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_tb)  #name
        print(exc_type)
        print(exc_val)  #Traceback
        return True  #返回TURE后，涵盖了前面三个
with Foo('a.txt') as f:
    print(f)
    print(abcdefg)
    print(f.name)
print('00000000000000000000')

=====>>>
执行enter
<__main__.Foo object at 0x00BED4B0>
执行exit
<traceback object at 0x00BF6AF8>
<class 'NameError'>
name 'abcdefg' is not defined
00000000000000000000