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", "SolServer", "RymdServer", "SkeppServer", "HavsServer", "SovServer" ]
parser = argparse.ArgumentParser()
parser.add_argument('-o', '--only', nargs='*', choices=servers, help='Space separated list of case sensitive server names to process')

Is it possible to force an item in the list to be unique, so that no duplicates are allowed?

Answer 1

The way to properly discard duplicates using argparse would be to create your own argparse.Action class that takes care of using set as suggestted by other answers:

import argparse

class UniqueAppendAction(argparse.Action):
    def __call__(self, parser, namespace, values, option_string=None):
        unique_values = set(values)
        setattr(namespace, self.dest, unique_values)

servers = ["ApaServer", "BananServer", "GulServer", "SolServer",
           "RymdServer", "SkeppServer", "HavsServer", "SovServer" ]
parser = argparse.ArgumentParser()
parser.add_argument('-o', '--only', nargs='*', choices=servers, action=UniqueAppendAction,
                    help='Space separated list of case sensitive server names to process')
print parser.parse_args()

Example output:

$ python test.py -o ApaServer ApaServer
Namespace(only=set(['ApaServer']))