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 cl

Solution 1:

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.

Solution 2:

class A(object): pass
class B(A): pass
class C(A): pass
class D(C): pass

# get the list of ancestors (assuming single inheritance!)
def ancl(cls):
    ret = []
    while cls is not object:
        ret.append(cls)
        cls = cls.__bases__[0]
    ret.append(object)
    return ret

def clcoancl(cls1, cls2):
    ancl1 = ancl(cls1)
    ancl2 = ancl(cls2)
    # find the first class present in both ancl1 and ancl2
    while len(ancl1) > 0 and len(ancl2) > 0 and ancl1[-1] == ancl2[-1]:
        ret = ancl1.pop(-1)
        ancl2.pop(-1)
    return ret

print clcoancl(A, B)
print clcoancl(B, C)
print clcoancl(C, D)

Whether you actually need this is a different matter, as pointed out by @DanielRoseman is his comment to your question.