New submission from James Corbett <james.h.corb...@gmail.com>:
The `argparse.ArgumentParser` sometimes rejects positional arguments with no arguments when `choices` is set and `nargs="*"`. When there are no arguments and `nargs` is `"*"`, the default value is chosen, or `[]` if there is no default value. This value is then checked against `choices` and an error message is printed if the value is not in `choices`. However, sometimes the value is intentionally not in `choices`, and this leads to problems. An example will explain this much better, and show that the issue only occurs with the particular combination of positionals, `nargs="*"`, and `choices`: ``` >>> import argparse >>> parser = argparse.ArgumentParser() >>> parser.add_argument("foo", choices=["a", "b", "c"], nargs="*") >>> parser.add_argument("--bar", choices=["d", "e", "f"], nargs="*") >>> parser.add_argument('--baz', type=int, choices=range(5, 10), default="20") >>> parser.parse_args("a --bar".split()) Namespace(foo=['a'], bar=[], baz=20) >>> parser.parse_args(["a"]) Namespace(foo=['a'], bar=None, baz=20) >>> parser.parse_args([]) usage: [-h] [--bar [{d,e,f} ...]] [--baz {5,6,7,8,9}] [{a,b,c} ...] : error: argument foo: invalid choice: [] (choose from 'a', 'b', 'c') ``` In this case I could have got around the last error by adding `[]` to choices, but that pollutes the help and usage messages. ---------- components: Library (Lib) messages: 371915 nosy: jameshcorbett priority: normal severity: normal status: open title: argparse: misbehavior when combining positionals and choices type: behavior versions: Python 3.9 _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue41047> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com