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.
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On Tue, Nov 26, 2013 at 9:38 AM, Guido van Rossum <guido@python.org> wrote:
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?

Sure. We have a Python script that serves as a gateway to another program. That other program has a "--sync" option. The gateway script has a "--sync-foo" option. When the gateway script is invoked with "--sync", we'd expect it to pass it to the program; instead, it matches it to its own "--sync-foo" and consumes the option.


Practically, this means a big caveat on exactly the use case parse_known_args was designed for: whenever I have a Python script using argparse and passing unknown arguments to other programs, I have to manually make sure there are no common prefixes between any commands to avoid this problem.


Frankly I don't see how the current behavior can be seen as the intended one.

Eli



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On Tue, Nov 26, 2013 at 9:30 AM, Eli Bendersky <eliben@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).

Eli


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