On Wed, Feb 25, 2009 at 4:24 AM, Nick Coghlan <ncogh...@gmail.com> wrote:
> An interesting discrepancy [1] has been noted when comparing
> contextlib.nested (and contextlib.contextmanager) with the equivalent
> nested with statements.
>
> Specifically, the following examples behave differently if
> cmB().__enter__() raises an exception which cmA().__exit__() then
> handles (and suppresses):
>
> with cmA():
> with cmB():
> do_stuff()
> # This will resume here without executing "Do stuff"
>
> �...@contextlib.contextmanager
> def combined():
> with cmA():
> with cmB():
> yield
>
> with combined():
> do_stuff()
> # This will raise RuntimeError complaining that the underlying
> # generator didn't yield
>
> with contextlib.nested(cmA(), cmB()):
> do_stuff()
> # This will raise the same RuntimeError as the contextmanager
> # example (unsurprising, given the way nested() is implemented)
>
> The problem arises any time it is possible to skip over the yield
> statement in a contextlib.contextmanager based context manager without
> raising an exception that can be seen by the code calling __enter__().
If the problem is just the yield, can't this just be fixed by
implementing contextlib.nested() as a class rather than as a
@contextmanager decorated generator? Or is this a problem with class
based context managers too?
Steve
--
I'm not *in*-sane. Indeed, I am so far *out* of sane that you appear a
tiny blip on the distant coast of sanity.
--- Bucky Katt, Get Fuzzy
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