How to represent class and nested class attributes in dict of dicts?

class Serial:
    def read(self):
        dic = {}
        for key, val in self.__class__.__dict__.items():
            if not key.startswith("__") and not callable(val):
                dic[key] = val
        for key, val in self.__dict__.items():
            dic[key] = val.read() if hasattr(val, "read") else val
        return dic


class A(Serial):
    aa = 1

class B(Serial):
    bb = 2

class C(Serial):
    c1 = 3
    a = A()
    def __init__(self):
        self.c2 = 4
        self.b = B()


c = C()
c.read()  # {'c1': 3, 'a': <__main__.A at 0x2b5f43ba1d0>, 'c2': 4, 'b': {'bb': 2}}

The above works only if I move c1 and a into __init__.

class C(Serial):
    def __init__(self):
        self.c1 = 3
        self.a = A()
        self.c2 = 4
        self.b = B()


c = C()
c.read()  # {'c1': 3, 'a': {'aa': 1}, 'c2': 4, 'b': {'bb': 2}}

Is there a simpler way?