Confusing error message for a missing classmethod decorator

[wrobell]

Please consider

import typing as tp
import enum

@enum.unique
class ColorIndex(enum.IntEnum):
    BLACK = 0
    GREY = 1
    WHITE = 2

    def _missing_(cls, value: object) -> tp.Any:
        return ColorIndex.BLACK

then mypy shows very confusing error (both signatures, for the subclass and the superclass, are the same)

$ mypy t.py
t.py:10: error: Signature of "_missing_" incompatible with supertype "Enum"
t.py:10: note:      Superclass:
t.py:10: note:          def _missing_(cls, value: object) -> Any
t.py:10: note:      Subclass:
t.py:10: note:          def _missing_(cls, value: object) -> Any
Found 1 error in 1 file (checked 1 source file)

The real issue is that _missing_ should be decorated with classmethod.

Python 3.10.1, mypy 0.930.

[sobolevn]

This works for me on latest master when I remove @unique decorator:

import typing as tp
import enum

# @enum.unique
class ColorIndex(enum.IntEnum):
    BLACK = 0

    def _missing_(cls, value: object) -> tp.Any:
        return ColorIndex.BLACK

» mypy out/ex.py --strict-optional --warn-unreachable --show-traceback
out/ex.py:7: error: Signature of "_missing_" incompatible with supertype "Enum"
out/ex.py:7: note:      Superclass:
out/ex.py:7: note:          @classmethod
out/ex.py:7: note:          def _missing_(cls, value: object) -> Any
out/ex.py:7: note:      Subclass:
out/ex.py:7: note:          def _missing_(cls, value: object) -> Any

[sobolevn]

No, I was wrong. It depends on the cache.

[sobolevn]

This is very hard to solve, but I've got a working prototype. I am going to finish it tomorrow.

[wrobell]

@sobolevn Thanks for the fix.

I wonder if it is also possible to improve the main error message?

Replace

Signature of "_missing_" incompatible with supertype "Enum"

with

Supertype "Enum" defines method "_missing_" as a class method

?

[sobolevn]

I think it is better to keep the existing format. It is way more informative.
When it works correctly, it shows everything we need to know.

[wrobell]

Agreed on the format. Still, the error message is going to be confusing.

Please consider:

import typing as tp
import enum

@enum.unique
class ColorIndex(enum.IntEnum):
    BLACK = 0
    GREY = 1
    WHITE = 2

    @classmethod
    def _missing_(cls, value: object) -> ColorIndex:
        return ColorIndex.BLACK

then

$ mypy t.py
Success: no issues found in 1 source file

Without classmethod decorator:

$ mypy t.py
t.py:10: error: Signature of "_missing_" incompatible with supertype "Enum"
t.py:10: note:      Superclass:
t.py:10: note:          @classmethod
t.py:10: note:          def _missing_(cls, value: object) -> Any
t.py:10: note:      Subclass:
t.py:10: note:          def _missing_(cls, value: object) -> ColorIndex
Found 1 error in 1 file (checked 1 source file)

The error message suggests I should change ColorIndex to Any, while it is missing classmethod decorator.

[GabrieleCalarota]

Any updates on this?