Different results for `isinstance` with `collecitons.abc.Collection` and `collections.abc.Mapping`

Question

I am having trouble understanding how isinstance is meant to work with the Abstract Base Classes from collections.abc . My class that implements all specified methods for collections.abc.Mapping does not make isinstance return True for my class, but does make isinstance return True for collections.abc.Collection . My class is not registered as a subclass with either ABC.

Running the following code (with Python 3.7, but I'm not sure if that matters):

class dictroproxy:
    def __init__(self, d): 
        self._d = d 
    def __getitem__(self, key):
        return self._d.__getitem__(key)
    def __contains__(self, key):
        return self._d.__contains__(key)
    def __len__(self):
        return self._d.__len__()
    def __iter__(self):
        return self._d.__iter__()
    def __eq__(self, other):
        if isinstance(other, dictroproxy):
            other = other._d
        return self._d.__eq__(other)
    def __ne__(self, other):
        return not self.__eq__(other)
    def get(self, key, default=None):
        return self._d.get(key, default)
    def keys(self):
        return self._d.keys()
    def values(self):
        return self._d.values()
    def items(self):
        return self._d.items()

if __name__ == "__main__":
    from collections.abc import Collection, Mapping
    dd = dictroproxy({"a": 1, "b": 2, "c": 3})
    print("Is collection?", isinstance(dd, Collection))
    print("Is mapping?", isinstance(dd, Mapping))

Gives me the following output:

Is collection? True
Is mapping? False

Am I missing something in my implementation, or do Collection and Mapping behave differently?

Answer 1

From my investigation into this I found that some ABCs implement a __subclasshook__ method to determine if a class seems like a subclass of the ABC. At least as of Python 3.9, some ABCs implement __subclasshook__ and some do not. Collection does and Mapping does not.

I haven't seen which ABCs this works for documented anywhere, so it may be that the only way to know is to try or review the source code.