python - 在另一个上下文管理器中处理上下文管理器的实例

Question

如何在 Python 中处理在另一个上下文管理器中创建的上下文管理器？

示例:假设您有充当​​上下文管理器的 A 类和也充当上下文管理器的 B 类。但是 B 类的实例必须实例化并使用 A 类的实例。我已经阅读了 PEP 343，这是我想到的解决方案:

class A(object):
    def __enter__(self):
        # Acquire some resources here
        return self

    def __exit__(seplf, exception_type, exception, traceback):
        # Release the resources and clean up
        pass


class B(object):
    def __init__(self):
        self.a = A()

    def __enter__(self):
        # Acquire some resources, but also need to "start" our instance of A
        self.a.__enter__()
        return self

    def __exit__(self, exception_type, exception, traceback):
        # Release the resources, and make our instance of A clean up as well
        self.a.__exit__(exception_type, exception, traceback)

这是正确的做法吗？还是我错过了一些陷阱？

Answer 1

如果您可以使用 @contextlib.contextmanager 装饰器，您的生活会变得更加轻松:

import contextlib

@contextlib.contextmanager
def internal_cm():
    try:
        print "Entering internal_cm"
        yield None
        print "Exiting cleanly from internal_cm"
    finally:
        print "Finally internal_cm"


@contextlib.contextmanager
def external_cm():
    with internal_cm() as c:
        try:
            print "In external_cm_f", c
            yield [c]
            print "Exiting cleanly from external_cm_f", c
        finally:
            print "Finally external_cm_f", c


if "__main__" == __name__:
    with external_cm():
        print "Location A"
    print
    with external_cm():
        print "Location B"
        raise Exception("Some exception occurs!!")