[Python-Dev] Allow enter() methods to skip the with statement body? (original) (raw)

Steven Bethard steven.bethard at gmail.com
Wed Feb 25 19:47:16 CET 2009

On Wed, Feb 25, 2009 at 4:24 AM, Nick Coghlan <ncoghlan at 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():  dostuff()  # This will resume here without executing "Do stuff"  @contextlib.contextmanager  def combined():  with cmA():  with cmB():  yield  with combined():  dostuff()  # This will raise RuntimeError complaining that the underlying  # generator didn't yield  with contextlib.nested(cmA(), cmB()):  dostuff()  # 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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