>> Do you guys have any good use case for such method?

> A use case was given in issue22998.
> My use case is JIT-compiling functions and function calls in Numba. We
> reimplement the function calls ourselves, so need a complete mapping of
> arguments to values.

This is a great use case ;-) Let's add it.

I propose the following method: BoundArguments.apply_defaults()

It will iterate through its parent Signature's parameters and assign default values to BoundArguments.arguments (when an arg is missing), including setting '()' for *args, and '{}' for **kwargs.

If you're OK with this, I can draft a patch.