Could an approach like this be used to avoid creating these functions? https://peps.python.org/pep-3119/#overloading-isinstance-and-issubclass

I don't think so, though I haven't read the PEP in detail.

However, it seems pretty advanced to use for a simple conversion.

It seems it'd be something like:

import sys

class GeneratorIsMeta(type):
    def __instancecheck__(cls, inst):
        return isinstance(inst, tuple(cls.gen_types()))

    def __subclasscheck__(cls, sub):
        return issubclass(sub, tuple(cls.gen_types()))


class DtTypes(metaclass=GeneratorIsMeta):
    @classmethod
    def gen_types(cls):
        yield dt.datetime
        yield dt.date
        if "numpy" in sys.modules:
            import numpy as np
            yield np.datetime64

class IntTypes(metaclass=GeneratorIsMeta):
    @classmethod
    def gen_types(cls):
        yield int
        if "numpy" in sys.modules:
            import numpy as np
            yield np.integer

Thank you.

I still thinks it is pretty advanced (even though you don't need it to be iterable in your example).

You could likely move the advanced logic into a decorator, but then you would need to import that into our other libraries. My main point with accepting iterator is not so much for param itself but for HoloViews.

My main point with accepting iterator is not so much for param itself but for HoloViews.

Oh yeah I agree users shouldn't have to pass this custom class, a generator seems appropriate. Instead I was wondering if there couldn't be a way to use the metaclass approach internally to avoid the special anyisinstance and anyissubclass functions, they seem to be easy to forget in the future.

I have switched to a metaclass approach with a decorator for easy access.