Augmented assignment on immutable vs mutable

Question

Accepted Answer

Tuples are immutable, so += cannot mutate the existing object in place. Instead, Python creates a brand-new tuple (1, 2, 3) and rebinds x to it, leaving y still pointing to the original (1, 2). Compare this to x += [3] on a list: lists are mutable, so += calls list.__iadd__ which mutates in place, and both names see the change. This difference in behavior between mutable and immutable types under += is a frequent interview question.