Python3 With as 語句如何了解

《 python 标準庫 》上這麼一句話：

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1. with open('filename', 'wt') as f:  
2.     f.write('hello, world!')  

我不明白為什麼這樣寫，下面這篇文章對此做出了解釋

原文位址：http://python.42qu.com/11155501

 With語句是什麼？       
         
Python’s with statement provides a very convenient way of dealing with the situation where you have to do a setup and teardown to make something happen. A very good example for this is the situation where you want to gain a handler to a file, read data from the file and the close the file handler.
有一些任務，可能事先需要設定，事後做清理工作。對于這種場景，Python的with語句提供了一種非常友善的處理方式。一個很好的例子是檔案處理，你需要擷取一個檔案句柄，從檔案中讀取資料，然後關閉檔案句柄。

Without the with statement, one would write something along the lines of:
如果不用with語句，代碼如下：
  
   
    
           

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file = open ( "/tmp/foo.txt" ) data = file .read() file .close()

There are two annoying things here. First, you end up forgetting to close the file handler. The second is how to handle exceptions that may occur once the file handler has been obtained. One could write something like this to get around this:

這裡有兩個問題。一是可能忘記關閉檔案句柄；二是檔案讀取資料發生異常，沒有進行任何處理。下面是處理異常的加強版本：

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file = open ( "/tmp/foo.txt" ) try : data = file .read() finally : file .close()

While this works well, it is unnecessarily verbose. This is where with is useful. The good thing about with apart from the better syntax is that it is very good handling exceptions. The above code would look like this, when using with:

雖然這段代碼運作良好，但是太冗長了。這時候就是with一展身手的時候了。除了有更優雅的文法，with還可以很好的處理上下文環境産生的異常。下面是with版本的代碼：

1 2	with open ( "/tmp/foo.txt" ) as file : data = file .read()

with如何工作？

while this might look like magic, the way Python handles with is more clever than magic. The basic idea is that the statement after with has to evaluate an object that responds to an __enter__() as well as an __exit__() function.

這看起來充滿魔法，但不僅僅是魔法，Python對with的處理還很聰明。基本思想是with所求值的對象必須有一個__enter__()方法，一個__exit__()方法。

After the statement that follows with is evaluated, the __enter__() function on the resulting object is called. The value returned by this function is assigned to the variable following as. After every statement in the block is evaluated, the __exit__() function is called.

緊跟with後面的語句被求值後，傳回對象的__enter__()方法被調用，這個方法的傳回值将被指派給as後面的變量。當with後面的代碼塊全部被執行完之後，将調用前面傳回對象的__exit__()方法。

This can be demonstrated with the following example:

下面例子可以具體說明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:

運作代碼，輸出如下

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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__()方法被執行

2. __enter__()方法傳回的值 - 這個例子中是"Foo"，指派給變量'sample'

3. 執行代碼塊，列印變量"sample"的值為 "Foo"

4. __exit__()方法被調用

with真正強大之處是它可以處理異常。可能你已經注意到Sample類的__exit__方法有三個參數- val, type 和 trace。 這些參數在異常進行中相當有用。我們來改一下代碼，看看具體如何工作的。

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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 / 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.

這個例子中，with後面的get_sample()變成了Sample()。這沒有任何關系，隻要緊跟with後面的語句所傳回的對象有__enter__()和__exit__()方法即可。此例中，Sample()的__enter__()方法傳回新建立的Sample對象，并指派給變量sample。

When executed:

代碼執行後：

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bash - 3.2 $ . / with_example02.py type : < type 'exceptions.ZeroDivisionError' > value: integer division or modulo by zero trace: <traceback object at 0x1004a8128 > Traceback (most recent call last): File "./with_example02.py" , line 19 , in <module> sample.do_something() File "./with_example02.py" , line 15 , in do_something bar = 1 / 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.

實際上，在with後面的代碼塊抛出任何異常時，__exit__()方法被執行。正如例子所示，異常抛出時，與之關聯的type，value和stack trace傳給__exit__()方法，是以抛出的ZeroDivisionError異常被列印出來了。開發庫時，清理資源，關閉檔案等等操作，都可以放在__exit__方法當中。

Thus, Python’s with is a nifty construct that makes code a little less verbose and makes cleaning up during exceptions a bit easier.

是以，Python的with語句是提供一個有效的機制，讓代碼更簡練，同時在異常産生時，清理工作更簡單。

I have put the code examples given here on

Github.

示例代碼可以在

Github上面找到。

譯注：本文原文見

Understanding Python's "With" Statement