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

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


argparse does prefix matching as long as there are no conflicts. For

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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