ROSALIND REVC

Given: A DNA string s of length at most 1000 bp.
Return: The reverse complement of s.

import sys
import re

def rev_comp(s):
 res = ''
 for i in s:
  if i == 'A':
   res += 'T'
  elif i == 'G':
   res += 'C'
  elif i == 'T':
   res += 'A'
  elif i == 'C':
   res += 'G'
 return res[::-1]

def main():
 if len(sys.argv) > 1:
  print rev_comp(sys.argv[1])
 else:
  print 'Enter your sequence!'

if __name__ == '__main__':
 main()