Annotating Enum attributes with Final causes surprising type violations

[yangskyboxlabs]

When Enum attributes are annoted with typing.Final, the "Type annotations are not allowed for enum members" warning is not raised as it would be for similar invalid annotations. Such attributes also also treated as having the annotated type instead of its Enum derived type.

For the code below:

from enum import Enum
from typing import Final


class BadEnum(Enum):
    VAL = "val"
    STR: str = "str"
    FINAL: Final = "final"


def get_enum(val: str) -> BadEnum:
    match val:
        case "val":
            return BadEnum.VAL
        case "str":
            return BadEnum.STR
        case "final":
            return BadEnum.FINAL
        case _:
            raise ValueError


assert type(get_enum("str")) is BadEnum
assert type(get_enum("final")) is BadEnum

pyright, in strict mode, will raise the following:

d:\pyright_scratch\enum_test.py
  d:\pyright_scratch\enum_test.py:7:5 - error: Type annotations are not allowed for enum members (reportGeneralTypeIssues)
  d:\pyright_scratch\enum_test.py:16:20 - error: Expression of type "str" is incompatible with return type "BadEnum"
    "str" is incompatible with "BadEnum" (reportReturnType)
  d:\pyright_scratch\enum_test.py:18:20 - error: Expression of type "Literal['final']" is incompatible with return type "BadEnum"
    "Literal['final']" is incompatible with "BadEnum" (reportReturnType)
3 errors, 0 warnings, 0 informations

To summarize:

1. FINAL attribute should also be flagged by reportGeneralTypeIssues?
2. BadEnum.STR and BadEnum.FINAL should still be treated as BadEnum instead of str and Literal['final']

Issue noticed after updating opentelemetry-semantic-convention to a version that contains open-telemetry/opentelemetry-python#3966

[erictraut]

Here's the applicable wording from the typing spec:

Members defined within an enum class should not include explicit type annotations. Type checkers should infer a literal type for all members. A type checker should report an error if a type annotation is used for an enum member because this type will be incorrect and misleading to readers of the code.

Based on that wording, an annotation of Final should probably be flagged as an error. I say "probably" because the spec says "type annotation", which implies that there's a supplied type expression. I authored that part of the spec, and when I wrote that sentence, I didn't consider that anyone would attempt to use a "naked" Final annotation with no type expression. In retrospect, I should have used the term "annotation expression" or just "annotation" to make it clear.

In any case, you shouldn't use Final in this context if you want your code to type check without errors.

I'll update pyright to emit an error for a Final annotation within an enum. This will be included in the next release.

[erictraut]

This is addressed in pyright 1.1.374.