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

Eli Bendersky eliben at gmail.com
Tue Nov 26 18:55:07 CET 2013

On Tue, Nov 26, 2013 at 9:46 AM, R. David Murray <rdmurray at bitdance.com>wrote:

> On Tue, 26 Nov 2013 09:30:10 -0800, 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).
> I'm not sure whether or not it is a bug, but there are a number of
> bugs involving argument prefix matching in the tracker.  Including one
> to be able to disable it, that has a patch but didn't get into 3.4
> (issue 14910).
> This topic seems more suited to the bug tracker than python-dev.
> Stephen hasn't been very active lately.  There is at least one
> non-committer who has been, but no committers have made time to review
> the patches and take action (they are on my list, but there is a lot
> of stuff ahead of them).  So if you'd like to help out in that regard,
> that would be great :)

Being able to disable prefix matching is certainly a good idea. I'll nosy
myself on Issue 14910 and will try to find some time for it.

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