Maybe, but this is a behaviour change so not what I tried to do here. It would require all subclasses of CTCBeam to have the same signature for the constructor, which it is not clear to me is desirable.

For the record, I think this is what the types would look like:

from typing import TypeVar, Type

Subclass = TypeVar('Subclass', bound='CTCBeam')

class CTCBeam:
    @classmethod
    def from_lm_beam(cls: Type[Subclass], lm_beam: "LMCTCBeam") -> Subclass:

Ah, well perhaps its better not to mess with it then. In that case LGTM