argparse on demand imports for types, choices etc

Question

I have quite a big program which has a CLI interaction based on argparse, with several sub parsers. The list of supported choices for the subparsers arguments a

Answer 1

To delay the fetching of choices, you could parse the command-line in two stages: In the first stage, you find only the subparser, and in the second stage, the subparser is used to parse the rest of the arguments:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('subparser', choices=['foo','bar'])

def foo_parser():
    parser = argparse.ArgumentParser()
    parser.add_argument('fooval', choices='123')
    return parser

def bar_parser():
    parser = argparse.ArgumentParser()
    parser.add_argument('barval', choices='ABC')
    return parser

dispatch = {'foo':foo_parser, 'bar':bar_parser}
args, unknown = parser.parse_known_args()
args = dispatch[args.subparser]().parse_args(unknown)
print(args)

It could be used like this:

% script.py foo 2
Namespace(fooval='2')

% script.py bar A
Namespace(barval='A')

Note that the top-level help message will be less friendly, since it can only tell you about the subparser choices:

% script.py -h
usage: script.py [-h] {foo,bar}
...

To find information about the choices in each subparser, the user would have to select the subparser and pass the -h to it:

% script.py bar -- -h
usage: script.py [-h] {A,B,C}

All arguments after the -- are considered non-options (to script.py) and are thus parsed by the bar_parser.