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<h1 class="entry-title">Python with语句</h1>
<p class="meta"><time datetime="2014-07-03T00:00:00+08:00" pubdate>Thu 03 July 2014</time></p>
</header>
<div class="entry-content"><p>感谢<a href="http://sdqali.in/blog/2012/07/09/understanding-pythons-with/">Sadique Ali</a>的总结分享</p>
<h3>With语句是什么？</h3>
<p>有一些任务，可能事先需要设置，事后做清理工作。对于这种场景，Python的with语句提供了一种非常方便的处理方式。 <br />
一个很好的例子是文件处理，你需要获取一个文件句柄，从文件中读取数据，然后关闭文件句柄。</p>
<p>如果不用with语句，代码如下：</p>
<div class="highlight"><pre><span class="n">file</span> <span class="o">=</span> <span class="n">open</span><span class="p">(</span><span class="s">"/tmp/foo.txt"</span><span class="p">)</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">file</span><span class="p">.</span><span class="n">read</span><span class="p">()</span>
<span class="n">file</span><span class="p">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
<p>这里有两个问题。一是可能忘记关闭文件句柄；二是文件读取数据发生异常，没有进行任何处理。下面是处理异常的加强版本：</p>
<div class="highlight"><pre><span class="n">file</span> <span class="o">=</span> <span class="n">open</span><span class="p">(</span><span class="s">"/tmp/foo.txt"</span><span class="p">)</span>
<span class="nl">try:</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">file</span><span class="p">.</span><span class="n">read</span><span class="p">()</span>
<span class="nl">finally:</span>
<span class="n">file</span><span class="p">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
<p>虽然这段代码运行良好，但是太冗长了。这时候就是with一展身手的时候了。除了有更优雅的语法，with还可以很好的处理上下文环境产生的异常。下面是with版本的代码：</p>
<div class="highlight"><pre><span class="n">with</span> <span class="n">open</span><span class="p">(</span><span class="s">"/tmp/foo.txt"</span><span class="p">)</span> <span class="n">as</span> <span class="n">file</span><span class="o">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">file</span><span class="p">.</span><span class="n">read</span><span class="p">()</span>
</pre></div>
<h3>with如何工作？</h3>
<p>这看起来充满魔法，但不仅仅是魔法，Python对with的处理还很聪明。基本思想是with所求值的对象必须有一个__enter__()方法，一个__exit__()方法。 </p>
<p>紧跟with后面的语句被求值后，返回对象的__enter__()方法被调用，这个方法的返回值将被赋值给as后面的变量。当with后面的代码块全部被执行完之后，将调用前面返回对象的__exit__()方法。</p>
<p>下面例子可以具体说明with如何工作：</p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre> 1
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19</pre></div></td><td class="code"><div class="highlight"><pre><span class="c">#!/usr/bin/env python</span>
<span class="c"># with_example01.py</span>
<span class="k">class</span> <span class="nc">Sample</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">__enter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">print</span> <span class="s">"In __enter__()"</span>
<span class="k">return</span> <span class="s">"Foo"</span>
<span class="k">def</span> <span class="nf">__exit__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="nb">type</span><span class="p">,</span> <span class="n">value</span><span class="p">,</span> <span class="n">trace</span><span class="p">):</span>
<span class="k">print</span> <span class="s">"In __exit__()"</span>
<span class="k">def</span> <span class="nf">get_sample</span><span class="p">():</span>
<span class="k">return</span> <span class="n">Sample</span><span class="p">()</span>
<span class="k">with</span> <span class="n">get_sample</span><span class="p">()</span> <span class="k">as</span> <span class="n">sample</span><span class="p">:</span>
<span class="k">print</span> <span class="s">"sample:"</span><span class="p">,</span> <span class="n">sample</span>
</pre></div>
</td></tr></table>
<p>运行代码，输出如下</p>
<div class="highlight"><pre><span class="n">bash</span><span class="o">-</span><span class="mf">3.2</span><span class="err">$</span> <span class="p">.</span><span class="o">/</span><span class="n">with_example01</span><span class="p">.</span><span class="n">py</span>
<span class="n">In</span> <span class="n">__enter__</span><span class="p">()</span>
<span class="nl">sample:</span> <span class="n">Foo</span>
<span class="n">In</span> <span class="n">__exit__</span><span class="p">()</span>
</pre></div>
<p>正如你看到的， </p>
<div class="highlight"><pre><span class="mf">1.</span> <span class="n">__enter__</span><span class="p">()</span><span class="err">方法被执行</span>
<span class="mf">2.</span> <span class="n">__enter__</span><span class="p">()</span><span class="err">方法返回的值</span> <span class="o">-</span> <span class="err">这个例子中是</span><span class="s">"Foo"</span><span class="err">，赋值给变量'</span><span class="n">sample</span><span class="err">'</span>
