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
