[Python] Enum of dataclass works but frozen attrs doesn't

The built-in enum provides a way to create enums of primitive types (IntEnum, StrEnum).

I'd like to create an enum of structured objects.

One way to do that is with dataclass, and it works:

from dataclasses import dataclass
from enum import Enum

from typing_extensions import assert_never

@dataclass(frozen=True)
class _BaseInstTypeDataclass:
instance_type: str
display_name: str


class InstTypeDataClass(_BaseInstTypeDataclass, Enum):
t2_micro = ("t2.micro", "t2.micro: Cheap!")
r7i_2xlarge = ("r7i.2xlarge", "r7i.2xlarge: Expensive!")


assert list(InstTypeDataClass) == [
InstTypeDataClass.t2_micro,
InstTypeDataClass.r7i_2xlarge,
]
assert isinstance(InstTypeDataClass.t2_micro, InstTypeDataClass)


# This function type checks
def f_dataclass(e: InstTypeDataClass):
if e == InstTypeDataClass.t2_micro:
...
elif e == InstTypeDataClass.r7i_2xlarge:
...
else:
assert_never(e)


Both the static- (via pyright) and runtime-behavior is as expected.

However, with attrs...:

import attrs


@attrs.define(frozen=True)
class _BaseInstTypeAttrs:
instance_type: str
display_name: str


class InstTypeAttrs(_BaseInstTypeAttrs, Enum):
t2_micro = ("t2.micro", "t2.micro: Cheap!")
r7i_2xlarge = ("r7i.2xlarge", "r7i.2xlarge: Expensive!")


# This function type checks
def f_attrs(e: InstTypeAttrs):
if e == InstTypeAttrs.t2_micro:
...
elif e == InstTypeAttrs.r7i_2xlarge:
...
else:
assert_never(e)


... the type checker is happy, but at run-time...:

$ python foo.py
_BaseInstTypeDataclass(instance_type='foo', display_name='bar')
Traceback (most recent call last):
File "foo.py", line 58, in <module>
class InstTypeAttrs(_BaseInstTypeAttrs, Enum):
File "/Users/.../python3.10/enum.py", line 287, in __new__
enum_member._value_ = value
File "/Users/.../python3.10/site-packages/attr/_make.py", line 551, in _frozen_setattrs
raise FrozenInstanceError()
attr.exceptions.FrozenInstanceError


Is this because... Enum and attrs.define(frozen=True) don't play nice...?

Continue reading...