On 3/24/06, Phillip J. Eby pje@telecommunity.com wrote:

At 03:27 PM 3/24/2006 -0800, Guido van Rossum wrote:

...

I guess I like the ambiguity -- to the outer __exit__, it shouldn't make any difference whether the exception was re-raised by the inner __exit__ or by the finally clause containing it. After all, if there *wasn't* an outer __exit__, there wouldn't be any difference to the user either, whether the re-raise came from __exit__ or from finally.

If you still disagree, can you produce a test case that's currently broken?

I discovered the issue when I updated to the latest implementation and it broke the tests for a context manager I had written. This context manager is basically a transaction manager that accumulates context managers for resources used in the transaction, then calls all their __exit__() methods from its __exit__. It has to be able to distinguish between an __exit__() that failed (which means a critical failure of the overall transaction!) and an exit that's merely allowing the original exception to propagate (which means that the other handlers should still be invoked, and that everything's working normally).

It seems you're proposing something that is *not* equivalent to

with A: with B: ...

since in that case a failure of the inner __exit__ (whether a re-raise or a real failure) would still invoke the outer __exit__. Is that a good idea?

I'm not against recommending in the PEP that __exit__ shouldn't re-raise but instead should return False to signal a re-raise, and fixing any existing code that re-raises in __exit__. But I'm still questioning your use case; why is it important not to call the outer __exit__ methods in your case?

-- --Guido van Rossum (home page: http://www.python.org/~guido/)