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<a href=".." >字符串</a>
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<h1 id="2-字符串和编码">2. 字符串和编码</h1>
<h2 id="21-字符编码">2.1 字符编码</h2>
<ul>
<li><code>ASCII</code>编码，早期的编码包含大小写英文字母、数字和一些符号，用一个字节（8位）表示。因编码量少可能会出现乱码问题。</li>
<li><code>Unicode</code>编码，统一各国语言编码，用两个字节（16位）表示。如果文本全是英文，则会比ASCII编码需要多一倍的存储空间，在存储和传输上不划算。</li>
<li><code>UTF-8</code>编码，即可变长的Unicode编码，UTF-8编码把一个Unicode字符根据不同的数字大小编码成1-6个字节，常用的英文字母被编码成1个字节，汉字通常是3个字节，只有很生僻的字符才会被编码成4-6个字节。如果你要传输的文本包含大量英文字符，用UTF-8编码能节省空间。</li>
</ul>
<blockquote>
<p>编码转换工具：<a href="https://tool.chinaz.com/tools/unicode.aspx" target="_blank">https://tool.chinaz.com/tools/unicode.aspx</a></p>
</blockquote>
<h2 id="22-字符编码使用场景">2.2 字符编码使用场景</h2>
<ul>
<li>在计算机内存中，统一使用Unicode编码。</li>
</ul>
<ul>
<li><p>当需要保存到硬盘或者需要传输的时候，就转换为UTF-8编码。</p>
<p>例如：</p>
<p>​ 用记事本编辑的时候，从文件读取的UTF-8字符被转换为Unicode字符到内存里，编辑完成后，保存的时候再把Unicode转换为UTF-8保存到文件。</p>
<p><img src="http://res.cloudinary.com/dqxtn0ick/image/upload/v1512810598/article/python/basics/UnicodeToUTF-8.png" width="40%"></p>
<p>​ 浏览网页的时候，服务器会把动态的生成的Unicode内容转换为UTF-8再传输到浏览器，所以很多网页的源码上会有类似<code><span class="token tag"><span class="token tag"><span class="token punctuation"><</span>meta</span> <span class="token attr-name">charset</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>UTF-8<span class="token punctuation">"</span></span> <span class="token punctuation">/></span></span></code>的信息，表示该网页正是用的UTF-8编码。</p>
<p><img src="http://res.cloudinary.com/dqxtn0ick/image/upload/v1512810599/article/python/basics/website-UTF-8.png" width="40%"></p>
</li>
</ul>
<h2 id="23-python的字符串">2.3 Python的字符串</h2>
<p>​ Python3的版本中，字符串是Unicode编码，即支持多语言。</p>
<p>​ 对于单个字符的编码，Python提供<code>ord()</code>函数获取字符的整数表示，<code>chr()</code>函数把编码转换为对应的字符。例如：</p>
<pre class="language-"><code class="lang-python"><span class="token operator">>></span><span class="token operator">></span> <span class="token builtin">ord</span><span class="token punctuation">(</span><span class="token string">'A'</span><span class="token punctuation">)</span>
<span class="token number">65</span>
<span class="token operator">>></span><span class="token operator">></span> <span class="token builtin">ord</span><span class="token punctuation">(</span><span class="token string">'中'</span><span class="token punctuation">)</span>
<span class="token number">20013</span>
<span class="token operator">>></span><span class="token operator">></span> <span class="token builtin">chr</span><span class="token punctuation">(</span><span class="token number">66</span><span class="token punctuation">)</span>
<span class="token string">'B'</span>
<span class="token operator">>></span><span class="token operator">></span> <span class="token builtin">chr</span><span class="token punctuation">(</span><span class="token number">25991</span><span class="token punctuation">)</span>
<span class="token string">'文'</span>
</code></pre>
<p>如果知道字符的整数编码，可以用十六进制写<code>str</code>：</p>
<pre class="language-"><code class="lang-python"><span class="token operator">>></span><span class="token operator">></span> <span class="token string">'\u4e2d\u6587'</span>
<span class="token string">'中文'</span>
</code></pre>
<h2 id="24-字符串编码转换">2.4 字符串(编码)转换</h2>
<p>​ Python的字符串类型是<code>str</code>，在内存中以Unicode表示，一个字符对应若干个字节。如果要在网络上传输，或者保存到磁盘上，就需要把<code>str</code>变为以字节为单位的<code>bytes</code>(UTF-8/ASCII编码)。</p>
<p>​ Python对<code>bytes</code>类型的数据用带<code>b</code>前缀的单引号或双引号表示：</p>
<pre class="language-"><code class="lang-Python">x = b'ABC'
</code></pre>
<h2 id="241-encodestr→bytes">2.4.1 encode()[str→bytes]</h2>
<p>​ 以Unicode表示的<code>str</code>通过<code>encode()</code>方法可以编码为指定的<code>bytes</code>，即Unicode编码转换成UTF-8编码或ASCII编码，例如：</p>
<pre class="language-"><code class="lang-python"><span class="token operator">>></span><span class="token operator">></span> <span class="token string">'ABC'</span><span class="token punctuation">.</span>encode<span class="token punctuation">(</span><span class="token string">'ascii'</span><span class="token punctuation">)</span>
