enum.IntFlag.name is typed as str|None

[adaamz]

Hello,
I would like to ask/report bug whether it is possible to have (enum.IntFlag)().name as type None or if it is incorrectly typed as str|None.

Bug Report
Looks like (enum.IntFlag)().name is typed as str|None but I think it should be just str.

To Reproduce

import enum


class ListType(enum.IntFlag):
    ABC = enum.auto()
    DEF = enum.auto()
    GHI = enum.auto()
    
    def get_names(self) -> list[str]:
        to_return = []

        for e in ListType:
            reveal_type(e)   # note: Revealed type is "__main__.ListType"
            reveal_type(e.name)  # note: Revealed type is "Union[builtins.str, None]"
            to_return.append(e.name)

        return to_return  # error: Incompatible return value type (got "list[str | None]", expected "list[str]")  [return-value]

https://onlinegdb.com/Rsv6ryZzf
https://mypy-play.net/?mypy=latest&python=3.11&gist=61af435c15e57ce5221abada2221008a

Expected Behavior
I would expect that (enum.IntFlag)().name is always string.

Success: no issues found in 1 source file

Actual Behavior

main.py:19: error: Incompatible return value type (got "list[str | None]", expected "list[str]")  [return-value]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.5.1
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.9, 3.10, 3.11

[JelleZijlstra]

I believe this is correct specifically for Flag. We have an explicit override for it: https://github.com/python/typeshed/blob/9e79a100b3a9f1ae931b46cea56dc3ef99122ae5/stdlib/enum.pyi#L235 Flags can have a None name because they can have values (combinations of basic flags) without an associated name.

In your case, you're iterating over the predefined members of the enum, which will always have a name, but I don't think there's a way to express that in the type system.

[adaamz]

@JelleZijlstra thank you for quick response. How can I create such enum without a name? I'm just curious how and why to do it, as I have never seen it 😉

[AlexWaygood]

Python 3.11.5 (tags/v3.11.5:cce6ba9, Aug 24 2023, 14:38:34) [MSC v.1936 64 bit (AMD64)] on win32
>>> from enum import Flag
>>> class MyFlags(Flag):
...     a = 1
...     b = 2
...
>>> MyFlags(0)
<MyFlags: 0>
>>> MyFlags(0).name is None
True

And on Python <=3.10:

Python 3.10.8 | packaged by conda-forge | (main, Nov 24 2022, 14:07:00) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from enum import Flag
>>> class MyFlags(Flag):
...     a = 1
...     b = 2
...
>>> MyFlags.a|MyFlags.b
<MyFlags.b|a: 3>
>>> _.name is None
True

[adaamz]

Thanks for examples, looks like enum is pretty strange in Python 🙂

I will just use cast to string around e.name on this single place.

[JelleZijlstra]

I would recommend using assert e.name is not None, for what it's worth; that communicates the same intent and is also safe at runtime.