old_contents: from __future__ import print_function
  import mass
  import argparse
  
  
  def main():
      parser = argparse.ArgumentParser(description='Calculate molar mass of a chemical compound.', prog='calcmass')
      parser.add_argument('compounds', metavar='C', nargs='+',
                          help='a compound to find the mass of')
      args = parser.parse_args()
      for a in args.compounds:
          val = mass.add_markers(a)
          elements = mass.strip_coeff(val)
          if len(elements) == 0:
              raise argparse.ArgumentParser.error(parser, 'Not a compound')
          if len(args.compounds) > 1:
              print(a + ':', end=' ')
          if type(mass.calculate(elements)) != str:
              print("%0.5f" % mass.calculate(elements))
          else:
              raise argparse.ArgumentParser.error(parser, 'Not a compound: ' +
                                                  mass.calculate(elements))
  
  
  if __name__ == "__main__":
      main()
  
new_contents: from __future__ import print_function
  from calcmass import mass
  import argparse
  
  
  def main():
      parser = argparse.ArgumentParser(description='Calculate molar mass of a chemical compound.', prog='calcmass')
      parser.add_argument('compounds', metavar='C', nargs='+',
                          help='a compound to find the mass of')
      args = parser.parse_args()
      for a in args.compounds:
          val = mass.add_markers(a)
          elements = mass.strip_coeff(val)
          if len(elements) == 0:
              raise argparse.ArgumentParser.error(parser, 'Not a compound')
          if len(args.compounds) > 1:
              print(a + ':', end=' ')
          if type(mass.calculate(elements)) != str:
              print("%0.5f" % mass.calculate(elements))
          else:
              raise argparse.ArgumentParser.error(parser, 'Not a compound: ' +
                                                  mass.calculate(elements))
  
  
  if __name__ == "__main__":
      main()
  
subject: Update __main__.py