python - 在上下文管理器 __enter__() 中捕获异常

Question

是否可以保证 __exit__() 方法在 __enter__() 有异常的情况下被调用？

>>> class TstContx(object):
...    def __enter__(self):
...        raise Exception('Oops in __enter__')
...
...    def __exit__(self, e_typ, e_val, trcbak):
...        print "This isn't running"
... 
>>> with TstContx():
...     pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in __enter__
Exception: Oops in __enter__
>>> 

编辑

这是我能得到的最接近...

class TstContx(object):
    def __enter__(self):
        try:
            # __enter__ code
        except Exception as e
            self.init_exc = e

        return self

    def __exit__(self, e_typ, e_val, trcbak):
        if all((e_typ, e_val, trcbak)):
            raise e_typ, e_val, trcbak

        # __exit__ code


with TstContx() as tc:
    if hasattr(tc, 'init_exc'): raise tc.init_exc

    # code in context

事后看来，上下文管理器可能不是最好的设计决策

Answer 1

像这样:

import sys

class Context(object):
    def __enter__(self):
        try:
            raise Exception("Oops in __enter__")
        except:
            # Swallow exception if __exit__ returns a True value
            if self.__exit__(*sys.exc_info()):
                pass
            else:
                raise


    def __exit__(self, e_typ, e_val, trcbak):
        print "Now it's running"


with Context():
    pass

要让程序继续其愉快的方式而不执行上下文 block ，您需要检查上下文 block 内的上下文对象，并且只有在 __enter__ 成功时才执行重要的操作。

class Context(object):
    def __init__(self):
        self.enter_ok = True

    def __enter__(self):
        try:
            raise Exception("Oops in __enter__")
        except:
            if self.__exit__(*sys.exc_info()):
                self.enter_ok = False
            else:
                raise
        return self

    def __exit__(self, e_typ, e_val, trcbak):
        print "Now this runs twice"
        return True


with Context() as c:
    if c.enter_ok:
        print "Only runs if enter succeeded"

print "Execution continues"

据我所知，您不能完全跳过 with block 。请注意，此上下文现在将 all 异常包含在其中。如果您不希望在 __enter__ 成功时吞下异常，请检查 __exit__ 中的 self.enter_ok 和 return False 如果是是的。