[Python] How to use Python argparse to have lazily loaded subcommands

I have a python package with a lot of sub commands and sub-sub commands. It is organized somewhat like this:

main.py

import argparse

from sum import prepare_arg_parser as prepare_sum_parser
from sub import prepare_arg_parser as prepare_sub_parser

parser = argparse.ArgumentParser()
sub_parsers = parser.add_subparsers(dest="command", required=True)

prepare_sum_parser(sub_parsers.add_parser("sum"))
prepare_sub_parser(sub_parsers.add_parser("sub"))

args = parser.parse_args()
args.func(args)


sum.py

import argparse


def prepare_arg_parser(parser):
parser.set_defaults(func=do_sum)
parser.add_argument("a", type=int)
parser.add_argument("b", type=int)


def do_sum(args):
print(f"{args.a} + {args.b} = {args.a + args.b}")


and so on.

However, as the list of module grows, the startup time also loads. I would like to lazily load each sub commands, so it only loads what's needed for the selected command.

I tried something like this:

import argparse

def do_sum(args):
from sum import prepare_arg_parser
parser = argparse.ArgumentParser()
prepare_arg_parser(parser)
sum_args = parser.parse_args(args)
sum_args.func(sum_args)

parser = argparse.ArgumentParser()
sub_parsers = parser.add_subparsers(dest="command", required=True)

sub_parsers.add_parser("sum").set_defaults(func=do_sum)

args, rest = parser.parse_known_args()
args.func(rest)


But then the help and error texts are not right.

Another way would be to split each sub commands into a "prepare argparse" module which would know all the args but lazily call the do_sum function for example, but I don't like it as it separate the argument declaration from their use. Also it would still load a lot of unnecessary files.

Is there have lazily-loaded sub commands?

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