Translating RNA into Protein (ROSALIND PROT)

Given: An RNA string s corresponding to a strand of mRNA (of length at most 10 kbp).
Return: The protein string encoded by s.

import sys

def codon_table():
 nukaa = {}
 nukaa['UUU'] = 'F'
 nukaa['CUU'] = 'L'
 nukaa['AUU'] = 'I'
 nukaa['GUU'] = 'V'
 nukaa['UUC'] = 'F'
 nukaa['CUC'] = 'L'
 nukaa['AUC'] = 'I'
 nukaa['GUC'] = 'V'
 nukaa['UUA'] = 'L'
 nukaa['CUA'] = 'L'
 nukaa['AUA'] = 'I'
 nukaa['GUA'] = 'V'
 nukaa['UUG'] = 'L'
 nukaa['CUG'] = 'L'
 nukaa['AUG'] = 'M'
 nukaa['GUG'] = 'V'
 nukaa['UCU'] = 'S'
 nukaa['CCU'] = 'P'
 nukaa['ACU'] = 'T'
 nukaa['GCU'] = 'A'
 nukaa['UCC'] = 'S'
 nukaa['CCC'] = 'P'
 nukaa['ACC'] = 'T'
 nukaa['GCC'] = 'A'
 nukaa['UCA'] = 'S'
 nukaa['CCA'] = 'P'
 nukaa['ACA'] = 'T'
 nukaa['GCA'] = 'A'
 nukaa['UCG'] = 'S'
 nukaa['CCG'] = 'P'
 nukaa['ACG'] = 'T'
 nukaa['GCG'] = 'A'
 nukaa['UAU'] = 'Y'
 nukaa['CAU'] = 'H'
 nukaa['AAU'] = 'N'
 nukaa['GAU'] = 'D'
 nukaa['UAC'] = 'Y'
 nukaa['CAC'] = 'H'
 nukaa['AAC'] = 'N'
 nukaa['GAC'] = 'D'
 nukaa['CAA'] = 'Q'
 nukaa['AAA'] = 'K'
 nukaa['GAA'] = 'E'
 nukaa['CAG'] = 'Q'
 nukaa['AAG'] = 'K'
 nukaa['GAG'] = 'E'
 nukaa['UGU'] = 'C'
 nukaa['CGU'] = 'R'
 nukaa['AGU'] = 'S'
 nukaa['GGU'] = 'G'
 nukaa['UGC'] = 'C'
 nukaa['CGC'] = 'R'
 nukaa['AGC'] = 'S'
 nukaa['GGC'] = 'G'
 nukaa['CGA'] = 'R'
 nukaa['AGA'] = 'R'
 nukaa['GGA'] = 'G'
 nukaa['UGG'] = 'W'
 nukaa['CGG'] = 'R'
 nukaa['AGG'] = 'R'
 nukaa['GGG'] = 'G'
 return nukaa


def main():
 if len(sys.argv) > 1:
  rna_string = sys.argv[1]
  nukaa = codon_table()
  nucl_str_len = len(rna_string) // 3
  nucl_string = ''
  i = 0
  while i < nucl_str_len:
   codon = rna_string[3*i:3*(i+1)]
   if codon=='UAA' or codon=='UAG' or codon=='UGA':
    break
   else:
     nucl_string += nukaa[codon]
   i+=1
  print nucl_string
 else:
  print 'Enter RNA sequence.'

if __name__ == '__main__':
 main()