Finds a DNA sequence complement

complement.py

#!/usr/bin/python2
"""

A DNA sequence is a string made up of the letters A, T, G, and C.
To find the complement of a DNA sequence, As are replaced by Ts,
Ts by As, Gs by Cs, and Cs by Gs. For example, the complement
of AATTGCCGT is TTAACGGCA.

This function finds the complement.

"""

from UserString import MutableString
from sys import argv

def complement(seq):
    result = MutableString(seq)
    for i in range(len(result)):
      if result[i] == 'A':
        result[i] = 'T'
      elif result[i] == 'T':
        result[i] = 'A'
      elif result[i] == 'G':
        result[i] = 'C'
      elif result[i] == 'C':
        result[i] = 'G'
    return result

def main():
  print repr(complement(argv[1]))

if __name__ == '__main__':
	main()