[issue5251] contextlib.nested inconsistent with, well, nested with statements due exceptions raised in __enter__

[Nick Coghlan]

Nick Coghlan <ncoghlan at gmail.com> added the comment:

Version 3 of patch attached
- now functionally complete with the correct semantics
- compiler package not updated yet
- documentation not updated yet

Unfortunately, getting the semantics right involves moving
context.__enter__ and context.__exit__ to temporary variables instead of
being able to leave them on the stack - that makes for something of a
speed hit.

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Added file: http://bugs.python.org/file13337/pep377_v3.diff

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