Parsing an unknown optional argument whose leading characters (e.g. '--user') match those of another, known optional argument (e.g. '--userdata') causes the unknown argument to be parsed as the known one.

For example - passing the unknown argument '--user' to a parser with the known argument '--userdata' assigns the passed '--user' value to '--userdata'.

Here's my setup info:
  Python: Python 3.6.3 :: Anaconda custom (64-bit)
  OS: MacOS High Sierra 10.13.6 

Wrote up some example cases here:
  https://gist.github.com/rob-dalton/9ec06b8d5e2e9083713ddc7884d16072

Would using `allow_abbrev=False` fix this for you?

https://docs.python.org/3/library/argparse.html#allow-abbrev

It does. Thanks! I feel dumb for not finding that.

Guess I'd just suggest making the default `false`, seems more intuitive to
me.

On Mon, Oct 1, 2018 at 2:00 PM Robert Dalton <rob.c.dalton@gmail.com> wrote:

> It does. Thanks! I feel dumb for not finding that.
>
> Guess I'd just suggest making the default `false`, seems more intuitive to
> me.
>
> On Mon, Oct 1, 2018 at 1:48 PM Zachary Ware <report@bugs.python.org>
> wrote:
>
>>
>> Zachary Ware <zachary.ware@gmail.com> added the comment:
>>
>> Would using `allow_abbrev=False` fix this for you?
>>
>> https://docs.python.org/3/library/argparse.html#allow-abbrev
>>
>> ----------
>> nosy: +zach.ware
>>
>> _______________________________________
>> Python tracker <report@bugs.python.org>
>> <https://bugs.python.org/issue34865>
>> _______________________________________
>>
>
>
> --
> Best,
> Rob Dalton
>
>
> (814) 571-0462 | robdalton.me | robcdalton@gmail.com
>

-- 
Best,
Rob Dalton

(814) 571-0462 | robdalton.me | robcdalton@gmail.com

As much as I agree that allow_abbrev=False would be a much better default, at this point we can't change it. Doing so would break who knows how many existing scripts when they upgraded Python versions.