If I run this script with -h
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from argparse import ArgumentParser
mapping = ['123456', '12345', '12345', '123']
p = ArgumentParser('11111111111111')
p.add_argument('-v', '--verbose', help='verbose mode', action='store_true')
p.add_argument('targets', help='installation targets',  nargs='+', metavar='\n'.join(mapping))
p.parse_args()
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I get an error:
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Traceback (most recent call last):
  File "tmp.py", line 7, in <module>
    p.parse_args()
  File "/usr/lib/python3.6/argparse.py", line 1730, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/usr/lib/python3.6/argparse.py", line 1762, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/usr/lib/python3.6/argparse.py", line 1968, in _parse_known_args
    start_index = consume_optional(start_index)
  File "/usr/lib/python3.6/argparse.py", line 1908, in consume_optional
    take_action(action, args, option_string)
  File "/usr/lib/python3.6/argparse.py", line 1836, in take_action
    action(self, namespace, argument_values, option_string)
  File "/usr/lib/python3.6/argparse.py", line 1020, in __call__
    parser.print_help()
  File "/usr/lib/python3.6/argparse.py", line 2362, in print_help
    self._print_message(self.format_help(), file)
  File "/usr/lib/python3.6/argparse.py", line 2346, in format_help
    return formatter.format_help()
  File "/usr/lib/python3.6/argparse.py", line 282, in format_help
    help = self._root_section.format_help()
  File "/usr/lib/python3.6/argparse.py", line 213, in format_help
    item_help = join([func(*args) for func, args in self.items])
  File "/usr/lib/python3.6/argparse.py", line 213, in <listcomp>
    item_help = join([func(*args) for func, args in self.items])
  File "/usr/lib/python3.6/argparse.py", line 334, in _format_usage
    assert ' '.join(pos_parts) == pos_usage
AssertionError
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