blocks|key|331282|text|在python中，在处理非托管资源(如文件流)时使用with关键字。它类似于VB.NET和C#中的using语句。它允许您确保在使用资源的代码完成运行时“清理”该资源，即使抛出异常也是如此。它为try/finally块提供了“语法糖”。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|331283|来自Python+Docs|331284|331285|+with语句澄清了以前使用try...finally块来确保执行清理代码的代码。在本节中，我将讨论该语句，因为它将经常使用。在下一节中，我将研究实现细节，并展示如何编写与此语句一起使用的对象。|blockquote|331286|331287|with语句是一个控制流结构，其基本结构为：|331288|将表达式作为变量:+with-block|331289|表达式被求值，并且它应该产生一个支持上下文管理协议的对象(即，具有__enter__()和__exit__()方法)。|331290|331291|根据Scott+Wisniewski的评论更新修复了VB标注。我确实把with和using搞混了。|331292|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.python.org/whatsnew/2.6.html#pep-343-the-with-statement^0|Q|4|1D|5|2P|B|0|2|B|0|0|0|1|4|E|D|0|0|0|4|0|0|X|B|19|A|0|0|Z|4|14|5|0^^$0|@$1|2|3|4|5|6|7|13|8|@$9|14|A|15|B|C]|$9|16|A|17|B|C]|$9|18|A|19|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|1A|8|@]|D|@$9|1B|A|1C|1|1D]]|E|$]]|$1|H|3|-4|5|6|7|1E|8|@]|D|@]|E|$]]|$1|I|3|J|5|K|7|1F|8|@$9|1G|A|1H|B|C]|$9|1I|A|1J|B|C]]|D|@]|E|$]]|$1|L|3|-4|5|6|7|1K|8|@]|D|@]|E|$]]|$1|M|3|N|5|6|7|1L|8|@$9|1M|A|1N|B|C]]|D|@]|E|$]]|$1|O|3|P|5|6|7|1O|8|@]|D|@]|E|$]]|$1|Q|3|R|5|6|7|1P|8|@$9|1Q|A|1R|B|C]|$9|1S|A|1T|B|C]]|D|@]|E|$]]|$1|S|3|-4|5|6|7|1U|8|@]|D|@]|E|$]]|$1|T|3|U|5|6|7|1V|8|@$9|1W|A|1X|B|C]|$9|1Y|A|1Z|B|C]]|D|@]|E|$]]|$1|V|3|-4|5|6|7|20|8|@]|D|@]|E|$]]]|W|$X|$5|Y|Z|10|E|$11|12]]]]

In python the <code>with</code> keyword is used when working with unmanaged resources (like file streams). It is similar to the <code>using</code> statement in VB.NET and C#. It allows you to ensure that a resource is &quot;cleaned up&quot; when the code that uses it finishes running, even if exceptions are thrown. It provides 'syntactic sugar' for <code>try/finally</code> blocks.
From <a href="http://docs.python.org/whatsnew/2.6.html#pep-343-the-with-statement" rel="noreferrer">Python Docs</a>:
<blockquote>
The <code>with</code> statement clarifies code that previously would use <code>try...finally</code> blocks to ensure that clean-up code is executed. In this section, I’ll discuss the statement as it will commonly be used. In the next section, I’ll examine the implementation details and show how to write objects for use with this statement.
The <code>with</code> statement is a control-flow structure whose basic structure is:
<pre><code>with expression [as variable]:
 with-block
</code></pre>
The expression is evaluated, and it should result in an object that supports the context management protocol (that is, has <code>__enter__()</code> and <code>__exit__()</code> methods).
</blockquote>
Update fixed VB callout per Scott Wisniewski's comment. I was indeed confusing <code>with</code> with <code>using</code>.

blocks|key|329777|text|Explanation+from+the+Preshing+on+Programming+blog|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|329778|329779|当你有两个相关的操作，你想成对执行，中间有一段代码时，它很方便。经典的例子是打开一个文件，操作文件，然后关闭它：|blockquote|329780|329781|with+open('output.txt'，'w')+as+f:+f.write('Hi+f.write！‘)|329782|上面的with语句将自动关闭嵌套代码块之后的文件。(继续阅读以了解关闭是如何发生的。)使用with语句的优点是，无论嵌套块如何退出，都可以保证关闭文件。如果在块结束之前发生异常，它将在异常被外部异常处理程序捕获之前关闭文件。如果嵌套块包含return语句、continue或break语句，则with语句在这些情况下也会自动关闭文件。|329783|entityMap|0|LINK|mutability|MUTABLE|url|http://preshing.com/20110920/the-python-with-statement-by-example/^0|0|1D|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@$A|V|B|W|1|X]]|C|$]]|$1|D|3|-4|5|6|7|Y|8|@]|9|@]|C|$]]|$1|E|3|F|5|G|7|Z|8|@]|9|@]|C|$]]|$1|H|3|-4|5|6|7|10|8|@]|9|@]|C|$]]|$1|I|3|J|5|6|7|11|8|@]|9|@]|C|$]]|$1|K|3|L|5|6|7|12|8|@]|9|@]|C|$]]|$1|M|3|-4|5|6|7|13|8|@]|9|@]|C|$]]]|N|$O|$5|P|Q|R|C|$S|T]]]]

<a href="http://preshing.com/20110920/the-python-with-statement-by-example/">Explanation from the Preshing on Programming blog</a>:

<blockquote>
 It’s handy when you have two related operations which you’d like to
 execute as a pair, with a block of code in between. The classic
 example is opening a file, manipulating the file, then 
 closing it:

<pre><code> with open('output.txt', 'w') as f:
 f.write('Hi there!')
</code></pre>
 
 The above with statement will automatically close the file after the
 nested block of code. (Continue reading to see exactly how the close
 occurs.) The advantage of using a with statement is that it is
 guaranteed to close the file no matter how the nested block exits. If
 an exception occurs before the end of the block, it will close the
 file before the exception is caught by an outer exception handler. If
 the nested block were to contain a return statement, or a continue or
 break statement, the with statement would automatically close the file
 in those cases, too.
</blockquote>

What is the python keyword "with" used for?

Example from: <a href="http://docs.python.org/tutorial/inputoutput.html" rel="noreferrer">http://docs.python.org/tutorial/inputoutput.html</a>

<pre><code>&gt;&gt;&gt; with open('/tmp/workfile', 'r') as f:
... read_data = f.read()
&gt;&gt;&gt; f.closed
True
</code></pre>

What is the python keyword "with" used for?

python关键字"with“是用来做什么的？示例来自：>>> with open('/tmp/workfile', 'r') as f:...     read_data = f.read()>>> f.closedTrue

问python关键字"with“是用来做什么的？
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