电脑/python/官方内置模块/json模块.html

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<html>
<head>
<!-- 2016-07-30 六 23:05 -->
<meta  charset="utf-8">
<meta  name="viewport" content="width=device-width, initial-scale=1">
<title>json模块</title>
<meta  name="generator" content="Org-mode">
<meta  name="author" content="万泽(德山书生)">
<meta  name="description" content="制作者邮箱：a358003542@gmail.com"
>
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</head>
<body>
<div id="content">
<header>
<h1 class="title">json模块</h1>
</header><nav id="table-of-contents">
<h2>Table of Contents</h2>
<div id="text-table-of-contents">
<ul>
<li><a href="#orgheadline3">1. json存储格式</a>
<ul>
<li><a href="#orgheadline1">1.1. 什么是json</a></li>
<li><a href="#orgheadline2">1.2. json存储格式语法</a></li>
</ul>
</li>
<li><a href="#orgheadline6">2. python的json模块</a>
<ul>
<li><a href="#orgheadline4">2.1. 存储字典值</a></li>
<li><a href="#orgheadline5">2.2. 定义自己的数据类型支持</a></li>
</ul>
</li>
<li><a href="#orgheadline7">3. dumps和loads函数</a></li>
<li><a href="#orgheadline8">4. 参考资料</a></li>
</ul>
</div>
</nav>


<div class="outline-2">
<h2 id="orgheadline3">json存储格式</h2>
<div class="outline-text-2" id="text-1">
</div><div class="outline-3">
<h3 id="orgheadline1">什么是json</h3>
<div class="outline-text-3" id="text-1-1">
<p>
json全称是JavaScript Object Notation，也就是JavaScript对象表示法。json是一种基于文本的，人类易读的数据存储交互格式。json文件保存使用后缀 <strong>.json</strong> 。虽然json是从javascript语言衍生出来，不过其作为数据存储和交互是独立于语言的。json和xml作为数据存储和交互方案相比有更易读和读写速度更快的特点。
</p>
</div>
</div>

<div class="outline-3">
<h3 id="orgheadline2">json存储格式语法</h3>
<div class="outline-text-3" id="text-1-2">
<p>
json存储格式的语法很简单，首先是最基本的数字开始，其支持两种数字类型，整数型和浮点型，其对应于python的int和float；字符串在双引号里面，其对应于python的字符串概念；布尔值true和false，其对应于python的True和False，然后还有一个null对应于python的None；json数据用 []表示，里面的元素用逗号分隔，其对应的正是python的列表概念；然后json的object对象用{}包围，其内是key:value这样的形式，其正对应于python的字典概念。
</p>

<p>
json就可以存储如上描述的这些数据类型，当然可以无限组合下去。这相比较于sql那样单纯的用table表格的形式来描述所有数据已经很灵活了。下面开始实际讲解如何利用python的json模块来读写数据。
</p>
</div>
</div>
</div>


<div class="outline-2">
<h2 id="orgheadline6">python的json模块</h2>
<div class="outline-text-2" id="text-2">
<p>
python语言已经内置了json模块，所以要读写json文件只需要简单 <code>import json</code> 即可。
</p>

<p>
首先让我们小试牛刀，把[1,2,3,4,5]这组数存( <strong>dump</strong> )进test.json文件里面去。
</p>

<div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">json</span>
<span class="n">lst</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">,</span><span class="mi">5</span><span class="p">]</span>

<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">&#39;test.json&#39;</span><span class="p">,</span><span class="n">mode</span><span class="o">=</span><span class="s1">&#39;w&#39;</span><span class="p">,</span><span class="n">encoding</span><span class="o">=</span><span class="s1">&#39;utf-8&#39;</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
    <span class="n">json</span><span class="o">.</span><span class="n">dump</span><span class="p">(</span><span class="n">lst</span><span class="p">,</span><span class="n">f</span><span class="p">)</span>
</pre></div>

<p>
json不支持元组(tuple)<sup><a id="fnr.1" class="footref" href="#fn.1">1</a></sup>和字节(bytes)类型，bytes类型一般不会去惊扰它，如果有tuple元组你需要存储，那么将其转换成列表即可。
</p>

