code: def is_valid_codon(codon, type='start'):
      
      if len(codon) != 3:
          return False
  
      if type == 'start':
          if codon != 'ATG':
              return False
      elif type == 'stop':
          if not any(_codon == codon for _codon in ('TGA', 'TAG', 'TAA')):
              return False
      else:
          logging.error("`{0}` is not a valid codon type. ".format(type) + \
              "Should be one of (`start` or `stop`)")
          sys.exit()
  
      return True
docs: Given a codon sequence, check if it is a valid start/stop codon