from abc import abstractproperty

from typing import Generic, TypeVar

T_co = TypeVar('T_co', covariant=True)
T = TypeVar('T')

class Dataset(Generic[T_co]):
    def __getitem__(self, index) -> T_co:
        raise NotImplementedError


class Base(Dataset):
    @abstractproperty
    def num_joints(self):
        return None

class Derived(Base):
    def t(self):
        pass

d = Derived()