Invalid typing assumption over variable with list(StrEnum) on python 3.11

[Nnonexistent]

Bug Report

On python 3.11 using StrEnum, if converting given enum to a list and assigning to a variable, mypy will wrongly assumes that given variable will be list[str], not list[StrEnum].

To Reproduce

from enum import StrEnum

class Choices(StrEnum):
    LOREM = "lorem"
    IPSUM = "ipsum"

# This is ok
def ok_func() -> list[Choices]:
    return list(Choices)

# However this produces an error
def error_func() -> list[Choices]:
    var = list(Choices)
    return var

https://mypy-play.net/?mypy=latest&python=3.11&gist=e73a15c8902092236567da4a4567f372

Expected Behavior

Mypy should assume list[Choices] type for var.

Actual Behavior

Incompatible return value type (got "List[str]", expected "List[Choices]") [return-value]

Your Environment

• Mypy version used: 1.0.0
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.11.1

[Apakottur]

In case it helps, I've just encountered the same issue in a slightly different situation:

from enum import StrEnum

class Choices(StrEnum):
    A = "a"

def my_func(obj: Choices) -> None:
    assert isinstance(obj, Choices)

for element in list(Choices):
    my_func(element)

Which gives:

error: Argument 1 to "my_func" has incompatible type "str"; expected "Choices"  [arg-type]

In my case I'm not assigning list(Choices) to anything, but the elements are inferred as str and not Choices.

[kikones34]

Are there any plans to work on this? We've had to silence this error many times in our codebase :(
I can confirm it still happens on Python 3.12 with Mypy 1.8.0.

[jhenly]

I was fiddling around with this issue and I found a, somewhat reasonable, workaround where you don't have to cast or silence the error (@kikones34).

from typing import overload, Self
from enum import StrEnum as _StrEnum

class StrEnum(_StrEnum):

    # mimic str.__new__'s overloads
    @overload
    def __new__(cls, object: object = ...) -> Self: ...
    @overload
    def __new__(cls, object: object, encoding: str = ..., errors: str = ...) -> Self: ...

    def __new__(cls, *values):
        # when we import enum, _StrEnum.__new__ gets moved to _new_member_ when
        # the "final" _StrEnum class is created via EnumType
        return _StrEnum._new_member_(cls, *values)

class Choices(StrEnum):
    LOREM = "lorem"
    IPSUM = "ipsum"

# this is still ok
def ok_func() -> list[Choices]:
    return list(Choices)

# and this no longer produces an error
def error_func() -> list[Choices]:
    var = list(Choices)
    return var

https://mypy-play.net/?mypy=latest&python=3.11&gist=3e91cdf18f5a07b47d6619cb43940b6c

This also seems to cover the issue @Apakottur found:
https://mypy-play.net/?mypy=latest&python=3.11&gist=f32efd891ff8b25fa333c937093682e1

You can then use it like so:

# file path: project_root/fixes/enum.py

from typing import overload, Self
from enum import StrEnum as _StrEnum

__all__ = ['StrEnum', ]

class StrEnum(_StrEnum):
    @overload
    def __new__(cls, object: object = ...) -> Self: ...
    @overload
    def __new__(cls, object: object, encoding: str = ..., errors: str = ...) -> Self: ...
    
    def __new__(cls, *values):
        return _StrEnum._new_member_(cls, *values)

# file path: project_root/main.py

from fixes.enum import StrEnum

class Choices(StrEnum):
    LOREM = "lorem"
    IPSUM = "ipsum"

if __name__ == '__main__':
    print(Choices.__members__)
    # outputs:
    # {'LOREM': <Choices.LOREM: 'lorem'>, 'IPSUM': <Choices.IPSUM: 'ipsum'>}

---

I'm not sure how mypy works, but I think this issue could be resolved if StrEnum in enum.pyi, in the typeshed:

# current enum.pyi

class StrEnum(str, ReprEnum):
    def __new__(cls, value: str) -> Self: ...
    _value_: str
    @_magic_enum_attr
    def value(self) -> str: ...
    @staticmethod
    def _generate_next_value_(name: str, start: int, count: int, last_values: list[str]) -> str: ...

