homepage

An alternative to Jason's example:
parser = argparse.ArgumentParser()
parser.add_argument('app')
parser.add_argument('--config')
parser.add_argument('app_args', nargs=argparse.REMAINDER)
args = parser.parse_args(['--config', 'bar', 'app'])
print vars(args)
# as expected: {'app': 'app', 'app_args': [], 'config': 'bar'}
When you have several positionals, one or more of which may have 0 arguments (*,?,...), it is best to put all of the optional arguments first. 
With 'app --config bar', parse_args identifies a 'AOA' pattern (argument, optional, argument). It then checks which positional arguments match. 'app' claims 1, 'app_args' claims 2 (REMAINDER means match everything that follows). That leaves nothing for '--config'.
What you expected was that 'app' would match with the 1st string, '--config' would match the next 2, leaving nothing for 'app_args'. 
In http://bugs.python.org/issue14191 I wrote a patch that would give the results you want if 'app_args' uses '*'. That is makes it possible to interleave positional and optional argument strings. But it does not change the behavior of REMAINDER.
parser.add_argument('app_args', nargs='*')
--------------
Maybe the documentation example for REMAINDER needs to modified to show just how 'greedy' REMAINDER is. Adding a:
parser.add_argument('--arg1',action='store_true')
does not change the outcome. REMAINDER still grabs '--arg1' even though it is a defined argument.
Namespace(arg1=False, args=['--arg1', 'XX', 'ZZ'], command='cmd', foo='B')