<p>
简单的读取是使用的json的 <strong>load</strong> 函数，如下所示：
</p>

<div class="highlight"><pre><span></span><span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">&#39;test.json&#39;</span><span class="p">,</span> <span class="n">mode</span><span class="o">=</span><span class="s1">&#39;r&#39;</span><span class="p">,</span> <span class="n">encoding</span><span class="o">=</span><span class="s1">&#39;utf-8&#39;</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
    <span class="n">lst2</span> <span class="o">=</span> <span class="n">json</span><span class="o">.</span><span class="n">load</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
</pre></div>

<p>
这样lst2就被赋值[1,2,3,4,5]了，方便后面的运算。
</p>
</div>


<div class="outline-3">
<h3 id="orgheadline4">存储字典值</h3>
<div class="outline-text-3" id="text-2-1">
<p>
上面的例子稍作修改即可以存储字典值了：
</p>
<div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">json</span>
<span class="n">dict01</span> <span class="o">=</span> <span class="p">{</span><span class="s1">&#39;a&#39;</span><span class="p">:</span><span class="mi">1</span><span class="p">,</span><span class="s1">&#39;b&#39;</span><span class="p">:</span><span class="mi">2</span><span class="p">,</span><span class="s1">&#39;c&#39;</span><span class="p">:[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">]}</span>

<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">&#39;test.json&#39;</span><span class="p">,</span> <span class="n">mode</span><span class="o">=</span><span class="s1">&#39;w&#39;</span><span class="p">,</span> <span class="n">encoding</span><span class="o">=</span><span class="s1">&#39;utf-8&#39;</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
    <span class="n">json</span><span class="o">.</span><span class="n">dump</span><span class="p">(</span><span class="n">dict01</span><span class="p">,</span><span class="n">f</span><span class="p">)</span>

<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">&#39;test.json&#39;</span><span class="p">,</span> <span class="n">mode</span><span class="o">=</span><span class="s1">&#39;r&#39;</span><span class="p">,</span> <span class="n">encoding</span><span class="o">=</span><span class="s1">&#39;utf-8&#39;</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
    <span class="n">dict02</span> <span class="o">=</span> <span class="n">json</span><span class="o">.</span><span class="n">load</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>

<span class="k">print</span><span class="p">(</span><span class="n">dict02</span><span class="p">)</span>
</pre></div>

<p>
上面的dump函数那句读者考虑加入 <strong>indent</strong> 选项：
</p>
<div class="highlight"><pre><span></span><span class="n">json</span><span class="o">.</span><span class="n">dump</span><span class="p">(</span><span class="n">dict01</span><span class="p">,</span><span class="n">f</span><span class="p">,</span><span class="n">indent</span><span class="o">=</span><span class="mi">4</span><span class="p">)</span>
</pre></div>

<p>
这样我们的test.json文件里面的数据会更好看一些了。此外 <strong>sort_keys</strong> 选项有时会很有用，默认是False，如果设置为True，则 <span class="underline">如果是字典值</span> 其将按照键来排序。
</p>


<p>
上面我们已经看到了json在引入进来的时候顶层元素一般要某是列表要某是字典值，有的程序员喜欢外围是字典值的情况，而有的喜欢外围是列表值的情况。由于受lxml模块的影响，其扫描xml文档，并将每个元素（node）视作字典，然后同一级的元素放入列表中，最大的元素是一个列表。总的来说我更喜欢这种数据表示方法。
</p>
</div>
</div>

<div class="outline-3">
<h3 id="orgheadline5">定义自己的数据类型支持</h3>
<div class="outline-text-3" id="text-2-2">
<p>
diveintopython3中介绍的time.struct_time类型失效了，最近版本的（python3.4）默认将类型tuple都转成了列表，而似乎time.struct_time实际上就是一个元组类型。
</p>

<p>
下面是关于一个关于复数类型的演示：
</p>
<div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">to_json</span><span class="p">(</span><span class="n">obj</span><span class="p">):</span>
    <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="nb">complex</span><span class="p">):</span>
        <span class="k">return</span> <span class="p">{</span><span class="s1">&#39;__class__&#39;</span><span class="p">:</span> <span class="s1">&#39;complex&#39;</span><span class="p">,</span>
                <span class="s1">&#39;__value__&#39;</span><span class="p">:</span> <span class="p">[</span><span class="n">obj</span><span class="o">.</span><span class="n">real</span><span class="p">,</span> <span class="n">obj</span><span class="o">.</span><span class="n">imag</span><span class="p">]}</span>                       
    <span class="k">raise</span> <span class="ne">TypeError</span><span class="p">(</span><span class="nb">repr</span><span class="p">(</span><span class="n">obj</span><span class="p">)</span> <span class="o">+</span> <span class="s1">&#39; is not JSON serializable&#39;</span><span class="p">)</span>  

