def invertdict(D):
    def keysof(V):
        return sorted(K for K in D.keys() if D[K] == V)
    return {V: keysof(V) for V in set(D.values())}
D = {'x': 'a', 'y': 'b', 'z': 'a'}
print(invertdict(D))

class MyList(list):
    def __getitem__(self, index):
        return super().__getitem__(index) * 2
m = MyList([1, 2])
print([m[i] + x for i, x in enumerate(m)])

class A: pass
class B(A): pass 
class C1(A): pass 
class C2: pass
class D1(B, C1): pass
class D2(B, C2): pass

X = list(D1.__mro__)
Y = list(D2.__mro__)
print(X[3] is Y[2])

int(2) is 2 * True

class A: attr = 1  
class B(A): pass
class C(B): pass
class D(A): attr = 2
class E(C, D): attr = C.attr  
x = E()
print(x.attr)

a = type([1, 2, 3]) == type(list)
b = [1, 2, 3].__class__ == list.__class__
print(a, b)

class A: attr = 1  
class B(A): pass     
class С(A): attr = 2
class D(B, С): pass   
x = D()
print(x.attr)

isinstance(type, object) + isinstance(object, type) + (type is object)

class С: pass 
c = С() 
a = type(c) == c.__class__
b = type(type(c)) is c.__class__.__class__
print(a, b)

f = type
print(f(f(f(f(f(None))))) is f )