Python中的with语句

Python的with语句提供了一种非常方便的处理方式。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。
例如：在没有with语句中的语句中的方法中我们可以看到对文件的处理方法（1）
file =   open("root/younamefolder/tmp/file.txt")
data = file.read()
file.close()

缺点是：在我们打开文件时，没有对异常进行处理，在文件处理结束后可能会对文件忘记进行关闭处理。

例如2：
在对异常进行处理的代码：
file =   open("root/younamefolder/tmp/file.txt")
try:
    data = file.read()
finally:
    file.close()
这样的好处就是我们在这里进行了异常处理的方法，但是在python这个惜字如金的编写程序的方式中。这样的
代码书写劣显有点冗余。
在使用我们的with语句中。我们会发现这个问题就很easy了。
with  open("root/younamefolder/tmp/file.txt") as file:
        data = file.read()

下面有一些我们关于with使用的一些技巧：
     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。
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
下面例子可以具体说明with如何工作：

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#!/usr/bin/env python # with_example01.py     class Sample:     def __enter__(self):         print "In __enter__()"         return "Foo"       def __exit__(self, type, value, trace):         print "In __exit__()"     def get_sample():     return Sample()     with get_sample() as sample:     print "sample:", sample

When executed, this will result in: 运行代码，输出如下

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bash-3.2$ ./with_example01.py In __enter__() sample: Foo In __exit__()

As you can see, The __enter__() function is executed The value returned by it - in this case "Foo" is assigned to sample The body of the block is executed, thereby printing the value of sample ie. "Foo" The __exit__() function is called. What makes with really powerful is the fact that it can handle exceptions. You would have noticed that the __exit__() function for Sample takes three arguments - val, type and trace. These are useful in exception handling. Let’s see how this works by modifying the above example. 正如你看到的， 1. __enter__()方法被执行 2. __enter__()方法返回的值 - 这个例子中是"Foo"，赋值给变量'sample' 3. 执行代码块，打印变量"sample"的值为 "Foo" 4. __exit__()方法被调用 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。

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#!/usr/bin/env python # with_example02.py     class Sample:     def __enter__(self):         return self       def __exit__(self, type, value, trace):         print "type:", type         print "value:", value         print "trace:", trace       def do_something(self):         bar = 1/0         return bar + 10   with Sample() as sample:     sample.do_something()

Notice how in this example, instead of get_sample(), with takes Sample(). It does not matter, as long as the statement that follows with evaluates to an object that has an __enter__() and __exit__() functions. In this case, Sample()’s __enter__() returns the newly created instance of Sample and that is what gets passed to sample. 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。 When executed: 代码执行后：

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bash-3.2$ ./with_example02.py type: value: integer division or modulo by zero trace: Traceback (most recent call last):   File "./with_example02.py", line 19, in     sample.do_something()   File "./with_example02.py", line 15, in do_something     bar = 1/0 ZeroDivisionError: integer division or modulo by zero

Essentially, if there are exceptions being thrown from anywhere inside the block, the __exit__() function for the object is called. As you can see, the type, value and the stack trace associated with the exception thrown is passed to this function. In this case, you can see that there was a ZeroDivisionError exception being thrown. People implementing libraries can write code that clean up resources, close files etc. in their __exit__() functions. 实际上，在with后面的代码块抛出任何异常时，__exit__()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。 Thus, Python’s with is a nifty construct that makes code a little less verbose and makes cleaning up during exceptions a bit easier. 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。 I have put the code examples given here on . 示例代码可以在上面找到。