In Python, a = [1,2,3]; b = a; b.append(4) leaves a with four elements, but a = 5; b = a; b += 1 leaves a at 5. Why do the two behave differently?

Question

Accepted Answer

Because append mutates the shared object while += rebinds a local name

Answer

Because lists are global by default and integers are local

Answer

Because the assignment operator behaves differently for container types

Answer

Because integers are passed by value and lists by reference

In Python, `a = [1,2,3]; b = a; b.append(4)` leaves `a` with four elements, but `a = 5; b = a; b += 1` leaves `a` at 5. Why do the two behave differently?