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Python with語句(轉)

1. 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.

Without the with statement, one would write something along the lines of:

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

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:

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

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:

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

2. 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.

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.

This can be demonstrated with the following example:

#!/usr/bin/env python

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:

bash-3.2$ ./
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.
1) __enter__()方法被執行
2) __enter__()方法返回的值 - 這個例子中是”Foo”,賦值給變量’sample’
3) 執行代碼塊,打印變量”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。可能你已經註意到Sample類的__exit()__方法有三個參數- val, type 和 trace。 這些參數在異常處理中相當有用。我們來改一下代碼,看看具體如何工作的。

#!/usr/bin/env python

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:

Notice how in this example, instead of getsample(), 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.

When executed:

bash-3.2$ ./
type: <type 'exceptions.ZeroDivisionError'>
value: integer division or modulo by zero
trace: <traceback object at 0x1004a8128>
Traceback (most recent call last):
  File "./", line 19, in <module>
  File "./", 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.
實際上,在with後面的代碼塊拋出任何異常時,__exit__()方法被執行。正如例子所示,異常拋出時,與之關聯的type,value和stack trace傳給__exit__()方法,因此拋出的ZeroDivisionError異常被打印出來了。開發庫時,清理資源,關閉文件等等操作,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.

I have put the code examples given here on Github.

譯註:本文原文見Understanding Python’s “With” Statement