Singleton as enum doesn't work as expected

[lmazuel]

Hello,

Based on the tag in the commit e818a96 that is the merge commit of #7000, that is declared to fix #1803, the singleton scenario as described in https://www.python.org/dev/peps/pep-0484/#support-for-singleton-types-in-unions should be supported in 0.720.

However, it's still not the case:

from typing import Union
from enum import Enum

class Empty(Enum):
    token = 0
_empty = Empty.token

def func(x: Union[int, None, Empty] = _empty) -> int:

    if x is _empty:
        return 0
    elif x is None:
        return 1
    else:  # At this point typechecker knows that x can only have type int
        return x * 2

does:

singleton.py:15: error: Unsupported operand types for * ("Empty" and "int")
singleton.py:15: note: Left operand is of type "Union[int, Empty]"

Have I misunderstood something?

Thank you!

[Michael0x2a]

You'll need to annotate _empty as being Final. For example, see this test case.

Without it, mypy thinks that it's possible for _empty to be later reassigned to some other variant of the Empty enum, in which case it'd be unsafe to narrow x to int within the else block. (Of course, that's impossible in this case since Empty contains only a single variant, but mypy doesn't treat single-variant enums any differently from other enums and so won't special-case this particular interaction.)

[lmazuel]

Gotcha. I have what I need then, thank you!
Out of curiosity, does it mean PEP484 would need an update (I literally just copied/pasted it to follow the recommendation here)?

[JukkaL]

I wonder if we should special case this in mypy. @Michael0x2a What do you think about this?

[Michael0x2a]

Yeah, I think special-casing could be reasonable, though probably it's not a big priority given that there's an easy workaround.

Actually, now that I'm thinking about it, I wonder if it might be as easy as just adding another case to the is_singleton_type and coerce_to_literal functions at the bottom of https://github.com/python/mypy/blob/master/mypy/checker.py. I'll try following up on this idea later this weekend. (@lmazuel -- or do you maybe want to try taking a stab at this?)

[lmazuel]

Hi @Michael0x2a sorry for the late answer, was in vacation :).
I don't know if I have the skills to do that honestly, so please assume I don't (that's safer :)) but I will try to find time to understand this (maybe :))
Thanks!