argparse support of/by argparse

[Dan Stromberg]

On Mon, Jul 12, 2021 at 12:34 PM Chris Angelico <rosuav at gmail.com> wrote:

> On Tue, Jul 13, 2021 at 5:22 AM lucas <lucas at bourneuf.net> wrote:
> > Running CPython on it will raise a TypeError, and running Mypy on it
> > will indicate that no issues were found.
> >
> > I was wondering if there is any way for me to have mypy detecting the
> > args.n type, based on the type keyword of the parser.add_argument
> function ?
> >
> > It appears that some type annotations were added to tierce party
> > modules, provided by mypy itself. Is there a technical issue preventing
> > such work to be made for argparse (or other CLI ; i didn't find anything
> > for others either)
> >
>
> Seems complicated, since it depends on a lot of run-time information.
> What if you flip the problem on its head? Instead of creating the
> argparser and letting that govern the types, maybe create a dataclass,
> and then programmatically build the parser.
>

This is why I eschew argparse.  I instead frequently do something like:

def usage(retval):
    """Output a usage message."""
    if retval:
        write = sys.stderr.write
    else:
        write = sys.stdout.write

    write(f'{sys.argv[0]} --age 50 --help\n')

    sys.exit(retval)


def main():
    """Compute maximum heart rate as a function of age."""
    age = -1
    while sys.argv[1:]:
        if sys.argv[1] == '--age':
            age = float(sys.argv[2])
            del sys.argv[1]
        elif sys.argv[1] in ('-h', '--help'):
            usage(0)
        else:
            sys.stderr.write(f'{sys.argv[0]}: unrecognized option:
{sys.argv[1]}\n')
            usage(1)
        del sys.argv[1]

    if age == -1:
        sys.stderr.write('--age is a required option\n')
        usage(1)


It is of course a little more typing, but tools like mypy and pylint are
much more effective this way.  This is a small program, but in a large one,
this approach can really help with correctness.

If someone has a mypy extension/plugin that can do argparse as well, I may
switch.  Until then....