How to specify argparse default file when type=argparse.FileType

Question

I'm attempting to allow an optional argument that takes a single command line argument of a filename:

parser.add_argument('--somefile', nargs=1, type=argparse.FileType('r'),
                     required=False, default='defaultfile.json')

The problem is when I specify a file using the --somefile foo.json I get back a handle in a list:

args.somefile=[<_io.TextIOWrapper name='foo.json' mode='r' encoding='UTF-8'>]

When I don't specify it and want the default, I get back a handle, but not in a list:

args.somefile=<_io.TextIOWrapper name='defaultfile.json' mode='r' encoding='UTF-8'>

How do I get args.somefile to return the same type of structure (handle either in a list or not in a list) consistently?

sKwa · Accepted Answer · 2018-02-07 11:20:24Z

1

Remove nargs argument.

Note that nargs=1 produces a list of one item. This is different from the default, in which the item is produced by itself.

UPDATE on comment

Removing nargs=1 would eliminate the requirement to specify the filename argument.

Lets check:

>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--somefile', type=argparse.FileType('r'), required=False, default='/tmp/data.json')
_StoreAction(option_strings=['--somefile'], dest='somefile', nargs=None, const=None, default='/tmp/data.json', type=FileType('r'), choices=None, help=None, metavar=None)
>>> args = parser.parse_args(['--somefile',])
usage: [-h] [--somefile SOMEFILE]
: error: argument --somefile: expected one argument

edited Feb 7, 2018 at 11:20

answered Feb 7, 2018 at 2:57

sKwa

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3 Comments

Omniver Over a year ago

Removing nargs=1 would eliminate the requirement to specify the filename argument.

sKwa Over a year ago

@Omniver, no, its not. Check it by yourself.

Omniver Over a year ago

Thank you. I did not read far enough in the documentation. 'nargs - The number of command-line arguments that should be consumed.' Later though: 'ArgumentParser objects usually associate a single command-line argument with a single action to be taken. The nargs keyword argument associates a different number of command-line arguments with a single action.'

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