Invariant Union?

[treykeown]

Is it possible to create a TypeAlias of a union, but where one of the specified types in a union must be matched exactly (invariant)?

Special: TypeAlias = Foo | Bar

Subclasses of Foo or Bar would not satisfy Special.

I believe I can create a TypeVar that achieves something similar:

T = TypeVar("T", Foo, Bar)

but my concern is that I'd be using a type variable where a "normal" annotation works best.

[hauntsaninja]

Probably not, could you talk more about your use case?

Note value constrained TypeVars may not work the way you expect them to: https://mypy-play.net/?mypy=latest&python=3.12&gist=6768354f1bab66850422a86eb5723a1f (the TypeVar is prevented from solving to SubFoo, not prevented from solving). Also note invariant generics don't prevent contained types from being subclasses either: https://mypy-play.net/?mypy=latest&python=3.12&gist=c29b3396aabe1ef94461301e7a6ede06

[treykeown]

My use case is a custom de/serializer. I only support a few specific types - it's not able to build subclasses. For now I work around this with runtime assertion:

from typing import get_args

AnySerializable: TypeAlias = Foo | Bar
serializable_types = get_args(AnySerializable)

def serialize(item: AnySerializable) -> bytes:
    assert type(item) in serializable_types
    ...

[erictraut]

There's no way to specify an "exact type" in any typing construct. From a type theory standpoint, any subclass of Foo is also a Foo.

One potential solution is to mark Foo as @final which would prevent any subclasses from being defined.