I have inherited a legacy project that has a Superclass which offers hooks for custom behaviors by invoking override functions in the subclasses. The superclass defines the attribute as None, and invokes the methods when the value is not None. It seems to be loosely based on ideas form zope.interface and protocols.

For example, a simplified version of the code is:

class Super:
    hook_bar: Optional[Callable] = None
    def foo(self) -> List:
        data = []
        if self.hook_bar is not None:
            data = self.hook_bar()
        return data

class Sub(Super):
    def hook_bar(self)->List:
        return hook_bar

I can’t seem to figure out how to type Super.hook_bar with a signature, which I’d like to do so I can ensure a consistent use of the interface with mypy.

I’ve tried a dozen techniques used for tangential concepts, but the only thing I’ve found that works with mypy is to define and unset the method – then have mypy ignore the unset:

class Super:
    def hook_bar(self)->List:
        return hook_bar
    hook_bar = None  # type: ignore

If I use Protocol to write classes that support the function and then inherit from that (as in the file-like mypy examples), I still need to ignore the = None on the superclass. I’ve been able to use typing.Protocol to define optional Callable+returns in method arguments, but that doesn’t seem to work as a classmethod.

Is there a better way to handle this?