merging two sorted lists

Function to merge two sorted lists in linear time. This function is half of the mergesort algorithm.

#! /usr/bin/env python

def merge_sorted_lists(list1, list2):
 r = []
 i, j = 0, 0
 while i < len(list1) and j < len(list2):
  if list1[i] < list2[j]:
   r.append(list1[i])
   i += 1
  else:
   r.append(list2[j])
   j += 1
 if i == len(list1):
  r.extend(list2[j:])
 elif j == len(list2):
  r.extend(list1[i:])
 else:
  print 'something\'s wrong'
 return r

print merge_sorted_lists([1,3,5], [2,4,6])

Sample output: