Python argparse: default argument stored as string, not list

Question

I cannot figure out this behaviour of argparse from the documentation:

import argparse

parser.add_argument("--host", metavar="", dest="host", nargs=1, default="localhost", help="Name of host for database.  Default is 'localhost'.")
args = parser.parse_args()
print(args)

Here is the output with and without an argument for "--host":

>> python demo.py
Namespace(host='localhost')

>> python demo.py --host host
Namespace(host=['host'])

In particular: why does the argument to "--host" get stored in a list when it is specified but not when the default is used?

Because you specified nargs=1. When nargs is present, because you can set nargs to + or a larger number, the results are stored in a list. But the default is given as a string. You can write default=["localhost"] and the default will be a list as well. — Xin Yin
– Xin Yin, Commented Aug 16, 2014 at 20:56
the default is added to the namespace as is (apart from any conversion that the type might do). nargs and action don't, for the most part, affect it. — hpaulj
– hpaulj, Commented Aug 16, 2014 at 23:20

Joohwan · Accepted Answer · 2020-03-14 05:00:06Z

35

Remove the "nargs" keyword argument. Once that argument is defined argparse assumes your argument is a list (nargs=1 meaning a list with 1 element)

edited Mar 14, 2020 at 5:00

answered Aug 16, 2014 at 21:08

Joohwan

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c24b · Accepted Answer · 2017-11-06 10:03:24Z

2

As an alternative and handy module: Docopt can be used for parsing command line arguments. Docopt transform a commandline into a dictionnary by defining values inside doc.

edited Nov 6, 2017 at 10:03

answered Aug 17, 2014 at 8:48

c24b

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Comments

Dave · Accepted Answer · 2021-01-29 11:53:49Z

1

The question title and the question body ask two different questions. This is potentially a sign of the confusion I shared with the OP.

The title: why is the default a string not a list? The body: why is the given value a list not a string?

The selected answer provides the solution to the question in the body. The answer to the question in the title is:

The entry default="localhost" sets default to "localhost", which is a sting. To set it as a list, you could use: default=["localhost"].

edited Jan 29, 2021 at 11:53

answered Jan 22, 2021 at 11:54

Dave

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2 Comments

hpaulj Over a year ago

It's a little unclear whether the OP is puzzled more by the value being a list (as you were), or the default not being a list. The final question could be interpreted either way.

Dave Over a year ago

Agreed - I've edited my answer to point out that confusion, rather than say it's misleading.

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