paul j3 added the comment:
This patch adds this `nargs='{m,n}'` or `nargs=(m,n)` feature.
It builds on the `better nargs error message` patch in
http://bugs.python.org/msg187754
It includes an argparse.rst paragraph, changes to argparse.py, and additions to
test_argparse.py.
The tests include those proposed by wm, rewritten to use the ParserTestCase
framework where possible. I did not give a lot of thought to test names. The
coverage could also use further thought.
As WM noted some range cases are the equivalent to existing nargs options
('{1,}'=='+'). I did not attempt to code or test such equivalences. Since the
'{0,}' form uses regex matching just like '*',
I don't think there is any advantage to making such a translation.
I convert the tuple version (m,n) to the re string '{m,n}' during the
add_argument() testing. So it is the string form that is added to the parser
action. This is also the format that appears in usage.
The documentation paragraph is:
'{m,n}'. m to n command-line arguments are gathered into a list. This is
modeled on the Regular Expression use. '{,n}' gathers up to n arguments. '{m,}'
gathers m or more. Thus '{1,}' is the equivalent to '+', and '{,1}' to '?'. A
tuple notation is also accepted, '(m,n)', '(None,n)', '(m,None)'. For example:
>>> parser = argparse.ArgumentParser(prog='PROG')
>>> parser.add_argument('--foo', nargs='{2,4}')
>>> parser.parse_args('--foo a b c'.split())
Namespace(foo=['a', 'b', 'c'])
>>> parser.parse_args('--foo a'.split())
usage: PROG [-h] [--foo FOO{2,4}]
PROG: error: argument --foo: expected {2,4} arguments
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Added file: http://bugs.python.org/file30187/nargsrange.patch
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