python argsparse data types

Question

I am running into a weird problem with args.parse in python 3.4 and I need some help. I am just starting with python, so it may be something stupid.

I am passing an argument

parser.add_argument('-solr', type=str, default='http://localhost:8983/solr/', nargs='+', help='Address of Solr server (ex: http://192.168.137.128:8983/solr/)')

It should be just one argument, all the time. If I actually pass the argument, it will get stored as an array and I will need to access it via args.solr[0]. However, if I don't pass any arguments to it, the default value will be used, but it will be stored as string, thus, args.solr[0] will be 'h'.

What gives? I've tried to play around with nargs, but even setting it to 1 doesn't seem to change anything.

Thanks in advance

Hari K · Accepted Answer · 2014-07-06 15:41:58Z

1

Use nargs = '?'. It stopped taking the input as an array.

parser.add_argument('--solr', type=str, default='http://localhost:8983/solr/',nargs='?',help='Address of Solr server (ex: http://192.168.137.128:8983/solr/)')

answered Jul 6, 2014 at 15:41

Hari K

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hpaulj Over a year ago

nargs='?' is more useful on positionals than optionals - unless you want --solr without any argument to mean something.

hpaulj · Accepted Answer · 2014-07-06 17:46:06Z

Since your argument is flagged (name starts with -) you don't need to set nargs at all - use its default None. It is optional (not required) unless you use the flag string.

parser.add_argument('--solr', default='http://localhost:8983/solr/', 
    help='Address of Solr server (ex: %(default)s)')

With this, sample inputs produce:

In [51]: parser.parse_args([])
Out[51]: Namespace(solr='http://localhost:8983/solr/')

In [52]: parser.parse_args(['--solr','test'])
Out[52]: Namespace(solr='test')

In [50]: parser.print_help()
usage: ipython [-h] [--solr SOLR]

optional arguments:
  -h, --help   show this help message and exit
  --solr SOLR  Address of Solr server (ex: http://localhost:8983/solr/)

With longer optional names, a -- is better than -. The default type is str, so you don't need to specify that. I've added %(default)s to the help, so you don't have to repeat the default value.

If you do want the result to be a list, e.g. ['test'], use nargs=1 and put the default in a list as well, e.g. default=['http...'].

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