class A:
    list1 = []
    number = 0

a, b = A(), A()

print(a.number, b.number, A.number)

a.number = 2
print(a.number, b.number, A.number) # understable that only a affected

A.number = 3
print(a.number, b.number, A.number) # understable that b.number and A.number affected only, because a.number overrides

print(a.list1, b.list1, A.list1)

a.list1.append(1)
print(a.list1, b.list1, A.list1) ## why all changed?

b.list1 = [2]
print(a.list1, b.list1, A.list1) # now b.list1 overrides