[Python-Dev] PEP 479: Change StopIteration handling inside generators (original) (raw)
Nick Coghlan ncoghlan at gmail.com
Wed Nov 26 08:06:29 CET 2014
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On 26 November 2014 at 04:04, Guido van Rossum <guido at python.org> wrote:
On Tue, Nov 25, 2014 at 9:49 AM, Chris Angelico <rosuav at gmail.com> wrote:
On Wed, Nov 26, 2014 at 4:45 AM, Isaac Schwabacher <ischwabacher at wisc.edu> wrote: > Yield can also raise StopIteration, if it's thrown in. The current > interaction of generator.throw(StopIteration) with yield from can't be > emulated under the PEP's behavior, though it's not clear that that's a > problem. > Hrm. I have absolutely no idea when you would use that, and how you'd go about reworking it to fit this proposal. Do you have any example code (production or synthetic) which throws StopIteration into a generator? Sounds like a good one for the obfuscated Python contest. :-) Unless the generator has a try/except surrounding the yield point into which the exception is thrown, it will bubble right out, and PEP 479 will turn this into a RuntimeError. I'll clarify this in the PEP (even though it logically follows from the proposal) -- I don't think there's anything to worry about.
This is actually the edge case that needs to be handled in contextlib
- a StopIteration raised by the with statement body gets thrown into the generator implementing the context manager. My current porting recommendation is to catch the RuntimeError & look at cause to see if it's the StopIteration instance that was thrown in, but an alternative option would be to just call gen.close() in that case, rather than gen.throw(exc).
Cheers, Nick.
-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia
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