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

This is a script that tests the idea of delaying the creation of a subparser until it is actually needed. In theory it might save start up time, by only creating the subparser that's actually needed.

I use the nargs=argparse.PARSER to replicate the subparser behavior in the main parser. help behavior is similar.

# lazy subparsers test
# lazy behaves much like a regular subparser case, but only creates one subparser
# for N=5 time differences do not rise above the noise

import argparse

def regular(N):
    parser = argparse.ArgumentParser()
    sp = parser.add_subparsers(dest='cmd')
    for i in range(N):
        spp = sp.add_parser('cmd%s'%i)
        spp.set_defaults(func='cmd%s'%(10*i))
        spp.add_argument('-f','--foo')
        spp.add_argument('pos', nargs='*')
    return parser

def lazy(N):
    parser = argparse.ArgumentParser()
    sp = parser.add_argument('cmd', nargs=argparse.PARSER, choices=[])
    for i in range(N):
        sp.choices.append('cmd%s'%i)
    return parser

def subpar(cmd):
    cmd, argv = cmd[0], cmd[1:]
    parser = argparse.ArgumentParser(prog=cmd)
    parser.add_argument('-f','--foo')
    parser.add_argument('pos', nargs='*')
    parser.set_defaults(func=cmd)
    args = parser.parse_args(argv)
    return args

N = 5
mode = True #False
argv = 'cmd1 -f1 a b c'.split()
if mode:
    args = regular(N).parse_args(argv)
    print(args)
else:
    args = lazy(N).parse_args(argv)
    print(args)
    if isinstance(args.cmd, list):
        sargs = subpar(args.cmd)
        print(sargs)

test runs with different values of mode (and N=5)

1004:~/mypy$ time python3 stack44315696.py 
Namespace(cmd='cmd1', foo='1', func='cmd10', pos=['a', 'b', 'c'])

real    0m0.052s
user    0m0.044s
sys 0m0.008s
1011:~/mypy$ time python3 stack44315696.py 
Namespace(cmd=['cmd1', '-f1', 'a', 'b', 'c'])
Namespace(foo='1', func='cmd1', pos=['a', 'b', 'c'])

real    0m0.051s
user    0m0.048s
sys 0m0.000s

N has to be much larger to start seeing a effect.