argparse: how to allow option values starting with dash

[Celelibi]

Documentation

The difference between nargs=None and nargs=1 should probably be explained in the docs in the section about arguments containing -.
Using nargs=1 actually allows to work around some issues with having an option value starting with a dash. Which has been a long standing issue with argparse as shown with the issue #53580.

Here is an example of the difference:

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument("--nargs-is-1", nargs=1)
>>> parser.parse_args(["--nargs-is-1", "-3,5-10"])
Namespace(nargs_is_none=None, nargs_is_1=['-3,5-10'])

>>> parser.add_argument("--nargs-is-none")
>>> parser.parse_args(["--nargs-is-none", "-3,5-10"])
usage: [-h] [--nargs-is-none NARGS_IS_NONE] [--nargs-is-1 NARGS_IS_1]
: error: argument --nargs-is-none: expected one argument

This trick, however, only covers the case where the number of arguments to gobble up is fixed. People wanting a variable number of arguments to their options would need a more involved disambiguation technique that argparse is incapable of for now. (Although I guess something could be implemented by labeling ambiguous arguments as ambiguous and introduce values of nargs for different treatment of those. But that's beside the scope of this issue.)

Incidentally, this trick has been made to work only one year ago by the commit 3f27153. Not sure if that was an intended consequence, but now it works and I'm here for it. :)

That documentation section (arguments containing -) suggests that argparse uses a heuristics to determine whether an argument is positional or optional. Maybe the gist of the heuristics could be laid out on so that users don't have to guess or read the code to get argparse to behave the way they want?

[brianschubert]

cc @serhiy-storchaka

[serhiy-storchaka]

Oh, I'm afraid that's a bug. The pattern for options should be the same as the pattern for positional arguments minus the "-". No existing test fails after fixing this bug, so it was definitely unintentional. Thank you for your report. This unintended effect will most likely be reverted. Don't rely on it.

The way which always works for nargs=None -- combine the option name and value using the "=" separator: --nargs-is-none=-3,5-10.

A good news is that in 0c5a48c, which was committed 10 days after 3f27153, the heuristics was changed, and now it recognizes "-3,5-10" as not option, unless you have options that start with a digit or a dot followed by a digit (like -0 or -.1). This does not solve the problem in general, but should help in your case. But this is only in 3.14.

>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument("--nargs-is-none")
_StoreAction(option_strings=['--nargs-is-none'], dest='nargs_is_none', nargs=None, const=None, default=None, type=None, choices=None, required=False, help=None, metavar=None, deprecated=False)
>>> parser.parse_args(["--nargs-is-none", "-3,5-10"])
Namespace(nargs_is_none='-3,5-10')

[Celelibi]

Oh, I'm afraid that's a bug. [...] This unintended effect will most likely be reverted. Don't rely on it.

So sad! :(
I guess it doesn't hurt to keep my code that way for now. The nargs=1 trick will work on python 3.13 and the negative number regex fix will make it work for python 3.14.

This whole issue is a bit moot then. Although the positional/optional heuristics may still benefit from a bit of explaining.

The way which always works for nargs=None -- combine the option name and value using the "=" separator: --nargs-is-none=-3,5-10.

Yeah, I know, that's just not the way I expect my program to be used most of the time.

A good news is that in 0c5a48c, which was committed 10 days after 3f27153, the heuristics was changed, and now it recognizes "-3,5-10" as not option, unless you have options that start with a digit or a dot followed by a digit (like -0 or -.1). This does not solve the problem in general, but should help in your case. But this is only in 3.14.

I guess the day I have a real use-case for the general case, I'll come back with a full fix. ^^