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

Guido van Rossum guido at python.org
Tue Nov 26 18:38:40 CET 2013

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.addargument('--sync-foo', action='storetrue') args = argparser.parseargs() 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 "parseknownargs" instead of "parseargs", the above happens too - --sync is recognized for --sync-foo and captured by the parser. But this is wrong! The whole idea of parseknownargs 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 parseknownargs) 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 parseknownargs is called. Again, at the very least this should be documented (for parseknownargs not less than a warning box, IMHO). Eli

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