b<span class="token string">'ABC'</span>
<span class="token operator">>></span><span class="token operator">></span> <span class="token string">'中文'</span><span class="token punctuation">.</span>encode<span class="token punctuation">(</span><span class="token string">'utf-8'</span><span class="token punctuation">)</span>
b<span class="token string">'\xe4\xb8\xad\xe6\x96\x87'</span>
<span class="token comment">#错误例子</span>
<span class="token operator">>></span><span class="token operator">></span> <span class="token string">'中文'</span><span class="token punctuation">.</span>encode<span class="token punctuation">(</span><span class="token string">'ascii'</span><span class="token punctuation">)</span>
Traceback <span class="token punctuation">(</span>most recent call last<span class="token punctuation">)</span><span class="token punctuation">:</span>
File <span class="token string">"<stdin>"</span><span class="token punctuation">,</span> line <span class="token number">1</span><span class="token punctuation">,</span> <span class="token keyword">in</span> <span class="token operator"><</span>module<span class="token operator">></span>
UnicodeEncodeError<span class="token punctuation">:</span> <span class="token string">'ascii'</span> codec can't encode characters <span class="token keyword">in</span> position <span class="token number">0</span><span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">:</span> ordinal <span class="token operator">not</span> <span class="token keyword">in</span> <span class="token builtin">range</span><span class="token punctuation">(</span><span class="token number">128</span><span class="token punctuation">)</span>
</code></pre>
<h2 id="242-decodebytes→str">2.4.2 decode()[bytes→str]</h2>
<p>​ 要把UTF-8编码或ASCII编码转换成Unicode编码，即bytes类型转换为str类型，可用decode()转换。例如：</p>
<pre class="language-"><code class="lang-python"><span class="token operator">>></span><span class="token operator">></span> b<span class="token string">'ABC'</span><span class="token punctuation">.</span>decode<span class="token punctuation">(</span><span class="token string">'ascii'</span><span class="token punctuation">)</span>
<span class="token string">'ABC'</span>
<span class="token operator">>></span><span class="token operator">></span> b<span class="token string">'\xe4\xb8\xad\xe6\x96\x87'</span><span class="token punctuation">.</span>decode<span class="token punctuation">(</span><span class="token string">'utf-8'</span><span class="token punctuation">)</span>
<span class="token string">'中文'</span>
<span class="token comment">#错误例子</span>
<span class="token operator">>></span><span class="token operator">></span> b<span class="token string">'\xe4\xb8\xad\xff'</span><span class="token punctuation">.</span>decode<span class="token punctuation">(</span><span class="token string">'utf-8'</span><span class="token punctuation">)</span>
Traceback <span class="token punctuation">(</span>most recent call last<span class="token punctuation">)</span><span class="token punctuation">:</span>
<span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>
UnicodeDecodeError<span class="token punctuation">:</span> <span class="token string">'utf-8'</span> codec can't decode byte <span class="token number">0xff</span> <span class="token keyword">in</span> position <span class="token number">3</span><span class="token punctuation">:</span> invalid start byte
</code></pre>
<p>​ 如果<code>bytes</code>中只有一小部分无效的字节，可以传入<code>errors='ignore'</code>忽略错误的字节：</p>
<pre class="language-"><code class="lang-python"><span class="token operator">>></span><span class="token operator">></span> b<span class="token string">'\xe4\xb8\xad\xff'</span><span class="token punctuation">.</span>decode<span class="token punctuation">(</span><span class="token string">'utf-8'</span><span class="token punctuation">,</span> errors<span class="token operator">=</span><span class="token string">'ignore'</span><span class="token punctuation">)</span>
