Except if __iadd__ doesn't exist on the type, then __add__ is called. The variable is always reassigned. Quick example:

x = (0, [1, 2])
try:
    x[1] += [3]  # calls list.__iadd__, then reassigns x[1]
except TypeError as e:
    print(e)

x += (4,)  # calls tuple.__add__, then reassigns x

print(x)

What is printed?