With语句是什么？
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。 如果不用with语句，代码如下：|
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file
=
open
(
"/tmp/foo.txt"
)
data
=
file
.read()
file
.close()
|
|
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file
=
open
(
"/tmp/foo.txt"
)
try
:
data
=
file
.read()
finally
:
file
.close()
|
|
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with
open
(
"/tmp/foo.txt"
) as
file
:
data
=
file
.read()
|
with如何工作？
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
|
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bash
-
3.2
$ .
/
with_example01.py
In __enter__()
sample: Foo
In __exit__()
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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()
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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
/
0
ZeroDivisionError: integer division
or
modulo by zero
|
（转）
