Counting Point Mutations (ROSALIND HAMM)

Given: Two DNA strings s and t of equal length (not exceeding 1 kbp).
Return: The Hamming distance dH(s,t).

import sys
def main():
 if len(sys.argv) > 1:
  s1 = sys.argv[1]
  s2 = sys.argv[2]
  diff_counter = 0
  i = 0
  while i < len(s1):
   if s1[i] != s2[i]:
    diff_counter += 1
   i += 1
  print diff_counter
 else:
  print 'Enter 2 strings.'

if __name__ == '__main__':
 main()