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Re: Why does argparse return None instead of [] if an append action isn't used?

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On 01/09/2015 03:44 PM, Adam Funk wrote:
> I noticed in use that if an option with the 'append' action isn't
> used, argparse assigns None to it rather than an empty list, &
> confirmed this interactively:
>
> #v+
>>>> import argparse
>>>> parser = argparse.ArgumentParser()
>>>> parser.add_argument('--foo', action='append')
> _AppendAction(option_strings=['--foo'], dest='foo', nargs=None, const=None, default=None, type=None, choices=None, help=None, metavar=None)
>>>> parser.add_argument('--bar', action='append')
> _AppendAction(option_strings=['--bar'], dest='bar', nargs=None, const=None, default=None, type=None, choices=None, help=None, metavar=None)
>>>> parser.parse_args('--foo 1 --foo 2'.split())
> Namespace(bar=None, foo=['1', '2'])
> #v-
>

Isn't that the exact behaviour documented here:

https://docs.python.org/3/library/argparse.html#default

where it says that the default for the default argument is None ?

I think Skip is right: you should be able to just add

default = []

to your arguments in the add_argument call.

Wolfgang

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Why does argparse return None instead of [] if an append action isn't used? Adam Funk <a24061@ducksburg.com> - 2015-01-09 14:44 +0000
  Re: Why does argparse return None instead of [] if an append action isn't used? Skip Montanaro <skip.montanaro@gmail.com> - 2015-01-09 08:57 -0600
    Re: Why does argparse return None instead of [] if an append action isn't used? Adam Funk <a24061@ducksburg.com> - 2015-01-09 15:25 +0000
  Re: Why does argparse return None instead of [] if an append action isn't used? Ned Batchelder <ned@nedbatchelder.com> - 2015-01-09 10:12 -0500
    Re: Why does argparse return None instead of [] if an append action isn't used? Adam Funk <a24061@ducksburg.com> - 2015-01-26 13:43 +0000
      Re: Why does argparse return None instead of [] if an append action isn't used? Peter Otten <__peter__@web.de> - 2015-01-26 15:04 +0100
        Re: Why does argparse return None instead of [] if an append action isn't used? Adam Funk <a24061@ducksburg.com> - 2015-01-26 15:50 +0000
          Re: Why does argparse return None instead of [] if an append action isn't used? Ian Kelly <ian.g.kelly@gmail.com> - 2015-01-26 09:13 -0700
            Re: Why does argparse return None instead of [] if an append action isn't used? Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2015-01-26 18:29 +0200
              Re: Why does argparse return None instead of [] if an append action isn't used? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-01-26 17:18 +0000
              Re: Why does argparse return None instead of [] if an append action isn't used? Ian Kelly <ian.g.kelly@gmail.com> - 2015-01-26 11:23 -0700
              Re: Why does argparse return None instead of [] if an append action isn't used? Ian Kelly <ian.g.kelly@gmail.com> - 2015-01-26 11:26 -0700
  Re: Why does argparse return None instead of [] if an append action isn't used? Wolfgang Maier <wolfgang.maier@biologie.uni-freiburg.de> - 2015-01-09 16:50 +0100
    Re: Why does argparse return None instead of [] if an append action isn't used? Adam Funk <a24061@ducksburg.com> - 2015-01-26 13:43 +0000

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