<span class="k">def</span> <span class="nf">from_json</span><span class="p">(</span><span class="n">obj</span><span class="p">):</span>                                  
    <span class="k">if</span> <span class="s1">&#39;__class__&#39;</span> <span class="ow">in</span> <span class="n">obj</span><span class="p">:</span>                            
        <span class="k">if</span> <span class="n">obj</span><span class="p">[</span><span class="s1">&#39;__class__&#39;</span><span class="p">]</span> <span class="o">==</span> <span class="s1">&#39;complex&#39;</span><span class="p">:</span>
            <span class="k">return</span> <span class="nb">complex</span><span class="p">(</span><span class="n">obj</span><span class="p">[</span><span class="s1">&#39;__value__&#39;</span><span class="p">][</span><span class="mi">0</span><span class="p">],</span><span class="n">obj</span><span class="p">[</span><span class="s1">&#39;__value__&#39;</span><span class="p">][</span><span class="mi">1</span><span class="p">])</span>          
    <span class="k">return</span> <span class="n">obj</span>
</pre></div>

<div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">json</span>

<span class="kn">from</span> <span class="nn">wanze.json</span> <span class="kn">import</span> <span class="n">to_json</span><span class="p">,</span> <span class="n">from_json</span>

<span class="n">lst</span> <span class="o">=</span> <span class="p">[{</span><span class="s1">&#39;a&#39;</span><span class="p">:</span><span class="mi">1</span><span class="p">,</span><span class="s1">&#39;f&#39;</span><span class="p">:</span><span class="mi">7</span><span class="o">+</span><span class="mi">8j</span><span class="p">,</span><span class="s1">&#39;c&#39;</span><span class="p">:[</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">]},</span><span class="mi">1</span><span class="o">+</span><span class="mi">1</span><span class="p">,</span><span class="mi">3</span><span class="o">+</span><span class="mi">5j</span><span class="p">]</span>

<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">&#39;test.json&#39;</span><span class="p">,</span> <span class="n">mode</span><span class="o">=</span><span class="s1">&#39;w&#39;</span><span class="p">,</span> <span class="n">encoding</span><span class="o">=</span><span class="s1">&#39;utf-8&#39;</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
    <span class="n">json</span><span class="o">.</span><span class="n">dump</span><span class="p">(</span><span class="n">lst</span><span class="p">,</span><span class="n">f</span><span class="p">,</span><span class="n">indent</span><span class="o">=</span><span class="mi">4</span><span class="p">,</span><span class="n">sort_keys</span><span class="o">=</span><span class="bp">True</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="n">to_json</span><span class="p">)</span>

<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">&#39;test.json&#39;</span><span class="p">,</span> <span class="n">mode</span><span class="o">=</span><span class="s1">&#39;r&#39;</span><span class="p">,</span> <span class="n">encoding</span><span class="o">=</span><span class="s1">&#39;utf-8&#39;</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
    <span class="n">lst2</span> <span class="o">=</span> <span class="n">json</span><span class="o">.</span><span class="n">load</span><span class="p">(</span><span class="n">f</span><span class="p">,</span><span class="n">object_hook</span><span class="o">=</span><span class="n">from_json</span><span class="p">)</span>

<span class="k">print</span><span class="p">(</span><span class="n">lst2</span><span class="p">)</span>
</pre></div>

<p>
<strong>default</strong> 选项连接前面定义的函数，这个函数对额外的对象类型进行转变——变成json可以支持的类型。比如这里的复数类型变成了列表输出，然后读取用 <strong>object_hook</strong> 函数连接的函数对obj进行额外的处理，这里的 <code>__class__</code> 还是你自己加上去的标记，然后return成某个python的值，如果不是则直接 <code>return obj</code> ，估计json模块还会有进一步的操作。
</p>

<p>
然后自定义对象按照上面的方法加入额外的条件判断语句即可，经过测试是可行的，目前还没有实用需求，最默认的这些数据类型觉得就听不错了，所以下面的讨论略过。
</p>
</div>
</div>
</div>


