[Python-Dev] Another PEP 343 contextmanager glitch

Guido van Rossum guido at python.org
Sat Mar 25 00:27:28 CET 2006

On 3/24/06, Phillip J. Eby <pje at telecommunity.com> wrote:
> At 03:05 PM 3/24/2006 -0800, Guido van Rossum wrote:
> >Oops, that's definitely a bug. Please go ahead and fix in both places.
> Will do.
> >(I'm still trying to fathom the other PEP 343 issue you brought up.)
> Here's the paragraph I'm proposing to add to the PEP to explain that issue:
>      NOTE: to facilitate chaining of contexts, __exit__() methods should
>      *not* reraise the error that is passed in, because it is always the
>      responsibility of the *caller* to reraise.  If an __exit__() method
>      exits with *any* exception, the caller of __exit__() should
>      interpret this as meaning that the __exit__() method itself
>      failed to perform the desired cleanup or other operations.
> Currently, the PEP is ambiguous on this point.  When "with" calls
> __exit__() this point is irrelevant because either way, the same exception
> will be raised/propagated.
> But if you are writing Python code that explicitly uses the context
> protocol (e.g. the 'nested' context manager), it should be clear as to
> whether a given __exit__() invocation failed or succeeded.  Thus,
> __exit__() should only raise an exception if it *failed*.
> (This then implies that the @contextmanager decorator must catch any
> re-raising done by the wrapped generator.)

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?

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

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