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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>16.5. struct</h1><div class="tutorial-body"><p>在Python中，比方说要把一个32位无符号整数变成字节，也就是4个长度的<code>bytes</code>，你得配合位运算符这么写：</p>

<pre class="hljs"><code class="language-plain">>>> n = 10240099

>>> b1 = (n & 0xff000000) >> 24

>>> b2 = (n & 0xff0000) >> 16

>>> b3 = (n & 0xff00) >> 8

>>> b4 = n & 0xff

>>> bs = bytes([b1, b2, b3, b4])

>>> bs

b'\x00\x9c@c'

</code></pre>

<p>非常麻烦。如果换成浮点数就无能为力了。</p>

<p>好在Python提供了一个<code>struct</code>模块来解决<code>bytes</code>和其他二进制数据类型的转换。</p>

<p><code>struct</code>的<code>pack</code>函数把任意数据类型变成<code>bytes</code>：</p>

<pre class="hljs"><code class="language-plain">>>> import struct

>>> struct.pack('>I', 10240099)

b'\x00\x9c@c'

</code></pre>

<p><code>pack</code>的第一个参数是处理指令，<code>'>I'</code>的意思是：</p>

<p><code>></code>表示字节顺序是big-endian，也就是网络序，<code>I</code>表示4字节无符号整数。</p>

<p>后面的参数个数要和处理指令一致。</p>

<p><code>unpack</code>把<code>bytes</code>变成相应的数据类型：</p>

<pre class="hljs"><code class="language-plain">>>> struct.unpack('>IH', b'\xf0\xf0\xf0\xf0\x80\x80')

(4042322160, 32896)

</code></pre>

<p>根据<code>>IH</code>的说明，后面的<code>bytes</code>依次变为<code>I</code>：4字节无符号整数和<code>H</code>：2字节无符号整数。</p>

<p>所以，尽管Python不适合编写底层操作字节流的代码，但在对性能要求不高的地方，利用<code>struct</code>就方便多了。</p>

<p><code>struct</code>模块定义的数据类型可以参考Python官方文档：</p>

<p><a href="https://docs.python.org/3/library/struct.html#format-characters" target="_blank">https://docs.python.org/3/library/struct.html#format-characters</a></p>

<p>Windows的位图文件（.bmp）是一种非常简单的文件格式，我们来用<code>struct</code>分析一下。</p>

<p>首先找一个bmp文件，没有的话用“画图”画一个。</p>

<p>读入前30个字节来分析：</p>

<pre class="hljs"><code class="language-plain">>>> s = b'\x42\x4d\x38\x8c\x0a\x00\x00\x00\x00\x00\x36\x00\x00\x00\x28\x00\x00\x00\x80\x02\x00\x00\x68\x01\x00\x00\x01\x00\x18\x00'

</code></pre>

<p>BMP格式采用小端方式存储数据，文件头的结构按顺序如下：</p>

<p>两个字节：<code>'BM'</code>表示Windows位图，<code>'BA'</code>表示OS/2位图；

一个4字节整数：表示位图大小；

一个4字节整数：保留位，始终为0；

一个4字节整数：实际图像的偏移量；

一个4字节整数：Header的字节数；

一个4字节整数：图像宽度；

一个4字节整数：图像高度；

一个2字节整数：始终为1；

一个2字节整数：颜色数。</p>

<p>所以，组合起来用<code>unpack</code>读取：</p>

<pre class="hljs"><code class="language-plain">>>> struct.unpack('<ccIIIIIIHH', s)

(b'B', b'M', 691256, 0, 54, 40, 640, 360, 1, 24)

</code></pre>

<p>结果显示，<code>b'B'</code>、<code>b'M'</code>说明是Windows位图，位图大小为640x360，颜色数为24。</p>

<p>请编写一个<code>bmpinfo.py</code>，可以检查任意文件是否是位图文件，如果是，打印出图片大小和颜色数。</p>

<pre class="hljs"><code class="language-python"><span class="hljs-keyword">import</span> base64, struct

bmp_data = base64.b64decode(<span class="hljs-string">'Qk1oAgAAAAAAADYAAAAoAAAAHAAAAAoAAAABABAAAAAAADICAAASCwAAEgsAA'</span> +

