b3 │
├─┬─┬─┬─┬─┬─┬─┬─┼─┬─┬─┬─┬─┬─┬─┬─┼─┬─┬─┬─┬─┬─┬─┬─┤
│ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │
├─┴─┴─┴─┴─┴─┼─┴─┴─┴─┴─┴─┼─┴─┴─┴─┴─┴─┼─┴─┴─┴─┴─┴─┤
│ n1 │ n2 │ n3 │ n4 │
└───────────┴───────────┴───────────┴───────────┘
</code></pre>
<p>这样我们得到4个数字作为索引，然后查表，获得相应的4个字符，就是编码后的字符串。</p>
<p>所以，Base64编码会把3字节的二进制数据编码为4字节的文本数据，长度增加33%，好处是编码后的文本数据可以在邮件正文、网页等直接显示。</p>
<p>如果要编码的二进制数据不是3的倍数，最后会剩下1个或2个字节怎么办？Base64用<code>\x00</code>字节在末尾补足后，再在编码的末尾加上1个或2个<code>=</code>号，表示补了多少字节，解码的时候，会自动去掉。</p>
<p>Python内置的<code>base64</code>可以直接进行base64的编解码：</p>
<pre class="hljs"><code class="language-plain">>>> import base64
>>> base64.b64encode(b'binary\x00string')
b'YmluYXJ5AHN0cmluZw=='
>>> base64.b64decode(b'YmluYXJ5AHN0cmluZw==')
b'binary\x00string'
</code></pre>
<p>由于标准的Base64编码后可能出现字符<code>+</code>和<code>/</code>，在URL中就不能直接作为参数，所以又有一种"url safe"的base64编码，其实就是把字符<code>+</code>和<code>/</code>分别变成<code>-</code>和<code>_</code>：</p>
<pre class="hljs"><code class="language-plain">>>> base64.b64encode(b'i\xb7\x1d\xfb\xef\xff')
b'abcd++//'
>>> base64.urlsafe_b64encode(b'i\xb7\x1d\xfb\xef\xff')
b'abcd--__'
>>> base64.urlsafe_b64decode('abcd--__')
b'i\xb7\x1d\xfb\xef\xff'
</code></pre>
<p>还可以自己定义64个字符的排列顺序，这样就可以自定义Base64编码，不过，通常情况下完全没有必要。</p>
<p>Base64是一种通过查表的编码方法，不能用于加密，即使使用自定义的编码表也不行。</p>
<p>Base64适用于小段内容的编码，比如数字证书签名、Cookie的内容等。</p>
<p>由于<code>=</code>字符也可能出现在Base64编码中，但<code>=</code>用在URL、Cookie里面会造成歧义，所以，很多Base64编码后会把<code>=</code>去掉：</p>
<pre class="hljs"><code class="language-plain"># 标准Base64:
'abcd' -> 'YWJjZA=='
# 自动去掉=:
'abcd' -> 'YWJjZA'
</code></pre>
<p>去掉<code>=</code>后怎么解码呢？因为Base64是把3个字节变为4个字节，所以，Base64编码的长度永远是4的倍数，因此，需要加上<code>=</code>把Base64字符串的长度变为4的倍数，就可以正常解码了。</p>
<h3>小结</h3>
<p>Base64是一种任意二进制到文本字符串的编码方法，常用于在URL、Cookie、网页中传输少量二进制数据。</p>
<h3>练习</h3>
<p>请写一个能处理去掉<code>=</code>的base64解码函数：</p>
<pre class="hljs"><code class="language-python"><span class="hljs-keyword">import</span> base64
<span class="hljs-keyword">def</span> <span class="hljs-title function_">safe_base64_decode</span>(<span class="hljs-params">s</span>):
<span class="hljs-keyword">pass</span>
<span class="hljs-comment"># 测试:</span>
<span class="hljs-keyword">assert</span> <span class="hljs-string">b'abcd'</span> == safe_base64_decode(<span class="hljs-string">'YWJjZA=='</span>), safe_base64_decode(<span class="hljs-string">'YWJjZA=='</span>)
<span class="hljs-keyword">assert</span> <span class="hljs-string">b'abcd'</span> == safe_base64_decode(<span class="hljs-string">'YWJjZA'</span>), safe_base64_decode(<span class="hljs-string">'YWJjZA'</span>)
<span class="hljs-built_in">print</span>(<span class="hljs-string">'ok'</span>)
</code></pre>
<h3>参考源码</h3>
<p><a href="do_base64.py">do_base64.py</a></p>
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