Python - Inversión de diccionario

from collections import defaultdict

def inverter(dic, key=None):

    newDict = defaultdict(dict)

    for k, v in dic.items():

        if not isinstance(v, dict):

            if not key:

                newDict[v] =  k

            else:

                newDict[k].update({key: v})

        else:

            v2 = inverter(v, k)

            for k2 in v2:

                if k2 in newDict:

                    newDict[k2].update(v2[k2])

                else:

                    newDict[k2] = v2[k2]

    return dict(newDict)

dic = {'Pos 1': {'Pos 2': 2, 'Pos 3': 3}, 'Pos 2': {'Pos 3': 4}}

dic_inv = inverter(dic)

print(dic_inv)

{'Pos 2': {'Pos 1': 2}, 'Pos 3': {'Pos 1': 3, 'Pos 2': 4}}