with obj as f: '代码块'
1.with obj ----》触发obj.__enter__(),拿到返回值
2.as f----->f=返回值、
3.with obj as f 等同于 f=obj.__enter__()
4.执行代码块一：没有异常的情况下，整个代码块运行完毕后去触发__exit__,它的三个参数都为None二：有异常的情况下，从异常出现的位置直接触发__exit__ a：如果__exit__的返回值为True，代表吞掉了异常 b：如果__exit__的返回值不为True，代表吐出了异常 c：__exit__的的运行完毕就代表了整个with语句的执行完毕
class Foo:
def __init__(self,name):
self.name=name
def __enter__(self):
print('执行enter')
return self
def __exit__(self, exc_type, exc_val, exc_tb):
print('执行exit')
print(exc_type)
print(exc_val)
print(exc_tb)
return True
with Foo('a.txt') as f:
print(f)
print(asdfsaasdfasdfasdfasdfasfasdfasdfasdfasdfasfdasfd) #触发__exit__
print(f.name)
print('-----------------')
print('-----------------')
print('-----------------')
print('-----------------')
')