<span class="mf">3.</span> <span class="err">执行代码块，打印变量</span><span class="s">"sample"</span><span class="err">的值为</span> <span class="s">"Foo"</span>
<span class="mf">4.</span> <span class="n">__exit__</span><span class="p">()</span><span class="err">方法被调用</span>
</pre></div>
<p>with真正强大之处是它可以处理异常。可能你已经注意到Sample类的__exit__方法有三个参数- val, type 和 trace。 这些参数在异常处理中相当有用。我们来改一下代码，看看具体如何工作的。</p>
<table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre> 1
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19</pre></div></td><td class="code"><div class="highlight"><pre><span class="c">#!/usr/bin/env python</span>
<span class="c"># with_example02.py</span>
<span class="k">class</span> <span class="nc">Sample</span><span class="p">:</span>
<span class="k">def</span> <span class="nf">__enter__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span>
<span class="k">def</span> <span class="nf">__exit__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="nb">type</span><span class="p">,</span> <span class="n">value</span><span class="p">,</span> <span class="n">trace</span><span class="p">):</span>
<span class="k">print</span> <span class="s">"type:"</span><span class="p">,</span> <span class="nb">type</span>
<span class="k">print</span> <span class="s">"value:"</span><span class="p">,</span> <span class="n">value</span>
<span class="k">print</span> <span class="s">"trace:"</span><span class="p">,</span> <span class="n">trace</span>
<span class="k">def</span> <span class="nf">do_something</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">bar</span> <span class="o">=</span> <span class="mi">1</span><span class="o">/</span><span class="mi">0</span>
<span class="k">return</span> <span class="n">bar</span> <span class="o">+</span> <span class="mi">10</span>
<span class="k">with</span> <span class="n">Sample</span><span class="p">()</span> <span class="k">as</span> <span class="n">sample</span><span class="p">:</span>
<span class="n">sample</span><span class="o">.</span><span class="n">do_something</span><span class="p">()</span>
</pre></div>
</td></tr></table>
<p>这个例子中，with后面的get_sample()变成了Sample()。这没有任何关系，只要紧跟with后面的语句所返回的对象有__enter__()和__exit__()方法即可。此例中，Sample()的__enter__()方法返回新创建的Sample对象，并赋值给变量sample。 </p>
<p>代码执行后：</p>
<div class="highlight"><pre><span class="nx">bash</span><span class="o">-</span><span class="mf">3.2</span><span class="err">$</span> <span class="nx">.</span><span class="p">/</span><span class="nx">with_example02.py</span>
<span class="k">type</span><span class="p">:</span> <span class="o"><</span><span class="k">type</span> <span class="s1">'exceptions.ZeroDivisionError'</span><span class="o">></span>
<span class="nb">value</span><span class="p">:</span> <span class="kt">integer</span> <span class="nx">division</span> <span class="ow">or</span> <span class="nx">modulo</span> <span class="k">by</span> <span class="nx">zero</span>
<span class="nb">trace</span><span class="p">:</span> <span class="o"><</span><span class="nx">traceback</span> <span class="nb">object</span> <span class="nx">at</span> <span class="mh">0x1004a8128</span><span class="o">></span>
<span class="nx">Traceback</span> <span class="p">(</span><span class="nx">most</span> <span class="nx">recent</span> <span class="nb">call</span> <span class="nb">last</span><span class="p">):</span>
<span class="nb">File</span> <span class="s2">"./with_example02.py"</span><span class="p">,</span> <span class="nb">line</span> <span class="mi">19</span><span class="p">,</span> <span class="k">in</span> <span class="o"><</span><span class="nx">module</span><span class="o">></span>
<span class="nx">sample.do_something</span><span class="p">()</span>
<span class="nb">File</span> <span class="s2">"./with_example02.py"</span><span class="p">,</span> <span class="nb">line</span> <span class="mi">15</span><span class="p">,</span> <span class="k">in</span> <span class="nx">do_something</span>
<span class="n">bar</span> <span class="o">=</span> <span class="mi">1</span><span class="p">/</span><span class="nx">0</span>
<span class="nx">ZeroDivisionError</span><span class="p">:</span> <span class="kt">integer</span> <span class="nx">division</span> <span class="ow">or</span> <span class="nx">modulo</span> <span class="k">by</span> <span class="nx">zero</span>
</pre></div>
<p>实际上，在with后面的代码块抛出任何异常时，<strong>exit</strong>()方法被执行。正如例子所示，异常抛出时，与之关联的type，value和stack trace传给__exit__()方法，因此抛出的ZeroDivisionError异常被打印出来了。开发库时，清理资源，关闭文件等等操作，都可以放在__exit__方法当中。</p>
<p>因此，Python的with语句是提供一个有效的机制，让代码更简练，同时在异常产生时，清理工作更简单。 </p></div>
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