How to determine the closest common ancestor class

Suppose I have four classes: A, B derived from A, C derived from A, and D derived from C. (So I always have single inheritance.) In python, what is the best way to determine the closest common ancestor of any two (instances of such) classes? Specifically, I need a function clcoancl(X,Y) for which clcoancl(A, B) == A, clcoancl(B, C) == A, and clcoancl(C, D) == C.

This should work for single or multiple inheritance, with any number of classes as input:

import inspect
from collections import defaultdict

def clcoancl(*cls_list):
    mros = [list(inspect.getmro(cls)) for cls in cls_list]
    track = defaultdict(int)
    while mros:
        for mro in mros:
            cur = mro.pop(0)
            track[cur] += 1
            if track[cur] == len(cls_list):
                return cur
            if len(mro) == 0:
                mros.remove(mro)
    return None # or raise, if that's more appropriate

As both NPE and Daniel Rossman have mentioned though, this is probably not the optimal solution to your root problem.