enum.nonmember Typing Issues

[max-muoto]

There are some issues with the typing for enum.nonmember:

import enum


class MyEnum(enum.StrEnum):
    VAL_1 = "val_1"
    VAL_2 = "val_2"

    _items = enum.nonmember({VAL_1: 1, VAL_2: 2})

    def to_int(self) -> int:
        return self._items[self]

Behavior like this is is fully supported at runtime, however, __getattr__ isn't defined on enum.nonmember, making this raise an error in Pyright. Previously, this was allowed, but now the typeshed definitions are fully relied on: microsoft/pyright#7924

Current types:

  class nonmember(Generic[_EnumMemberT]):
      def __init__(self, value: _EnumMemberT) -> None: ...
      def __getattr__(self, name: str) -> _EnumMemberT: ...

Additionally, using .value here actually does lead to a runtime error, but types for nonmember would indicate that being fine.

# Fine
MyEnum.VAL_1.to_int()

Type-checker would except this, but this fails at runtime:

import enum


class MyEnum(enum.StrEnum):
    VAL_1 = "val_1"
    VAL_2 = "val_2"

    _items = enum.nonmember({VAL_1: 1, VAL_2: 2})

    def to_int(self) -> int:
        return self._items.value[self]

#  'dict' object has no attribute 'value'
MyEnum.VAL_1.to_int()

[srittau]

The stubs for nonmember are a 1:1 representation of the runtime class. The reason that this works at runtime is some magic in EnumDict:

https://github.com/python/cpython/blob/81c3130c51a2b1504842cb1a93732cc03ddbbd79/Lib/enum.py#L402-L404

This is something that the stubs can't reproduce, but needs to be hard-coded into type checkers, unfortunately.