class A:
    def __init__(self, a, b):
        self.a = a
        self.b = b

    def add(self):
        return self.a + self.b

    def sub(self):
        return self.a - self.b
    
a = A(1, 2)
assert a.add() == 3
assert a.sub() == -1

assert A.__base__ is object

class B(A):
    def __init__(self, a, b, c):
        super(B, self).__init__(a, b)
        self.c = c

    def add(self):
        return self.a + self.b + self.c

    def sub(self):
        return self.a - self.b - self.c

assert B.__base__ is A    

b = B(1, 2, 3)
assert b.add() == 6
assert b.sub() == -4

class C(B):
    def __init__(self, a, b, c, d):
        super(C, self).__init__(a, b, c)
        self.d = d

    def add(self):
        return self.a + self.b + self.c + self.d

    def sub(self):
        return self.a - self.b - self.c - self.d
    
assert C.__base__ is B

c = C(1, 2, 3, 4)
assert c.add() == 10
assert c.sub() == -8

class D(C):
    def __init__(self, a, b, c, d, e):
        super(D, self).__init__(a, b, c, d)
        self.e = e

    def add(self):
        return super(D, self).add() + self.e

    def sub(self):
        return super(D, self).sub() - self.e
    
assert D.__base__ is C

d = D(1, 2, 3, 4, 5)
assert d.add() == 15
assert d.sub() == -13

assert isinstance(1, int)
assert isinstance(1, object)
assert isinstance(C, type)
assert isinstance(C, object)
assert isinstance(d, object)
assert isinstance(d, C)
assert isinstance(d, B)
assert isinstance(d, A)
assert isinstance(object, object)
assert isinstance(type, object)

class A:
    a = 1
    b = 2

assert A.a == 1
assert A.b == 2

class B(A):
    b = 3
    c = 4

# assert B.a == 1  ...bug here
assert B.b == 3
assert B.c == 4

from c import void_p

class A(void_p):
    pass
    
a = A()
assert repr(a) == '<void* at (nil)>'