test.py

class A(dict):
    def __new__(cls):
        ret = dict.__new__(cls)
        ret.a = 1
        return ret


class B(A):
    def __new__(cls, a):
        print('A new', a)
        #return _A.__new__(cls) # 注释掉这句，a.a则会报错
        return dict.__new__(cls)
    
    def __init__(cls, a): pass # 若无这个，B(1) 也会报错，这也算是个问题

a = B(1)
print(type(a))
#print(a.a)