220

I have created a script using argparse.

The script needs to take a configuration file name as an option, and user can specify whether they need to proceed totally the script or only simulate it.

The args to be passed: ./script -f config_file -s or ./script -f config_file.

It's ok for the -f config_file part, but It keeps asking me for arguments for the -s which is optionnal and should not be followed by any.

I have tried this:

parser = argparse.ArgumentParser()
parser.add_argument('-f', '--file')
#parser.add_argument('-s', '--simulate', nargs = '0')
args = parser.parse_args()
if args.file:
    config_file = args.file
if args.set_in_prod:
        simulate = True
else:
    pass

With the following errors:

File "/usr/local/lib/python2.6/dist-packages/argparse.py", line 2169, in _get_nargs_pattern
nargs_pattern = '(-*%s-*)' % '-*'.join('A' * nargs)
TypeError: can't multiply sequence by non-int of type 'str'

And same errror with '' instead of 0.

Stoic
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philippe
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2 Answers2

335

As @Felix Kling suggested use action='store_true':

>>> from argparse import ArgumentParser
>>> p = ArgumentParser()
>>> _ = p.add_argument('-f', '--foo', action='store_true')
>>> args = p.parse_args()
>>> args.foo
False
>>> args = p.parse_args(['-f'])
>>> args.foo
True
Community
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jfs
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    Shouldn't it be `-s` instead of `-f` ? AFAIK the -f option should have a filename as an option and the -s option trigger the dry-run behaviour. Or am I understanding it the wrong way? – runlevel0 May 06 '22 at 13:45
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    `-f` is short for `--foo` as in [`foobar`](https://en.wikipedia.org/wiki/Foobar) -- just a placeholder/dummy name -- no semantics. – jfs May 06 '22 at 14:12
135

To create an option that needs no value, set the action [docs] of it to 'store_const', 'store_true' or 'store_false'.

Example:

parser.add_argument('-s', '--simulate', action='store_true')
Felix Kling
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  • Hello;Thanks for your answer. I have tried as explained in the doc: >>> parser.add_argument('--foo', '-f', action='store_true') _StoreTrueAction(option_strings=['--foo', '-f'], dest='foo', nargs=0, const=True, default=False, type=None, choices=None, help=None, metavar=None) >>> parser.parse_args() Namespace(foo=False) >>> print args.foo Traceback (most recent call last): File "", line 1, in AttributeError: 'Namespace' object has no attribute 'foo' >>>` This does not seem to work; I Don't know how to retrieve the value after assignation. Thanks for helping me! – philippe Mar 11 '11 at 09:41
  • @philippe: you've forgot to bind `parser.parse_args()` returned value to `args` http://stackoverflow.com/questions/5262702/argparse-module-how-to-add-option-without-any-argument/5271692#5271692 – jfs Mar 11 '11 at 10:26
  • @philippe: `args = parser.parse_args()` as you already have in your code. You just have to replace the commented line and then you can access the value with `args.simulate`. – Felix Kling Mar 11 '11 at 10:34