<span class="hljs-string">'AAAAAAAAAAA/3//f/9//3//f/9//3//f/9//3//f/9//3//f/9//3//f/9//3//f/9//3/'</span> +

<span class="hljs-string">'/f/9//3//f/9//3//f/9/AHwAfAB8AHwAfAB8AHwAfP9//3//fwB8AHwAfAB8/3//f/9/A'</span> +

<span class="hljs-string">'HwAfAB8AHz/f/9//3//f/9//38AfAB8AHwAfAB8AHwAfAB8AHz/f/9//38AfAB8/3//f/9'</span> +

<span class="hljs-string">'//3//fwB8AHz/f/9//3//f/9//3//f/9/AHwAfP9//3//f/9/AHwAfP9//3//fwB8AHz/f'</span> +

<span class="hljs-string">'/9//3//f/9/AHwAfP9//3//f/9//3//f/9//38AfAB8AHwAfAB8AHwAfP9//3//f/9/AHw'</span> +

<span class="hljs-string">'AfP9//3//f/9//38AfAB8/3//f/9//3//f/9//3//fwB8AHwAfAB8AHwAfAB8/3//f/9//'</span> +

<span class="hljs-string">'38AfAB8/3//f/9//3//fwB8AHz/f/9//3//f/9//3//f/9/AHwAfP9//3//f/9/AHwAfP9'</span> +

<span class="hljs-string">'//3//fwB8AHz/f/9/AHz/f/9/AHwAfP9//38AfP9//3//f/9/AHwAfAB8AHwAfAB8AHwAf'</span> +

<span class="hljs-string">'AB8/3//f/9/AHwAfP9//38AfAB8AHwAfAB8AHwAfAB8/3//f/9//38AfAB8AHwAfAB8AHw'</span> +

<span class="hljs-string">'AfAB8/3//f/9/AHwAfAB8AHz/fwB8AHwAfAB8AHwAfAB8AHz/f/9//3//f/9//3//f/9//'</span> +

<span class="hljs-string">'3//f/9//3//f/9//3//f/9//3//f/9//3//f/9//3//f/9//3//f/9//38AAA=='</span>)

<span class="hljs-keyword">def</span> <span class="hljs-title function_">bmp_info</span>(<span class="hljs-params">data</span>):

<span class="hljs-keyword">return</span> {

<span class="hljs-string">'width'</span>: <span class="hljs-number">200</span>,

<span class="hljs-string">'height'</span>: <span class="hljs-number">100</span>,

<span class="hljs-string">'color'</span>: <span class="hljs-number">24</span>

}

bi = bmp_info(bmp_data)

<span class="hljs-keyword">assert</span> bi[<span class="hljs-string">'width'</span>] == <span class="hljs-number">28</span>

<span class="hljs-keyword">assert</span> bi[<span class="hljs-string">'height'</span>] == <span class="hljs-number">10</span>

<span class="hljs-keyword">assert</span> bi[<span class="hljs-string">'color'</span>] == <span class="hljs-number">16</span>

<span class="hljs-built_in">print</span>(<span class="hljs-string">'ok'</span>)

</code></pre>

<p><a href="check_bmp.py">check_bmp.py</a></p>

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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="72.html" style="color: var(--primary, #3b82f6); text-decoration: none; font-weight: 500;">← 16.4. base64</a><a href="74.html" style="color: var(--primary, #3b82f6); text-decoration: none; font-weight: 500;">16.6. hashlib →</a></div><p style="margin-top: 16px; font-size: 13px; color: var(--text-secondary, #999);">原文：<a href="https://liaoxuefeng.com/books/python/built-in-modules/struct/index.html" style="color: var(--primary, #3b82f6);">廖雪峰 - 16.5. struct</a></p></div></div>

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