How to fix python recursion function

Question

I am trying to write a recursive function which returns a copy of a list where neighbouring elements have been swapped. For example, swapElements([2, 3, 4, 9]) would return [3, 2, 9, 4].
This is my code as of now:

def swapElements(mylist):
    if len(mylist) == 1:
        pass
    if len(mylist) == 2:
        mylist[0], mylist[1] = mylist[1], mylist[0]
    else:
        mylist[0], mylist[1] = mylist[1], mylist[0]
        swapElements(mylist[2:])
    return mylist

When I run this function it only returns the list with the first two elements swapped, does anyone know why this function is not swapping any other elements other than the first two and how I could fix it?

Answer 1

In your question it says you wish to return a copy . Your function mutates mylist in-place. Additionally, pass has a return value of None . You can simplify the function like so -

def swap (a = []):
  if len(a) < 2:
    return a
  else:
    return [ a[1], a[0] ] + swap(a[2:])

print(swap([2, 3, 4, 9]))
# [2, 3, 4, 9]

print(swap([2, 3, 4, 9, 6]))
# [2, 3, 4, 9, 6]

print(swap([1]))
# [1]

print(swap())
# []

Answer 2

I would let the recursion do the work and, if this is Python 3, do something less index-oriented like:

def swapPairs(array):
    if len(array) < 2:
        return array

    a, b, *rest = array

    return [b, a, *swapPairs(rest)]