Generate a weak ref to a python callable accounting for loose functions, bound methods and callable classes.

callable_weak_ref.py

"""
Generate a weak ref to a python callable accounting for loose functions, bound methods and callable classes.

This allows creating references to python functions without increasing their reference countm and creating
circular dependencies.
"""
import typing
import weakref
import types


def callable_weak_ref(callback: typing.Callable):
    if not callable(callback):
        raise TypeError("callback must be callable")

    elif isinstance(callback, types.FunctionType):
        return weakref.ref(callback)

    elif isinstance(callback, types.MethodType) and callback.__self__ is not None:
        return weakref.WeakMethod(callback)
    elif isinstance(callback, types.MethodType) and callback.__self__ is None:
        raise TypeError(
            "callback must be a bound method, a function, or a callable class"
        )
    else:
        # otherwise it's a callable class.
        return weakref.ref(callback)