New submission from zachrahan: In python 3.6 (and several versions back), using argparse with required subparsers will cause an unhelpful TypeError if the 'dest' parameter is not explicitly specified, and no arguments are provided.
Test case: import argparse parser = argparse.ArgumentParser() subparsers = parser.add_subparsers() subparsers.required = True args = parser.parse_args([]) Observed result: TypeError: sequence item 0: expected str instance, NoneType found If the line above is changed to: subparsers = parser.add_subparsers(dest='function') Then the following is printed to stderr: usage: python [-h] {} ... python: error: the following arguments are required: function This issue goes back at least several years: http://stackoverflow.com/questions/23349349/argparse-with-required-subparser/23354355 Though it seems odd to not specify a dest in the add_subparsers line, the pattern is not completely useless. The below works fine without setting a 'dest' in add_subparsers, except when argv is empty: sub1 = subparsers.add_parser('print') sub1.set_defaults(function=print) However, an empty argv produces the unexpected TypeError above. I'm not sure if argparse should provide a more useful exception in this case, or if there is a clean way to do the right thing without a dest specified. ---------- components: Library (Lib) messages: 285646 nosy: zachrahan priority: normal severity: normal status: open title: argparse fails with required subparsers, un-named dest, and empty argv type: behavior versions: Python 3.6 _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue29298> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com