(original) (raw)

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 <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

_______________________________________________

Python-Dev mailing list

Python-Dev@python.org

https://mail.python.org/mailman/listinfo/python-dev

Unsubscribe: https://mail.python.org/mailman/options/python-dev/eliben%40gmail.com