Martin Panter added the comment:

I suggest this is a duplicate of Issue 15112. The same problem also happens 
with nargs="*", and that issue apparently has a patch to handle both cases.

For the record, this is the resulting error, and a demo that it works if the 
option comes before the positional arguments:

>>> print("test 2:", ap.parse_args(["abc", "--option", "mmm"]))
usage: [-h] [--option] arg_1 [arg_2]
: error: unrecognized arguments: mmm
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/proj/python/cpython/Lib/", line 1729, in parse_args
    self.error(msg % ' '.join(argv))
  File "/home/proj/python/cpython/Lib/", line 2385, in error
    self.exit(2, _('%(prog)s: error: %(message)s\n') % args)
  File "/home/proj/python/cpython/Lib/", line 2372, in exit
  File "/home/", line 345, in exit
    raise SystemExit(code)
__main__.SystemExit: 2
>>> ap.parse_args(["--option", "abc", "mmm"])
Namespace(arg_1='abc', arg_2='mmm', option=True)

nosy: +vadmium
resolution:  -> duplicate
stage:  -> resolved
status: open -> closed
superseder:  -> argparse: nargs='*' positional argument doesn't accept any 
items if preceded by an option and another positional
title: argparse parsing bug -> argparse parsing (mingling --option and optional 
positional argument)

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