[Python-Dev] Another PEP 343 contextmanager glitch

[Phillip J. Eby]

It appears that the contextmanager implementation (both in the stdlib and 
as described in the PEP) doesn't correctly handle the case where the 
generator yields a new value in response to throw().  It responds by 
suppressing the original exception, but IIRC earlier versions of the PEP 
threw a RuntimeError for this scenario.

Unless there are any objections, I'd like to restore this by replacing the 
"return True" that follows the throw(), with:

     raise RuntimeError("generator didn't stop after throw()")

and again, update the PEP accordingly.