iter doesn't work on TypeVar with bound of Enum

Bug Report

A function that takes an argument of Type[_E] where _E = TypeVar("_E", bound=Enum) does not accept calling iter on that argument. Calling iter on a concrete enum class is accepted, which is why I think this isn't purely a typeshed issue. Using the class in a loop or calling the __iter__ member is also accepted.

To Reproduce

Run mypy on the following:

#!/usr/bin/env python3

import socket
from enum import Enum
from typing import Iterator, Type, TypeVar

_E = TypeVar("_E", bound=Enum)

def enum_iter(cls: Type[_E]):
    return iter(cls)

for value in enum_iter(socket.SocketKind):
    print(value)

Expected Behavior

Expected no mypy errors.

Actual Behavior

mypy_enum_iter.py:10: error: No overload variant of "iter" matches argument type "Type[_E]"
mypy_enum_iter.py:10: note: Possible overload variants:
mypy_enum_iter.py:10: note:     def [_SupportsNextT <: SupportsNext[Any]] iter(_SupportsIter[_SupportsNextT]) -> _SupportsNextT
mypy_enum_iter.py:10: note:     def [_T] iter(Callable[[], Optional[_T]], None) -> Iterator[_T]
mypy_enum_iter.py:10: note:     def [_T] iter(Callable[[], _T], object) -> Iterator[_T]
Found 1 error in 1 file (checked 1 source file)

What's even weirder is that adding an Iterator[_E] return annotation to enum_iter makes it pass.

Your Environment

Mypy version used: 0.942
Mypy command-line flags: none
Mypy configuration options from mypy.ini (and other config files): none
Python version used: 3.8.10
Operating system and version: Ubuntu 20.04

I think this is a repro that doesn't involve enums, revealing that the issue is a general one with mypy's understanding of metaclasses rather than an issue specific to the enum plugin:

from typing import Iterator, TypeVar

M = TypeVar("M")

class Meta(type):
    def __iter__(self: type[M]) -> Iterator[M]: ...

class Foo(metaclass=Meta): ...

F = TypeVar("F", bound=Foo)

def foo_iter(cls: type[F]):
    return iter(cls)

Mypy output:

main.py:13: error: No overload variant of "iter" matches argument type "Type[F]"
main.py:13: note: Possible overload variants:
main.py:13: note:     def [_SupportsNextT <: SupportsNext[Any]] iter(__iterable, _SupportsIter[_SupportsNextT]) -> _SupportsNextT
main.py:13: note:     def [_T] iter(__function, Callable[[], Optional[_T]], None) -> Iterator[_T]
main.py:13: note:     def [_T] iter(__function, Callable[[], _T], object) -> Iterator[_T]
Found 2 errors in 1 file (checked 1 source file)

It probably doesn't even have anything to do with TypeVars. I can reproduce a similar error with this snippet:

from typing import Iterator, TypeVar

M = TypeVar("M")

class Meta(type):
    def __iter__(self: type[M]) -> Iterator[M]: ...

class Foo(metaclass=Meta): ...

def foo_iter(cls: type[Foo]):
    return iter(cls)  # error: Argument 1 to "iter" has incompatible type "Type[Foo]"; expected "_SupportsIter[<nothing>]"

I guess a good question is whether subclasses of Foo are expected to have a metaclass that is a subclass (in the Liskov substitution sense) of Meta. If not, then the error would be legitimate since cls could be an instance of a subclass of Foo whose metaclass doesn't provide __iter__.

Support protocol inference for Type[T] via metaclass (#14554)

Navigation Menu

Repository navigation

iter doesn't work on TypeVar with bound of Enum #12553

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

iter doesn't work on TypeVar with bound of Enum #12553

Description

Activity

AlexWaygood commented on Apr 9, 2022

AlexWaygood commented on Apr 10, 2022

bmerry commented on Apr 11, 2022

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Issue actions