Python--with的用法详解
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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.有一些任务，可能事先需要设置，事后做清理工作。对于这种场景，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。Without the with statement, one would write something along the lines of:如果不用with语句，代码如下：| 1 2 3 |
file
=
open
(
"/tmp/foo.txt"
)
data
=
file
.read()
file
.close()
|
| 1 2 3 4 5 |
file
=
open
(
"/tmp/foo.txt"
)
try
:
data
=
file
.read()
finally
:
file
.close()
|
| 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.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。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.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。This can be demonstrated with the following example:下面例子可以具体说明with如何工作：| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 |
#!/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
|