add type hints

[nicrie]

That should support code maintenance and documentation in the future.

In our case of subclasses we probably need TypeVar and Generic.

Here's an example:

from abc import ABC, abstractmethod
from typing import TypeVar, Generic

T = TypeVar('T')

class A(ABC, Generic[T]):
    @abstractmethod
    def method(self, input: T) -> T:
        pass

class B(A[int]):
    def method(self, input: int) -> int:
        # Implement the method specifically for ints
        pass

class C(A[float]):
    def method(self, input: float) -> float:
        # Implement the method specifically for floats
        pass

In this example, A is an abstract base class and uses a type variable T. It declares an abstract method method that takes an argument of type T and returns a value of type T. B and C inherit from A, specializing T to int and float respectively. Each of them implements method for their respective types.

This approach ensures that the subclasses B and C will work with the specific types you want (int and float respectively), while maintaining the generality of the base class A.

If we want to ensure that A should only be instantiated with certain types, we can add constraints to the TypeVar:

T = TypeVar('T', int, float)

With this, T can only be int or float, and trying to create a class that inherits from A[str] for example, would result in a type error.

[nicrie]

fixed by #91