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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.