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.
--
Regards,
Benjamin
_______________________________________________
Python-Dev mailing list
Python-Dev@python.org
http://mail.python.org/mailman/listinfo/python-dev
Unsubscribe:
http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
