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

[R. David Murray]

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 :)

--David