[ty] Support iterating over enums

[sharkdp]

Summary

Infer the correct type in a scenario like this:

class Color(Enum):
    RED = 1
    GREEN = 2
    BLUE = 3

for color in Color:
    reveal_type(color)  # revealed: Color

We should eventually support this out-of-the-box when astral-sh/ty#501 is implemented. For this reason, @AlexWaygood would prefer to keep things as they are (we currently infer Unknown, so false positives seem unlikely). But it seemed relatively easy to support, so I'm opening this for discussion.

part of astral-sh/ty#183

Test Plan

Adapted existing test.

Ecosystem analysis

- warning[unused-ignore-comment] rotkehlchen/chain/aggregator.py:591:82: Unused blanket `type: ignore` directive

This unused-ignore-comment goes away due to a new true positive.

[github-actions]

mypy_primer results

Changes were detected when running on open source projects

rotki (https://github.com/rotki/rotki)
- warning[unused-ignore-comment] rotkehlchen/chain/aggregator.py:591:82: Unused blanket `type: ignore` directive
- Found 1580 diagnostics
+ Found 1579 diagnostics

No memory usage changes detected ✅

[AlexWaygood]

The code looks good but you already know my opinion on whether it's worth adding this branch in the first place :-)

[MichaReiser]

Alex said the code looks good ;)

[sharkdp]

It doesn't look like a huge maintenance burden to me, so I'm merging this to complete my list of "high priority" enum tasks.

[AlexWaygood]

I suppose the logic here doesn't account for the fact that enums can have subclasses of EnumMeta as their metaclass. E.g. we reveal Foo here, but at runtime iterating over this enum class yields ints:

from typing import Iterator, reveal_type
import enum

class Annoying(enum.EnumMeta):
    def __iter__(cls) -> Iterator[int]:
        yield from range(4)

class Foo(enum.Enum, metaclass=Annoying):
    A = 1
    B = 2

for f in Foo:
    reveal_type(f)

https://play.ty.dev/3347bb64-35ba-469a-9359-b5fe504bb81d

Having said that, we'll emit a Liskov violation on Annoying.__iter__ once we've implemented astral-sh/ty#166. In fact, I don't think it's possible to override EnumMeta.__iter__ and change its return type in a Liskov-compatible way. So this probably isn't a serious issue.

[carljm]

FWIW, mypy issues the Liskov error, pyright does not. Both mypy and pyright do correctly infer that the iteration yields int in that example.

[carljm]

But I think once we fix astral-sh/ty#501 and remove this temporary approach, that should fall out anyway, so not worth worrying about now.