[Python-Dev] PEP 479: Change StopIteration handling inside generators (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Thu Nov 27 12:25:13 CET 2014

On 27 November 2014 at 09:50, Guido van Rossum <guido at python.org> wrote:

On Wed, Nov 26, 2014 at 3:15 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

This is actually the second iteration of this bug: the original implementation always suppressed StopIteration. PJE caught that one before Python 2.5 was released, but we didn't notice that 3.3 had brought it back in a new, more subtle form :(

It's worth noting that my "allowimplicitstop" idea in the other thread wouldn't affect subgenerators - those would still convert StopIteration to RuntimeError unless explicitly silenced. You've lost me in this subthread. Am I right to conclude that the PEP change doesn't cause problems for contextlib(*), but that the PEP change also probably wouldn't have helped diagnose any contextlib bugs?

I think the PEP 479 semantics would have made both bugs (the one PJE found in 2.5, and the newer one Isaac pointed out here) less cryptic, in that they would have caused RuntimeError to be raised, rather than silently consuming the StopIteration and continuing execution after the with statement body.

With the new semantics, contextlib just needs to be updated to cope with the StopIteration -> RuntimeError conversion, and Isaac's "spurious success" bug will be fixed*.

Without PEP 479, I believe my only recourse to systematically eliminate the risk of generator based context managers silently consuming StopIteration would be to implement the "StopIteration -> gen.close()" workaround, and that would be a backwards incompatible change in its own right.

Cheers, Nick.

P.S. *(This does mean I was wrong about allow_implicit_stop being useful to contextlib, but I still think the decorator is useful for cases where StopIteration is being used to terminate the generator on purpose)

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

More information about the Python-Dev mailing list