[ParamSpec] Duplicating the signature from one method to another method in ABC

[randolf-scholz]

Consider that we have some abstract base class

class Distribution(Protocol[T_co]):
    """Represents a random distribution."""

    @abstractmethod
    def pdf(self, x: ArrayLike, /) -> T_co:
        """Returns the value of the probability density function at x."""
        ...
    
    
    def logpdf(self, x: ArrayLike, /) -> T_co:
        """Returns the value of the log-probability density function at x."""
        try:
            return self.pdf(x).log()
        except AttributeError as exc:
            raise NotImplementedError from exc

The default implementation of logpdf is likely not useful when a subclass decides to add optional arguments. Instead we might want to do something like:

    def logpdf(self, x: ArrayLike, /, *args, **kwargs) -> T_co:
        """Returns the value of the log-probability density function at x."""
        try:
            return self.pdf(x, *args, **kwargs).log()
        except AttributeError as exc:
            raise NotImplementedError from exc

However, this would mean that subclasses really do have to always allow *args and **kwargs. What we really want to say is that the signature of logpdf has to be identical to pdf. (So something like def logpdf(self, x: ArrayLike, /, *args: P.args, **kwargs: P.kwargs) where P is a ParamSpec bound to self.pdf)

Maybe one could do some wrapper trickery in __init_subclass__, but that seems complicated. Is there any way to directly tell type checkers the intention here?

[randolf-scholz]

Some related issues:

• Inherit type signature from another method mypy#2003
• Method signature forwarding for subclasses #1079