Argparse: "default" interacts incorrectly/non-intuitively with action="count"

[MJKirk]

Very similar to issue #110131 - the documentation is not clear on how default and count interact.

Consider

parser = argparse.ArgumentParser()
parser.add_argument('--verbose', '-v', action='count', default=3)
parser.parse_args(['-v'])

My intuition, and how I read the documentation, is that you should end up with verbose=1, but actually you get verbose=4.
I haven't checked the source code, but I assume under the hood something similar to append/extend is happening.

My take is that it would be better for default to really act as a default, and get overwritten rather than added to by the passed arguments. But if not, the documentation should be changed to something similar to what is written for action="append".

Linked PRs

• gh-127296: Document that argparse count action starts from a non-zero default #156908

[ericvsmith]

I'm not sure that's a great assumption. I think of "-v" in this case as "increment the current value by one", and here the current value initially would be the default.

In any event, this can't be changed due to backward compatibility, although I guess we could add another option to control this behavior. Something along the lines of "initial value to be used if this option ever processed", and you'd use 0 in this case.

Good job finding #110131! I agree it's basically the same, except for action="count".

[terryjreedy]

Behavior same as sum([1], 3) == 4, with parameter names default and start being synonyms. Perhaps add "The count starts with the default when the latter is not 0" . Note that start can be negative (same for default?), so that the result is the deficit or surplus with respect to start. I don't know if this is a sensible usage for argparse.

[MJKirk]

I can see for backwards compatibility reasons the argument name default cannot be changed, but it seems something like initial would convey the behaviour better.

"The count starts with the default when the latter is not 0"

Something like this would make the documentation clearer 👍

[rhettinger]

"The count starts with the default when the latter is not 0"

+1 for this edit.

[ericvsmith]

+1 for this edit.

Agreed. And if we do this here, we should also do it for #110131.