上下文協議管理

with obj as f:

'**塊'

1.with obj ----》觸發obj.__enter__(),拿到返回值

2.as f----->f=返回值、

3.with obj as f 等同於 f=obj.__enter__()

4.執行**塊

一：沒有異常的情況下，整個**塊執行完畢後去觸發__exit__,它的三個引數都為none

二：有異常的情況下，從異常出現的位置直接觸發__exit__

a：如果__exit__的返回值為true，代表吞掉了異常

b：如果__exit__的返回值不為true，代表吐出了異常

c：__exit__的的執行完畢就代表了整個with語句的執行完畢

class

foo:
def__init__
(self,name):
self.name=name
def__enter__
(self):
print('
執行enter')
return
self
def__exit__
(self, exc_type, exc_val, exc_tb):
print('
執行exit')
print
(exc_type)
print
(exc_val)
print
(exc_tb)
return
true
with foo(
'a.txt
') as f:
print
(f)    
print(asdfsaasdfasdfasdfasdfasfasdfasdfasdfasdfasfdasfd)  #
觸發__exit__
print
(f.name)
print('
-----------------')
print('
-----------------')
print('
-----------------')
print('
-----------------')
print('
-----------------')
print('
-----------------')
print('
-----------------')
print('
000000000000000000000000000000000000000000000
')

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