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.