argparse optional argument before positional argument

Question

I was wondering if it is possible to have a positional argument follow an argument with an optional parameter. Ideally the last argument entered into the command line would always apply toward 'testname'.

import argparse
parser = argparse.ArgumentParser(description='TAF')
parser.add_argument('-r','--release',nargs='?',dest='release',default='trunk')
parser.add_argument('testname',nargs='+')
args = parser.parse_args()

I would like both of these calls to have smoketest apply to testname, but the second one results in an error.

>> python TAF.py -r 1.0 smoketest
>> python TAF.py -r smoketest
TAF.py: error: too few arguments

I realize that moving the positional argument to the front would result in the correct behavior of the optional parameter, however this is not quite the format I am looking for. The choices flag looks like an attractive alternative, however it throws an error instead of ignoring the unmatched item.

EDIT: I've found a hacky way around this. If anyone has a nicer solution I would appreciate it.

import argparse
parser = argparse.ArgumentParser(description='TAF')
parser.add_argument('-r','--release',nargs='?',dest='release',default='trunk')
parser.add_argument('testname',nargs=argparse.REMAINDER)
args = parser.parse_args()

if not args.testname:
    args.testname = args.release
    args.release = ''

Answer 1

As stated in the documentation:

'?'. One argument will be consumed from the command line if possible, and produced as a single item. If no command-line argument is present, the value from default will be produced. Note that for optional arguments, there is an additional case - the option string is present but not followed by a command-line argument. In this case the value from const will be produced.

So, the behaviour you want is not obtainable using '?'. Probably you could write some hack using argparse.Action and meddling with the previous results.(1)

I think the better solution is to split the functionality of that option. Make it an option that requires an argument(but the option itself is optional) and add an option without argument that sets the release to 'trunk'. In this way you can obtain the same results without any hack. Also I think the interface is simpler.

In your example:

python TAF.py -r smoketest

It's quite clear that smoketest will be interpreted as an argument to -r. At least following unix conventions. If you want to keep nargs='?' then the user must use --:

$ python TAF.py -r -- sometest
Namespace(release=None, testname=['sometest'])   #parsed result

(1) An idea on how to do this: check if the option has an argument. If it has one check if it is a valid test name. If so put into by hand into testname and set release to the default value. You'll also have to set a "flag" that tells you that this thing happened.

Now, before parsing sys.argv you must redirect sys.stderr. When doing the parsing you must catch SystemExit, check the stderr and see if the error was "too few arguments", check if the flag was set, if so ignore the error and continue running, otherwise you should reprint to the original stderr the error message and exit.

This approach does not look robust, and it's probably buggy.