Python argparse: Force a list item to be unique

Question

Being able to validate the list items using choices=servers below is nice.

servers = [ \"ApaServer\", \"BananServer\", \"GulServer\", \"SolServ         


        

Answer 1

Here's an excerpt from some code that I use for a similar purpose:

def parse_args(argv):
    class SetAction(argparse.Action):
        """argparse.Action subclass to store distinct values"""
        def __call__(self, parser, namespace, values, option_string=None):
            try:
                getattr(namespace,self.dest).update( values )
            except AttributeError:
                setattr(namespace,self.dest,set(values))
    ap = argparse.ArgumentParser(
        description = __doc__,
        formatter_class = argparse.ArgumentDefaultsHelpFormatter,
        )
    ap.add_argument('--genes', '-g',
                    type = lambda v: v.split(','),
                    action = SetAction,
                    help = 'select by specified gene')

This is similar in spirit to jcollado's reply, with a modest improvement: multiple options can be specified, with comma separated values, and they are deduped (via set) across all options.

For example:

snafu$ ./bin/ucsc-bed -g BRCA1,BRCA2,DMD,TNFA,BRCA2 -g BRCA1
Namespace(genes=set([u'BRCA1', u'BRCA2', u'DMD', u'TNFA']))

Note that there are two -g args. BRCA2 is specified twice in the first, but appears only once. BRCA1 is specified in the first and second -g opts, but also appears only once.