Skip to content

Option with optional argument

Tom de Geus

Suppose that I have

My program

Usage:
  myprog [options]

Options:
  -h, --help        Show this screen.
      --version     Show version.
      --files=<arg> Files. [default: foo.txt]

I would like to distinguish in my code:

  • --files not specified.
  • --files specified, but with no argument to accept the default.
  • --files myfile, i.e. --files specified with custom argument.

With the current docstring I can either

  • Not specify --files.
  • Specify --files with an argument.

So I’m missing:

  • The option to specify --files without an argument.
  • Distinguish if --files was specified, or if the user specified --files foo.txt

Advertisement

Answer

You will need to specify the --files argument in the main usage string. For example:

# dopt.py
from docopt import docopt

dstr = """My program

Usage:
  myprog [--files [FNAME]] [options]

Options:
  -h, --help        Show this screen.
      --version     Show version.
"""

if __name__ == '__main__':
    arguments = docopt(dstr)
    print(arguments)

This essentially makes --files a true/false argument and adds another argument FNAME to hold the file name. Usage:

$ python dopt.py
{'--files': False,
 '--help': False,
 '--version': False,
 'FNAME': None}

$ python dopt.py --files
{'--files': True,
 '--help': False,
 '--version': False,
 'FNAME': None}

$ python dopt.py --files abc.txt
{'--files': True,
 '--help': False,
 '--version': False,
 'FNAME': 'abc.txt'}

Then, you can use the value of --files and FNAME from the returned dict to infer what to do:

if not arguments['--files']:
    print("Not using files")
elif not arguments['FNAME']:
    print("Using default file foo.txt")
else:
    print(f"Using file {arguments['FNAME']}")

A pitfall to remember: you can also specify FNAME independently of --files. So this also works, and it might interfere with other arguments, so be sure to test all combinations thoroughly:

$ python dopt.py abc.txt
{'--files': False,
 '--help': False,
 '--version': False,
 'FNAME': 'abc.txt'}
Not using files

Personally, I prefer using argparse because it’s less ambiguous. It builds the doc string from the prescribed arguments, not the other way round.

In argparse, an argument can have a default value and you can specify that it can take zero or one argument using nargs="?". Then, you can specify a const="foo.txt" value which the argument will take if no values are given. For example:

# dopt.py
import argparse

parser = argparse.ArgumentParser()
parser.add_argument("--files", required=False, default=None, nargs="?", const="foo.txt")
p = parser.parse_args()
print(p)

And running this:

$ python dopt.py
Namespace(files=None)

$ python dopt.py --files
Namespace(files='foo.txt')

$ python dopt.py --files abc.txt
Namespace(files='abc.txt')

And it even handles the “no --files” case correctly:

$ python dopt.py abc.txt
usage: dopt.py [-h] [--files [FILES]]
dopt.py: error: unrecognized arguments: abc.txt