<span class="token string">'中'</span>
</code></pre>
<h2 id="243-len">2.4.3 len()</h2>
<p>​ 对于<code>str</code>类型len()计算的是字符数，对于<code>bytes</code>类型len()计算的是字节数。</p>
<pre class="language-"><code class="lang-python"><span class="token comment">#对于str类型</span>
<span class="token operator">>></span><span class="token operator">></span> <span class="token builtin">len</span><span class="token punctuation">(</span><span class="token string">'ABC'</span><span class="token punctuation">)</span>
<span class="token number">3</span>
<span class="token operator">>></span><span class="token operator">></span> <span class="token builtin">len</span><span class="token punctuation">(</span><span class="token string">'中文'</span><span class="token punctuation">)</span>
<span class="token number">2</span>
<span class="token comment">#对于bytes类型</span>
<span class="token operator">>></span><span class="token operator">></span> <span class="token builtin">len</span><span class="token punctuation">(</span>b<span class="token string">'ABC'</span><span class="token punctuation">)</span>
<span class="token number">3</span>
<span class="token operator">>></span><span class="token operator">></span> <span class="token builtin">len</span><span class="token punctuation">(</span>b<span class="token string">'\xe4\xb8\xad\xe6\x96\x87'</span><span class="token punctuation">)</span>
<span class="token number">6</span>
<span class="token operator">>></span><span class="token operator">></span> <span class="token builtin">len</span><span class="token punctuation">(</span><span class="token string">'中文'</span><span class="token punctuation">.</span>encode<span class="token punctuation">(</span><span class="token string">'utf-8'</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token number">6</span>
</code></pre>
<p>​ 1个中文字符经过UTF-8编码后通常会占用3个字节，而1个英文字符只占用1个字节。</p>
<p>​ 为了避免中文乱码，一般使用UTF-8编码对<code>str</code>和<code>bytes</code>进行转换，即Unicode→UTF-8。所以一般在Python源文件开头写以下信息：</p>
<pre class="language-"><code class="lang-python"><span class="token comment">#!/usr/bin/env python3</span>
<span class="token comment"># -*- coding: utf-8 -*-</span>
</code></pre>
<p>第一行注释是为了告诉Linux/OS X系统，这是一个Python可执行程序，Windows系统会忽略这个注释；</p>
<p>第二行注释是为了告诉Python解释器，按照UTF-8编码读取源代码，否则，你在源代码中写的中文输出可能会有乱码。</p>
<h2 id="25-格式化">2.5 格式化</h2>
<h2 id="251-的方式">2.5.1 %的方式</h2>
<p>​ 使用<code>%</code>来格式化字符串，其中<code>%s</code>永远起作用，可以把任何数据类型转换成字符串，如果字符串中包含<code>%</code>，则使用<code>%%</code>转义表示字符串中的<code>%</code>。常用占位符如下：</p>
<table>
<thead>
<tr>
<th>占位符</th>
<th>替换内容</th>
</tr>
</thead>
<tbody>
<tr>
<td>%s</td>
<td>字符串</td>
</tr>
<tr>
<td>%d</td>
<td>整数</td>
</tr>
<tr>
<td>%f</td>
<td>浮点数</td>
</tr>
<tr>
<td>%x</td>
<td>十六进制整数</td>
</tr>
</tbody>
</table>
<p>例如：</p>
<pre class="language-"><code class="lang-python"><span class="token operator">>></span><span class="token operator">></span> <span class="token string">'Hello, %s'</span> <span class="token operator">%</span> <span class="token string">'world'</span>
<span class="token string">'Hello, world'</span>
<span class="token operator">>></span><span class="token operator">></span> <span class="token string">'Hi, %s, you have $%d.'</span> <span class="token operator">%</span> <span class="token punctuation">(</span><span class="token string">'Michael'</span><span class="token punctuation">,</span> <span class="token number">1000000</span><span class="token punctuation">)</span>
<span class="token string">'Hi, Michael, you have $1000000.'</span>
<span class="token comment">#整数和浮点数</span>
