ROSALIND DNA

Given: A DNA string s of length at most 1000 nt.
Return: Four integers (separated by spaces) counting the respective number of times that the symbols 'A', 'C', 'G', and 'T' occur in s.

I've wrote even two functions to count lettes:

import sys
import re

def cycle(s):
 a_num = 0
 t_num = 0
 g_num = 0
 c_num = 0
 for i in s:
  if i == 'A':
   a_num += 1
  elif i == 'T':
   t_num += 1
  elif i == 'G':
   g_num += 1
  elif i == 'C':
   c_num += 1
 return str(a_num) + ' ' + str(c_num) + ' ' + str(g_num) + ' ' + str(t_num)

def reg_expr(s):
 A = re.findall(r'A', s)
 C = re.findall(r'C', s)
 G = re.findall(r'G', s)
 T = re.findall(r'T', s)
 return str(len(A)) + ' ' + str(len(C)) + ' ' + str(len(G)) + ' ' + str(len(T))

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

if __name__ == '__main__':
 main()