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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).