<span class="token operator">>></span><span class="token operator">></span> <span class="token keyword">print</span><span class="token punctuation">(</span><span class="token string">'%2d-%02d'</span> <span class="token operator">%</span> <span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token operator">>></span><span class="token operator">></span> <span class="token keyword">print</span><span class="token punctuation">(</span><span class="token string">'%.2f'</span> <span class="token operator">%</span> <span class="token number">3.1415926</span><span class="token punctuation">)</span>
<span class="token comment"># %s的通用性</span>
<span class="token operator">>></span><span class="token operator">></span> <span class="token string">'Age: %s. Gender: %s'</span> <span class="token operator">%</span> <span class="token punctuation">(</span><span class="token number">25</span><span class="token punctuation">,</span> <span class="token boolean">True</span><span class="token punctuation">)</span>
<span class="token string">'Age: 25. Gender: True'</span>
<span class="token comment">#转义字符串中的%</span>
<span class="token operator">>></span><span class="token operator">></span> <span class="token string">'growth rate: %d %%'</span> <span class="token operator">%</span> <span class="token number">7</span>
<span class="token string">'growth rate: 7 %'</span>
</code></pre>
<h2 id="252-format">2.5.2 format()</h2>
<p>​ 可以使用字符串的<code>format()</code>方法，它会用传入的参数依次替换字符串内的占位符<code>{0}</code>、<code>{1}</code>……。但建议直接使用<code>%</code>的方式。</p>
<pre class="language-"><code class="lang-python"><span class="token operator">>></span><span class="token operator">></span> <span class="token string">'Hello, {0}, 成绩提升了 {1:.1f}%'</span><span class="token punctuation">.</span><span class="token builtin">format</span><span class="token punctuation">(</span><span class="token string">'小明'</span><span class="token punctuation">,</span> <span class="token number">17.125</span><span class="token punctuation">)</span>
<span class="token string">'Hello, 小明, 成绩提升了 17.1%'</span>
</code></pre>
<h2 id="26-json-中的中文解析">2.6 json 中的中文解析</h2>
<p>使用<code>json.dumps(data, ensure_ascii=False)</code>解析json中的中文。</p>
<pre class="language-"><code class="lang-python"><span class="token comment">#!/usr/bin/python</span>
<span class="token comment"># -*- coding: utf8 -*-</span>
<span class="token keyword">import</span> json
<span class="token keyword">import</span> sys
<span class="token builtin">reload</span><span class="token punctuation">(</span>sys<span class="token punctuation">)</span>
sys<span class="token punctuation">.</span>setdefaultencoding<span class="token punctuation">(</span><span class="token string">'utf-8'</span><span class="token punctuation">)</span>
data <span class="token operator">=</span> <span class="token punctuation">[</span> <span class="token punctuation">{</span> <span class="token string">'a'</span> <span class="token punctuation">:</span> <span class="token string">'中文'</span> <span class="token punctuation">}</span> <span class="token punctuation">]</span>
result <span class="token operator">=</span> json<span class="token punctuation">.</span>dumps<span class="token punctuation">(</span>data<span class="token punctuation">,</span> ensure_ascii<span class="token operator">=</span><span class="token boolean">False</span><span class="token punctuation">)</span>
<span class="token keyword">print</span> result
</code></pre>
<p>输出结果：</p>
<pre class="language-"><code class="lang-bash"><span class="token punctuation">[</span><span class="token punctuation">{</span><span class="token string">"a"</span><span class="token keyword">:</span> <span class="token string">"中文"</span><span class="token punctuation">}</span><span class="token punctuation">]</span>
</code></pre>
<p>参考：</p>
<ul>
<li><a href="https://www.liaoxuefeng.com/wiki/1016959663602400/1017075323632896" target="_blank">https://www.liaoxuefeng.com/wiki/1016959663602400/1017075323632896</a></li>
</ul>
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