<div class="outline-2">
<h2 id="orgheadline7">dumps和loads函数</h2>
<div class="outline-text-2" id="text-3">
<p>
此外json模块还有dumps函数其对应dump函数不过没有文件操作，然后loads函数对应load函数不过没有文件操作。
</p>

<p>
dumps和loads函数可以简单理解为：dumps函数能够简单将python对象字符串化，而loads函数可以简单理解为将某一字符串python对象化。和dump还有load函数比较，其对应的文本数据风格更偏向一行行的相似数据，这些一行行的数据之间彼此并无什么关系（包括简单的顺序关系）；而dump和load函数其对应的文本数据风格更倾向于整个文本数据是一个整体，包括[1,2,3&#x2026;]这样的列表风格数据，其内都暗含顺序关系。
</p>

<p>
对于上面谈论的一行行独立数据的json文件，推荐使用dumps和loads函数来管理。写入json文件如下所示：
</p>

<div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">json</span>

<span class="n">data1</span> <span class="o">=</span> <span class="p">{</span><span class="s1">&#39;color&#39;</span><span class="p">:</span><span class="s1">&#39;red&#39;</span><span class="p">,</span><span class="s1">&#39;count&#39;</span><span class="p">:</span><span class="mi">200</span><span class="p">}</span>
<span class="n">data2</span> <span class="o">=</span> <span class="p">{</span><span class="s1">&#39;color&#39;</span><span class="p">:</span><span class="s1">&#39;yellow&#39;</span><span class="p">,</span><span class="s1">&#39;count&#39;</span><span class="p">:</span><span class="mi">100</span><span class="p">}</span>
<span class="n">data3</span> <span class="o">=</span> <span class="p">{</span><span class="s1">&#39;color&#39;</span><span class="p">:</span><span class="s1">&#39;blue&#39;</span><span class="p">,</span><span class="s1">&#39;count&#39;</span><span class="p">:</span><span class="mi">500</span><span class="p">}</span>

<span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">&#39;test.json&#39;</span><span class="p">,</span><span class="s1">&#39;w&#39;</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
    <span class="k">for</span> <span class="n">data</span> <span class="ow">in</span> <span class="p">[</span><span class="n">data1</span><span class="p">,</span><span class="n">data2</span><span class="p">,</span><span class="n">data3</span><span class="p">]:</span>
        <span class="k">print</span><span class="p">(</span><span class="n">json</span><span class="o">.</span><span class="n">dumps</span><span class="p">(</span><span class="n">data</span><span class="p">),</span><span class="nb">file</span><span class="o">=</span><span class="n">f</span><span class="p">)</span>
</pre></div>

<p>
然后一行行读取如下所示：
</p>
<div class="highlight"><pre><span></span><span class="k">with</span> <span class="nb">open</span><span class="p">(</span><span class="s1">&#39;test.json&#39;</span><span class="p">,</span><span class="s1">&#39;r&#39;</span><span class="p">)</span> <span class="k">as</span> <span class="n">f</span><span class="p">:</span>
    <span class="n">data</span> <span class="o">=</span> <span class="p">[</span><span class="n">json</span><span class="o">.</span><span class="n">loads</span><span class="p">(</span><span class="n">line</span><span class="p">)</span> <span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="n">f</span><span class="p">]</span>
    <span class="k">print</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
</pre></div>
<p>
之后你可以使用列表解析来单独对每一行的每个元素执行某种操作。
</p>
</div>
</div>


<div class="outline-2">
<h2 id="orgheadline8">参考资料</h2>
<div class="outline-text-2" id="text-4">
<ol class="org-ol">
<li><a href="http://www.w3school.com.cn/json/index.asp">w3school的json教程</a></li>
<li><a href="http://www.diveintopython3.net/serializing.html">diveintopython3-serializing</a></li>
<li><a href="https://docs.python.org/3.4/library/json.html">json模块官方文档</a></li>
</ol>
</div>
</div>
<div id="footnotes">
<h2 class="footnotes">Footnotes: </h2>
<div id="text-footnotes">

<div class="footdef"><sup><a id="fn.1" class="footnum" href="#fnr.1">1</a></sup> <div class="footpara">原则上不支持，如果你一定要使用也不会报错，其会对应生成json的数组，但如果你再读取json文件，则又会返回列表，也就是元组默认被当作列表处理了。</div></div>


</div>
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