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

Eli Bendersky eliben at gmail.com
Tue Nov 26 18:30:10 CET 2013

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).

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