have 1.py as:
import argparse
import sys
print (sys.argv)
parser = argparse.ArgumentParser(prog='demo')
parser.add_argument('--host -h', help='host for the server of %(prog)', nargs=1, required=True, metavar='<host>')
parser.add_argument('--port -p', help='port for the server of %(prog)', nargs=1, required=True, metavar='<port>')
parser.parse_args(sys.argv)


and run C:\Python34\python.exe 1.py -h 1.
Will obtain:

Traceback (most recent call last):
  File "1.py", line 7, in <module>
    parser.parse_args(sys.argv)
  File "C:\Python34\lib\argparse.py", line 1717, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "C:\Python34\lib\argparse.py", line 1749, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "C:\Python34\lib\argparse.py", line 1955, in _parse_known_args
    start_index = consume_optional(start_index)
  File "C:\Python34\lib\argparse.py", line 1895, in consume_optional
    take_action(action, args, option_string)
  File "C:\Python34\lib\argparse.py", line 1823, in take_action
    action(self, namespace, argument_values, option_string)
  File "C:\Python34\lib\argparse.py", line 1016, in __call__
    parser.print_help()
  File "C:\Python34\lib\argparse.py", line 2348, in print_help
    self._print_message(self.format_help(), file)
  File "C:\Python34\lib\argparse.py", line 2332, in format_help
    return formatter.format_help()
  File "C:\Python34\lib\argparse.py", line 278, in format_help
    help = self._root_section.format_help()
  File "C:\Python34\lib\argparse.py", line 208, in format_help
    func(*args)
  File "C:\Python34\lib\argparse.py", line 208, in format_help
    func(*args)
  File "C:\Python34\lib\argparse.py", line 515, in _format_action
    help_text = self._expand_help(action)
  File "C:\Python34\lib\argparse.py", line 602, in _expand_help
    return self._get_help_string(action) % params
ValueError: incomplete format

You are missing an 's' after the parens.  It should be: %(prog)s

It's essentially the same issue as http://bugs.python.org/issue21666.

An error in the 'help' parameter is not caught until 'print_help' is called (via 'parse_args').  

A possible improvement is to test the 'help' string during 'add_argument'.  Similar testing for 'nargs' and 'metavar' has been proposed.