上下文管理器——with语句的实现

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<h1 id="-" style="margin: 20px 0px 10px; padding: 0px; font-weight: bold; color: black; font-size: 24px; border-bottom: 2px solid #aaaaaa;">前言
<p style="margin: 0px 0px 1.2em !important; font-size: 16px; line-height: 1.75em; padding-right: 0.5em; padding-left: 0.5em;">with语句的使用给我们带来了很多的便利，最常用的可能就是关闭一个文件，释放一把锁。

 :
     :
        
<span class="hljs-function" style="color: #bbdaff;"&gt;<span class="hljs-keyword" style="color: #ebbbff;"&gt;def</span> <span class="hljs-title" style="color: #7285b7;"&gt;produce</span><span class="hljs-params" style="color: #ffc58f;"&gt;(self)</span>:</span>
    <span class="hljs-keyword" style="color: #ebbbff;"&gt;raise</span> RuntimeError(<span class="hljs-string" style="color: #d1f1a9;"&gt;'ERROR'</span>)
<span class="hljs-class"><span class="hljs-keyword" style="color: #ebbbff;">class <span class="hljs-title" style="color: #7285b7;">MyWith:

<span class="hljs-function" style="color: #bbdaff;"><span class="hljs-keyword" style="color: #ebbbff;">def <span class="hljs-title" style="color: #7285b7;">enter<span class="hljs-params" style="color: #ffc58f;">(self):

<span class="hljs-keyword" style="color: #ebbbff;">return ErrorProduce()
<span class="hljs-function" style="color: #bbdaff;"&gt;<span class="hljs-keyword" style="color: #ebbbff;"&gt;def</span> <span class="hljs-title" style="color: #7285b7;"&gt;__exit__</span><span class="hljs-params" style="color: #ffc58f;"&gt;(self,exc_tb)</span>:</span>
    <span class="hljs-keyword" style="color: #ebbbff;"&gt;if</span> exc_type:
        print(<span class="hljs-string" style="color: #d1f1a9;"&gt;"有错"</span>)
        <span class="hljs-comment" style="color: #7285b7;"&gt;#print(exc_val)</span>
        <span class="hljs-comment" style="color: #7285b7;"&gt;#return True</span>
    <span class="hljs-keyword" style="color: #ebbbff;"&gt;else</span>:
        print(<span class="hljs-string" style="color: #d1f1a9;"&gt;"完美执行"</span>)
<span class="hljs-keyword" style="color: #ebbbff;">with MyWith() <span class="hljs-keyword" style="color: #ebbbff;">as E:

E.produce()

 contextlib  contextmanager
<span class="hljs-class"><span class="hljs-keyword" style="color: #ebbbff;">class <span class="hljs-title" style="color: #7285b7;">ErrorProduce:

<span class="hljs-function" style="color: #bbdaff;"><span class="hljs-keyword" style="color: #ebbbff;">def <span class="hljs-title" style="color: #7285b7;">init<span class="hljs-params" style="color: #ffc58f;">(self):

<span class="hljs-keyword" style="color: #ebbbff;">pass
<span class="hljs-function" style="color: #bbdaff;"&gt;<span class="hljs-keyword" style="color: #ebbbff;"&gt;def</span> <span class="hljs-title" style="color: #7285b7;"&gt;produce</span><span class="hljs-params" style="color: #ffc58f;"&gt;(self)</span>:</span>
    <span class="hljs-keyword" style="color: #ebbbff;"&gt;raise</span> RuntimeError(<span class="hljs-string" style="color: #d1f1a9;"&gt;'ERROR'</span>)
<span class="hljs-decorator">@contextmanager

<span class="hljs-function" style="color: #bbdaff;"><span class="hljs-keyword" style="color: #ebbbff;">def <span class="hljs-title" style="color: #7285b7;">my_with<span class="hljs-params" style="color: #ffc58f;">():

<span class="hljs-keyword" style="color: #ebbbff;">try:

<span class="hljs-keyword" style="color: #ebbbff;">yield ErrorProduce()

<span class="hljs-keyword" style="color: #ebbbff;">except Exception <span class="hljs-keyword" style="color: #ebbbff;">as e:

print(e)

　　　　 <span class="hljs-comment" style="color: #7285b7;">#raise

<span class="hljs-keyword" style="color: #ebbbff;">finally:

print(<span class="hljs-string" style="color: #d1f1a9;">'收尾')
<span class="hljs-keyword" style="color: #ebbbff;">with my_with() <span class="hljs-keyword" style="color: #ebbbff;">as E:

E.produce()

print(<span class="hljs-string" style="color: #d1f1a9;">'aa')

（编辑：李大同）

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