To argp the subparser argument looks just like another positional, one that takes choices (the names of the subparsers). Also argp knows nothing about pos_arg1. That's in tmpp's list of arguments.
When argp sees filename command1 otherarg, filename and command1 satisfy its 2 positionals. otherarg is then passed on the tmpp.
With command1 otherarg, again 2 strings, 2 argp positionals. command is assigned to inputfile. There's no logic to backtrack and say command1 fits subcommands better, or that `tmpp' needs one of those strings.
You could change the 1st positional to an optional, --inputfile.
Or you could inputfile another positional of tmpp. If a number of the subparsers need it it, consider using parents.
argparse isn't a smart as you, and can't 'think ahead' or 'backtrack'. If it appears to do something smart it's because it uses re pattern matching to handle nargs values (e.g. ?, *, +).
EDIT
One way to 'trick' argparse into recognizing the first positional as the subparser is to insert an optional after it. With command1 -b xxx otherarg, -b xxx breaks up the list of positional strings, so only command1 is matched against inputfile and subcommands.
p=argparse.ArgumentParser()p.add_argument('file',nargs='?',default='foo')sp = p.add_subparsers(dest='cmd')spp = sp.add_parser('cmd1')spp.add_argument('subfile')spp.add_argument('-b')p.parse_args('cmd1 -b x three'.split())# Namespace(b='x', cmd='cmd1', file='foo', subfile='three')While argparse allows variable length positionals in any order, how it handles them can be confusing. It's easier to predict what argparse will do if there is only one such positional, and it occurs at the end.
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