上下文管理协议

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")

with Foo(‘a.txt‘) as f:
    print(f)
    print(f.name)
    print("======>")
    print("======>")

print("000000000000000")

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) #异常类
        print(exc_val)  #异常值
        print(exc_tb)  #追踪信息

with Foo(‘a.txt‘) as f:
    print(f)
    print(f.name)
    # print(jinling)  #触发 __exit__()
    print("======>")
    print("======>")

print("000000000000000")

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) #异常类
        print(exc_val)  #异常值
        print(exc_tb)  #追踪信息
        # return True

with Foo(‘a.txt‘) as f:
    print(f)
    print(f.name)
    print(jinling)  #触发 __exit__()
    print("======>")
    print("======>")

print("000000000000000")

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) #异常类
        print(exc_val)  #异常值
        print(exc_tb)  #追踪信息
        return True

with Foo(‘a.txt‘) as f:
    print(f)
    print(f.name)
    print(jinling)  #触发 __exit__()
    print("======>")
    print("======>")

print("000000000000000")   #因为__exit__ 吞掉了异常，所以最后一句00000000000正常输出，但是with块内异常下面的代码不能正常输出