Was updated to account for str.__new__'s overloads:

# proposed enum.pyi

class StrEnum(str, ReprEnum):
    @overload
    def __new__(cls, object: object = ...) -> Self: ...
    @overload
    def __new__(cls, object: ReadableBuffer, encoding: str = ..., errors: str = ...) -> Self: ...
    _value_: str
    @_magic_enum_attr
    def value(self) -> str: ...
    @staticmethod
    def _generate_next_value_(name: str, start: int, count: int, last_values: list[str]) -> str: ...

I understand that the documentation for StrEnum.__new__ states "values must already be of type str", but if you look at the actual code you'll notice that StrEnum.__new__ is accounting for all of the current parameters to str.__new__:

def __new__(cls, *values):
    "values must already be of type `str`"
    if len(values) > 3:
        raise TypeError('too many arguments for str(): %r' % (values, ))
    if len(values) == 1:
        # it must be a string
        if not isinstance(values[0], str):
            raise TypeError('%r is not a string' % (values[0], ))
    if len(values) >= 2:
        # check that encoding argument is a string
        if not isinstance(values[1], str):
            raise TypeError('encoding must be a string, not %r' % (values[1], ))
    if len(values) == 3:
        # check that errors argument is a string
        if not isinstance(values[2], str):
            raise TypeError('errors must be a string, not %r' % (values[2]))
    value = str(*values)
    member = str.__new__(cls, value)
    member._value_ = value
    return member

---

Edit

Updating the enum.pyi typeshed file with the proposed updates doesn't seem to fix anything. Hopefully the workaround described above will help someone until the underlying issue gets resolved.

[kikones34]

@jhenly your fix works, thanks! Although I really don't like having to import a custom StrEnum, we'll probably just keep silencing the errors for now.

[jarmstrong-atlassian]

I don't think this has anything to do with assigning StrEnum to a list. Contrary to the documentation, StrEnum just doesn't work (although on a closer reading I notice that the example is specific to Enum): https://mypy.readthedocs.io/en/stable/literal_types.html#enums

from enum import (
    StrEnum,
    Enum,
)
from typing import reveal_type


class TestEnum(Enum):
    ONE = 'one'

class TestStrEnum(StrEnum):
    ONE = 'one'

reveal_type(TestEnum.ONE)
reveal_type(TestStrEnum.ONE)

def test_enum(param: TestEnum):
    print(param)

def test_str_enum(param: TestStrEnum):
    print(param)

test_enum(TestEnum.ONE)
test_str_enum(TestStrEnum.ONE)

The output is:

mypy test_enum.py
test_enum.py:24: error: Argument 1 to "test_str_enum" has incompatible type "str"; expected "TestStrEnum"  [arg-type]

Python 3.11.7 (main, Jan 18 2024, 12:21:46) [GCC 11.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> from test_enum import TestEnum
Runtime type is 'TestEnum'
Runtime type is 'TestStrEnum'
TestEnum.ONE
one

[kikones34]

@jarmstrong-atlassian how are you performing the test? I get the following output with Python 3.11.1 and Mypy 1.9.0:

enum_test.py:14: note: Revealed type is "Literal[enum_test.TestEnum.ONE]?"
enum_test.py:15: note: Revealed type is "Literal[enum_test.TestStrEnum.ONE]?"
Success: no issues found in 1 source file

[jarmstrong-atlassian]

@kikones34 My bad. I was running mypy enum_test.py, but this was still in the directory with my current mypy.ini which was still setting python_version = 3.10. Removing that line, I get the expected result you posted. Sorry for the confusion.

[tamird]

Another weird case + a workaround that appeases mypy:

from collections.abc import Iterable
from enum import StrEnum

def takes_enum(e: StrEnum) -> StrEnum:
    return e

def takes_enum_type(type_: type[StrEnum]) -> StrEnum:
    return next(iter(type_)) # error: Incompatible return value type (got "str", expected "StrEnum")  [return-value]
    
def takes_enum_type_with_trick(type_: type[StrEnum]) -> StrEnum:
    return next(iter(mypy_type_hint_trick(type_))) # no error!
    
def mypy_type_hint_trick(type_: type[StrEnum]) -> Iterable[StrEnum]:
    return type_

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