Support enum iteration

[gvanrossum]

Fixes python/mypy#2305.

Example:

import enum
class Colors(enum.Enum):
    RED = 0
    BLUE = 1
print([color.name for color in Colors])  # This used to fail, now it works

(I found this unfinished project. Maybe it was waiting for some mypy improvement, or maybe I got distracted. Anyway, it seems to work now.)

[JelleZijlstra]

In the enum.py code it inherits from type, not ABCMeta. Is there a reason to use ABCMeta here?

[gvanrossum]

Good point, let me see if that works.

[gvanrossum]

This should do it.

[JelleZijlstra]

I thought this might break because of the new bound on _T, since this decorator applies to Enum classes, not Enum instances. But I tried it and it seems to work with mypy.

[gvanrossum]

Oh, hm, that might be by accident, or because something defaults to Any when mypy doesn't understand it. Would be nice to try this and see what the reveal_type(unique(Color)) actually says.

[JelleZijlstra]

$ cat ../bin/enumu.py 
import enum

@enum.unique
class X(enum.Enum):
    a = 1
    b = 2
reveal_type(X)

$ mypy --custom-typeshed-dir . ../bin/enumu.py 
../bin/enumu.py:7: error: Revealed type is 'def (value: Any) -> enumu.X'

This looks right to me.

[gvanrossum]

Hm...

import enum
class Colors(enum.Enum):
  RED = 1
  GREEN = 2
  BLUE= 3
X = unique(Colors)
reveal_type(X)

Gives

__tmp__.py:12: error: Type argument 1 of "unique" has incompatible value "Colors"
__tmp__.py:13: error: Revealed type is 'def (value: Any) -> __tmp__.Colors'

(Sorry the line numbers are offset, it's about the last two lines.)

[JelleZijlstra]

Interesting, so it works differently when used as a decorator. That seems like a mypy bug.

[gvanrossum]

Oh, that's not how it's supposed to be used. As a class decorator it does indeed work as expected. Not sure why or how.

[JelleZijlstra]

Maybe to be safe we should give unique its own TypeVar for now.

[JelleZijlstra]

Thanks! This will fix some # type: ignores in my code.