Specify valid values for arguments using docopt

You can specify them in a separate part of the __doc__ , as an Example -

Test Control Program

Usage: 
  test.py bitwrite ([--off=<bits>...][--on=<bits>...])
  test.py regwrite ([regA | regB]<value>)

Options:
  -o <bits>... --off <bits>...  #Turn bits off
  -i <bits>... --on <bits>...   #Turns bits on

Arguments:

      bits: 1 - on
            0 - off

Please note Arguments is not any special name recognized by docopt , its just any name, you can use any such appropriate name you want.

In the above the Arguments section would show the valid values for bits, and other arguments .

Then when running the above doc with --help option, you would get result as -

>>>> python a.py --help
Test Control Program

Usage:
  test.py bitwrite ([--off=<bits>...][--on=<bits>...])
  test.py regwrite ([regA | regB]<value>)

Options:
  -o <bits>... --off <bits>...  #Turn bits off
  -i <bits>... --on <bits>...   #Turns bits on

Arguments:

      bits: 1 - on
            0 - off

If you want to show the complete __doc__ without having to specify the --help or -h option, you can catch DocoptExit exception when calling the docopt() function and print the __doc__ at that time.

Example -

"""Test Control Program

Usage: 
  test.py bitwrite ([--off=<bits>...][--on=<bits>...])
  test.py regwrite ([regA | regB]<value>)

Options:
  -o <bits>... --off <bits>...  #Turn bits off
  -i <bits>... --on <bits>...   #Turns bits on

Arguments:

      bits: 1 - on
            0 - off

"""
from docopt import docopt, DocoptExit


if __name__ == '__main__':
    try:
        arguments = docopt(__doc__, version='Test Control Program')
        print(arguments)
    except DocoptExit:
        print(__doc__)