I think this example is quite deceptive. When setting obj.a and obj.b, what is your intention? If you want to set the instance attributes, you should have created self.a and self.b in the class constructor.
class A:
a = (1,)
b = [1,]
def __init__(self):
self.a = (1,)
self.b = [1,]
A.a # this is the class attribute
(1,)
A.b # class attribute
[1]
obj = A()
obj.a # instance attribute
(1,)
obj.b
[1]
obj.a += (2,)
obj.b += [2,]
A.a
(1,) # still the same class attribute
A.b
[1]
obj.a
(1, 2) # instance attribute appended
obj.b
[1, 2]
