On Tue, Aug 7, 2012 at 10:02 AM, Benjamin Peterson <benja...@python.org> wrote:
> 2012/8/6 Dino Viehland <di...@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.
Cheers,
Nick.
--
Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia
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