Python: Merge two lists recursivley alternating between the heads of each list

merge.py

def merge_lists_into_string(a, b, string):
    '''
    Takes two lists of varying size and merges them like so:
        ([1,2,3], [10, 20, 30, 40, 50]) >> [1, 10, 2, 20, 3, 30, 40, 50]
    '''
    if a:
        string += a.pop(0)
    if b:
        string += b.pop(0)
    if a or b:
        string = merge_lists_into_string(a, b, string)
    return string