[Python-Dev] yield from, user defined iterators, and StopIteration w/ a value...

Nick Coghlan ncoghlan at gmail.com
Tue Aug 7 03:54:00 CEST 2012

On Tue, Aug 7, 2012 at 10:02 AM, Benjamin Peterson <benjamin at python.org> wrote:
> 2012/8/6 Dino Viehland <dinov at microsoft.com>:
>> I'm trying to create an object which works like a generator and delegates to a generator for its implementation, but can also participate in yield from using 3.3 beta.  I want my wrapper object to be able to cache some additional information - such as whether or not the generator has completed - as well as have it provide some additional methods for interacting with the state of the generator.  But currently this doesn't seem possible because raising StopIteration from a user defined iterator has its value ignored as far as yield from is concerned.  Here's a simplified example of the problem:
>> class C:
>>     def __iter__(self): return self
>>     def __next__(self):
>>             raise StopIteration(100)
>> def g():
>>     if False:
>>         yield 100
>>     return 100
>> def f(val):
>>     x = yield from val
>>     print('x', x)
>> print(list(f(C())))
>> print(list(f(g())))
>> Which outputs:
>> x None
>> []
>> x 100
>> []
>> So you can see for the C case the value I raise from StopIteration is ignored, but the value from the generator is propagated out.  From my reading of PEP 380 the behavior here is incorrect for the user defined iterator case.  next(iter(C())) raises StopIteration exception with a value and that should be the resulting value of the yield from expression according to the formal semantics.
> Looks like a bug to me. Please file an issue.
>> Ok, looking at the implementation this seems to be because PyIter_Next clears the exception which prevents it from being seen in the yield from code path.   So should yield from just be doing "(*iter->ob_type->tp_iternext)(iter);" directly and avoid the error checking code?  Or am I wrong and this is the intended behavior?
> This is probably the simpliest fix. In C, returning NULL from __next__
> with no exception set is shorthand for StopIteration.

+1 from me for the simple fix.


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

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