class VersionAction(argparse.Action):
def __call__(self, parser, namespace, values, option_string):
print(VERSION)
exit()

...

parser.add_argument("-v", "--version", nargs=0, action=VersionAction)

Thanks. An action class didn't occur to me. I looked briefly at the
code for argparse to see how it handled --help. The added argument
seemed normal, so gave up, figuring there was some special handling of
that option.

Skip

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ummm, hate to say this, but have you checked the documentation? this
case is supported using an action named 'version' without doing very much.

Thanks, Mats.

I actually searched all over the argparse docs. (There's a lot to digest. Honestly, if I wasn't attempting to be sort of up-to-date I'd just continue using getopt.) It never occurred to me that "version" would be special, so
I didn't specifically search for it, or realize there would be a specific action devoted to it. I knew (the default) "help" was special, so I focused
my search for it. Obviously, "--help" is a pretty bad search term.

Skip



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Thanks. It occurs to me that instead of providing two special actions
("help" and "version"), it might be worthwhile to provide a standard way of saying, "if present, process this option and exit before considering other details of the command line." Matt's example action works well enough for
my needs, but it would be nice if more than one such option could be given.
For example:

import argparse

parser = argparse.ArgumentParser()
parser.add_argument("url")
parser.add_argument("--version", version="777", action="version")

args = parser.parse_args(["--help", "--version"])

Which option is processed depends on their order on the command line. I
don't believe it's possible to run the script and see them both processed. That's probably a secondary consideration though. My script is working well enough in this regard now.

Skip

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On Tue, 29 Nov 2022 at 12:37, Loris Bennett <loris.bennett@fu-berlin.de> wrote:

Mats Wichmann <mats@wichmann.us> writes:

On 11/27/22 16:40, Skip Montanaro wrote:

I have a script to which I'd like to add a --version flag. It should print >> the version number then exit, much in the same way --help prints the help >> text then exits. I haven't been able to figure that out. I always get a
complaint about the required positional argument.
I think I could use something like nargs='*', but that would push
off
detection of the presence of the positional arg to the application.
Shouldn't I be able to tell argparse I'm going to process --verbose, then >> exit?

ummm, hate to say this, but have you checked the documentation? this
case is supported using an action named 'version' without doing very
much.

I hadn't noticed the action 'version'. I just use

parser.add_argument(
"-v", "--version", action="store_true", dest="version",
help="print version"
)

That's still going to validate the rest of the args though - notably,
you can't omit any mandatory arguments (like a subcommand).

The version and help actions are implemented pretty simply, actually.
They're just little action classes that do their action immediately on
getting triggered. It should be easy enough to make any action you
want that way.

ChrisA

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