Python 2.7. Parse input parameters

Question

Good time of the day! First of all, let me say that I'm a newbie in Python world. I've problems with parsing input parameters. At this moment I'm using Python 2.7 and module which is called argparse . I'm trying to design simple application which will be able to parse simple input parameters. Here is a short example:

my_app.py sync --force

Second example:

my_app.py patch --branch

I see that for this I can use add_argument which can work with positional and optional arguments. Like in my case I want to have few positional (but optional at the same time) and few optional arguments. To do that I've design small script

if __name__ == '__main__':
    parser = argparse.ArgumentParser(description='My App')
    parser.add_argument('sync', type=bool, const=True, nargs='?')
    parser.add_argument('-f', '--force', dest='sync_force', type=bool, const=True, nargs='?')
    parser.add_argument('-b', '--branch', type=str, const=True, nargs='?')
    parser.add_argument('-u', '--url', type=str, const=True, nargs='?')
    parser.add_argument('patch', type=bool, const=True, nargs='?')
    parser.add_argument('revert', type=bool, const=True, nargs='?')
    parser.add_argument('verify', type=bool, const=True, nargs='?')

    values = parser.parse_args()

    if values.revert:
        handler.revert()
    else:
        parser.print_help()

I see that I can use nargs='?' to specify the positional parameter as optional, but each time when I'm calling my script it shows like I got ' sync ' as input parameter, even if I specified ' patch '. So, I think that it shows just first added element. Could you tell me what's wrong and where is a problem?

Update: I'm trying to achieve a situation when I will be able to have only one positional argument at the same time(and at least one, but with any additional optional parameters). For example

my_app.py sync

my_app.py path

my_app.py verify --force

my_app.pyrevert --branch

Answer 1

With:

import argparse

if __name__ == '__main__':

    parser = argparse.ArgumentParser(description='My App')
    parser.add_argument('cmd', choices=['sync','patch', 'revert','verify'])
    parser.add_argument('-f', '--force', action='store_true')
    parser.add_argument('-b', '--branch')
    parser.add_argument('-u', '--url')

    args = parser.parse_args()
    print(args)
    if args.cmd in ['revert']:
        print('handler.revert()')
    else:
        parser.print_help()

testing

1243:~/mypy$ python stack60327766.py 
usage: stack60327766.py [-h] [-f] [-b BRANCH] [-u URL]
                        {sync,patch,revert,verify}
stack60327766.py: error: too few arguments
1244:~/mypy$ python stack60327766.py revert
Namespace(branch=None, cmd='revert', force=False, url=None)
handler.revert()
1244:~/mypy$ python stack60327766.py revert -f -b foobar -u aurl
Namespace(branch='foobar', cmd='revert', force=True, url='aurl')
handler.revert()
1244:~/mypy$ python stack60327766.py verify-f -b foobar -u aurl
usage: stack60327766.py [-h] [-f] [-b BRANCH] [-u URL]
                        {sync,patch,revert,verify}
stack60327766.py: error: argument cmd: invalid choice: 'verify-f' (choose from 'sync', 'patch', 'revert', 'verify')
1245:~/mypy$ python stack60327766.py verify -f -b foobar -u aurl
Namespace(branch='foobar', cmd='verify', force=True, url='aurl')
usage: stack60327766.py [-h] [-f] [-b BRANCH] [-u URL]
                        {sync,patch,revert,verify}

My App

positional arguments:
  {sync,patch,revert,verify}

optional arguments:
  -h, --help            show this help message and exit
  -f, --force
  -b BRANCH, --branch BRANCH
  -u URL, --url URL

Answer 2

argparse has native support for subcommands .

Adapting from the above-linked documentation,

parser = argparse.ArgumentParser()
parser.add_argument('--foo')
subparsers = parser.add_subparsers()
sync_parser = subparsers.add_parser('sync')
patch_parser = subparsers.add_parser('patch')
revert_parser = subparsers.add_parser('revert')
verify_parser = subparsers.add_parser('verify')

etc. is probably what you're looking for.

Beyond that, I can only recommend the Click library for a more fluent interface for CLIs.