first attempt at a merge sort

merge_sort.py

def merge_sort(l, first_iteration=False):
    data = []
    if first_iteration:
        return merge_sort([[i] for i in l])
    else:
        for i in xrange(0, len(l), 2):
            left, right = l[i], l[i+1] if i != len(l)-1 else []
            subdata = []
            for l_item in list(left):
                for r_item in list(right):
                    if l_item < r_item:
                        subdata.append(l_item)
                        left.remove(l_item)
                        break
                    elif r_item < l_item:
                        subdata.append(r_item)
                        right.remove(r_item)
            subdata += left + right
            data.append(subdata)
        return [y for x in data for y in x] if len(data) == 1 else merge_sort(data)

if __name__ == '__main__':
    my_list = [1, 12, 4, 7, 3, 5, 2, 6, 11, 10, 9, 13, 8]
    print(merge_sort(my_list, True))