Mandatory Iterator with "yield" #633

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1. rockneurotiko commented on Apr 13, 2015

   

   rockneurotiko

   on Apr 13, 2015

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   Hi!

   Like you say, 'yield from' check the return type and allow it if it's a subtype of 'Iterable' like you can see here:
   https://github.com/JukkaL/mypy/blob/master/mypy/checker.py#L1384

   The 'yield' word only allows 'Iterator' like you can see here:
   https://github.com/JukkaL/mypy/blob/master/mypy/checker.py#L1357

   Actually, yield and yield from, explained in the PEPs (225 and 380), return an Iterator (because both have a next function).

   So, I think that the change should be change 'yield from' to only allow an Iterator as return type more than let 'yield' allow Iterable subclass.
2. fcolas commented on Apr 13, 2015

   

   fcolas

   on Apr 13, 2015

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   But the thing is that Iterator is already an Iterable:

   >>> import typing
   >>> issubclass(typing.Iterator, typing.Iterable)
   True
   

   In the same way that my first example returns a List which is a subtype of Iterable, I'd expect my function to be correctly typed if I advertise an Iterable implemented as an Iterator. What am I missing?
3. rockneurotiko commented on Apr 13, 2015

   

   rockneurotiko

   on Apr 13, 2015

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   Yes, it's a subclass, and I've rediscovered why I wrote the 'yield from' support to allow Iterable subtype.

   >>> def a():
           yield 1
           yield 2
           yield 3
   
   >>> b = a()
   >>> dir(b)
   [..., '__iter__', ... '__next__',...]   # wrapped
   

   >>> def a():
            yield from [1,2,3]
   
   >>> b = a()
   >>> dir(b)
   [..., '__iter__', ...,  '__next__',...]  # wrapped
   

   Both, 'yield' and 'yield from' have next (iterator) and iter (iterable), so, changing whan I've said in my previous comment, seems correct to allow iterable subtypes.
4. gvanrossum

   closed this as completedin #1137on Jan 20, 2016