len(Enum) error #2891

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1. ilevkivskyi commented on Feb 22, 2017

   

   ilevkivskyi

   on Feb 22, 2017

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   This is probably related to #2305

   Now that we have some support for metaclasses, both these should be not difficult to fix.
   @elazarg what do you think?
2. elazarg commented on Feb 22, 2017

   

   elazarg

   on Feb 22, 2017

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   Guido seems to be working on this here. I have also tried implementing it myself. len is easy; the problem is with declaring EnumMeta as an iterable, since it should be iterable over the concrete subclass, which is not declared yet. Selftype solves this nicely for the method definition, but not for the subclassing declaration. The closest I've got is

   T = TypeVar('T', bound='Enum')
   
   class EnumMeta(abc.ABCMeta, Iterable['Enum'], Sized):  # ABCMeta fixes an unrelated issue
       def __iter__(self: Type[T]) -> Iterator[T]: ...
       def __getitem__(self: Type[T], item: str) -> T: ...
       def __len__(self) -> int: ...

   This gives us list(Letters) as List[Enum], which is not very helpful.

   But in a way, metaclasses are generic in the concrete base class, so we should be writing something like

   T = TypeVar('T', metaclass='EnumMeta')
   class EnumMeta(Type[T], Iterable[T], Sized):  # Type[T] is a declaration like Generic[T]
       def __iter__(self: EnumMeta[T]) -> Iterator[T]: ...
       def __getitem__(self: EnumMeta[T], item: str) -> T: ...
       def __len__(self) -> int: ...
   
   class Enum(metaclass=EnumMeta):
      ...

   We can probably give some ad-hoc fix for protocols such as Iterable - possibly by inferring the generic parameter instead of defaulting to Any - but it would be nice to have the "right" solution somehow.

   To summarize:

   • len and __getitem__ seems to be easy
   • We might need "inference" for protocols like Iterable[Subclass] since we can't express this today
3. JukkaL commented on Feb 24, 2017

   

   JukkaL

   on Feb 24, 2017

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   __getitem__ already works (#2812). Making the iteration item type Any would be a reasonable first step to unblock the use case. It looks like inferring the item type would require some special casing in mypy.
4. JukkaL

   added

   feature

   priority-1-normal

   on Feb 24, 2017
5. MeGotsThis commented on Mar 1, 2017

   

   MeGotsThis

   on Mar 1, 2017

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   Also here are some other lines I just ran into:

   print(Letters.A in Letters)

   produces in mypy

   enum-test.py:8: error: Unsupported right operand type for in ("Letters")
   
6. MeGotsThis commented on Mar 2, 2017

   

   MeGotsThis

   on Mar 2, 2017

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   Found another case

   print(list(Letters))
   print(list(reversed(Letters)))

   produces in mypy 0.470

   enum-test.py:9: error: No overload variant of "list" matches argument types [def (value: Any) -> enum-test.Letters]
   enum-test.py:10: error: No overload variant of "reversed" matches argument types [def (value: Any) -> enum-test.Letters]
   
7. gvanrossum commented on Apr 21, 2017

   

   gvanrossum

   on Apr 21, 2017

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   Fixed by #1136.
8. gvanrossum

   closed this as completedon Apr 21, 2017
9. ilevkivskyi commented on Apr 21, 2017

   

   ilevkivskyi

   on Apr 21, 2017

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   There is a small remainder:

   No overload variant of "reversed" matches argument types [def (value: Any) -> tst22.Letters]
   

   But it looks like a purely typeshed issue (EnumMeta should inherit from Reversible).