Extend is a method of list, which modifies it but doesn't return self (returning None instead). If you need the modified value as the expression value, use +, as in [2]+[].
>>> x = [2]>>> x.extend([]) # Nothing is printed because the return value is None>>> x == [2]True>>> x[2]
They do this on purpose so that you will remember that the extend function is actually modifying the list in-place. Same with sort(). It always returns None.