Eli did give his use case... a front end for a program that has a
parameter "--sync", and a front end preprocessor of some sort was trying
to use "--sync-foo" as an argument, and wanted "--sync" to be left in
the parameters to send on to the back end program.
Design of the front-end might better be aware of back end parameters and
not conflict, but the documentation could be improved, likely.
It might also be possible to add a setting to disable the prefix
matching feature, for code that prefers it not be done. Whether that is
better done as a global setting, or a per-parameter setting I haven't
thought through. But both the constructor and the parameter definitions
already accept a variable number of named parameters, so I would think
it would be possible to add another, and retain backward compatibility
via an appropriate default.
On 11/26/2013 9:38 AM, Guido van Rossum 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?
On Tue, Nov 26, 2013 at 9:30 AM, Eli Bendersky <eli...@gmail.com
<mailto:eli...@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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