class Adder:
    def add(self, x, y):
        print('not implemented!')
    def __init__(self, start=[]):
        self.data = start
    def __add__(self, other):                    # Or in subclasses?
        return self.add(self.data, other)        # Or return type?

class ListAdder(Adder):
    def add(self, x, y):
        return x + y

class DictAdder(Adder):
    def add(self, x, y):
        new = {}
        for k in x.keys(): new[k] = x[k]
        for k in y.keys(): new[k] = y[k]
        return new