python上下文是什么

with不是上下文管理器。with只是一个语句。实现了__enter__和__exit__这两个上下文管理协议方法的对象，都被称作上下文管理器。

上下文管理器被with调用时，会创建运行时上下文，__enter__在语句体执行之前进入运行时上下文，__exit__负责在语句执行完成

之后退出。

with A() as a,B() as b:

  suite

class TraceBlock:

    def message(self, arg):

        print('running ' + arg)

    def __enter__(self):

        print('starting with block')

        return self

    def __exit__(self, exc_type, exc_val, exc_tb):

        if exc_type is None:

            print('exited normally')

        else:

            print('raise an exception' + str(exc_type))

            return False

if __name__ == '__main__':

    with TraceBlock() as action:

        action.message('test1')

        raise Exception

        print('reached')

@contextmanager

def message(arg):

    print('running' + arg)

    yield

    print('hello')

if __name__ == '__main__':

    with message('hell'):

        print('ok')

@contextmanager

def make_context():

    print('enter')

    try:

        yield {}

    except Exception as err:

        print('error', err)

    finally:

        print('exit')

if __name__ == '__main__':

    with make_context() as value:

        raise Exception

        print(value)