[Python-Dev] potential argparse problem: bad mix of parse_known_args and prefix matching

Eli Bendersky eliben at gmail.com
Tue Nov 26 19:16:56 CET 2013

On Tue, Nov 26, 2013 at 9:53 AM, Glenn Linderman <v+python at g.nevcal.com>wrote:

>  Eli did give his use case... a front end for a program that has a
> parameter "--sync", and a front end preprocessor of some sort was trying to
> use "--sync-foo" as an argument, and wanted "--sync" to be left in the
> parameters to send on to the back end program.
> Design of the front-end might better be aware of back end parameters and
> not conflict, but the documentation could be improved, likely.
> It might also be possible to add a setting to disable the prefix matching
> feature, for code that prefers it not be done. Whether that is better done
> as a global setting, or a per-parameter setting I haven't thought through.
> But both the constructor and the parameter definitions already accept a
> variable number of named parameters, so I would think it would be possible
> to add another, and retain backward compatibility via an appropriate
> default.
FWIW I'm not advocating a breaking behavior change here - I fully realize
the ship has sailed. I'm interested in mitigation actions, though. Making
the documentation explain this explicitly + adding an option to disable
prefix matching (in 3.5 since we're past the 3.4 features point) will go a
long way for alleviating this gotcha.


> On 11/26/2013 9:38 AM, Guido van Rossum wrote:
>  I think matching on the shortest unique prefix is common for command
> line parsers in general, not just argparse. I believe optparse did this
> too, and even the venerable getopt does! I think all this originated in the
> original (non-Python) GNU standard for long option parsing. All that
> probably explains why the docs hardly touch upon it.
>  As to why parse_known_args also does this, I can see the reasoning behind
> this behavior: to the end user, "--sync" is a valid option, so it would be
> surprising if it didn't get recognized under certain conditions.
>  I suppose you were badly bitten by this recently? Can you tell us more
> about what happened?
> On Tue, Nov 26, 2013 at 9:30 AM, Eli Bendersky <eliben at gmail.com> wrote:
>>   Hello,
>>  argparse does prefix matching as long as there are no conflicts. For
>> example:
>> argparser = argparse.ArgumentParser()
>> argparser.add_argument('--sync-foo', action='store_true')
>> args = argparser.parse_args()
>>  If I pass "--sync" to this script, it recognizes it as "--sync-foo".
>> This behavior is quite surprising although I can see the motivation for it.
>> At the very least it should be much more explicitly documented (AFAICS it's
>> barely mentioned in the docs).
>> If there's another argument registered, say "--sync-bar" the above will
>> fail due to a conflict.
>> Now comes the nasty part. When using "parse_known_args" instead of
>> "parse_args", the above happens too - --sync is recognized for --sync-foo
>> and captured by the parser. But this is wrong! The whole idea of
>> parse_known_args is to parse the known args, leaving unknowns alone. This
>> prefix matching harms more than it helps here because maybe the program
>> we're actually acting as a front-end for (and hence using parse_known_args)
>> knows about --sync and wants to get it.
>>  Unless I'm missing something, this is a bug. But I'm also not sure
>> whether we can do anything about it at this point, as existing code *may*
>> be relying on it. The right thing to do would be to disable this prefix
>> matching when parse_known_args is called.
>>  Again, at the very least this should be documented (for parse_known_args
>> not less than a warning box, IMHO).
>> Eli
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