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.
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?
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