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[Python-ideas] Propagating StopIteration value

Serhiy Storchaka storchaka at gmail.com
Tue Oct 9 17:28:14 CEST 2012 
On 09.10.12 16:07, Oscar Benjamin wrote:
> I really should have checked this before posting but I didn't have
> Python 3.3 available:

Generator expression also eats the StopIteration value:

 >>> next(x for x in f())
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
StopIteration

> These next two seem wrong since there are two iterables (but I don't
> think they can be done differently):
>
>>>> def g():
> ....     return 'From the other generator...'
> ....     yield
> ....
>>>> next(itertools.compress(f(), g()))
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> StopIteration: Returned from generator!
>>>> next(zip(f(), g()))
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> StopIteration: Returned from generator!

 >>> def h():
...     yield 42
...     return 'From the another generator...'
...
 >>> next(zip(f(), h()))
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
StopIteration: Returned from generator!
 >>> next(zip(h(), f()))
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
StopIteration: Returned from generator!

This is logical. Value returned from the first exhausted iterator.

> I guess this should be treated as undefined behaviour? Perhaps it
> should be documented as such so that anyone who chooses to rely on it
> was warned.

This should be treated as implementation details now.
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