argparse: How can I allow multiple values to override a default

Question

This is slightly related to the topic covered in a question about allowing an argument to be specified multiple times.

I'd like to be able to specify an option multiple times like this:

 tool --foo 1 --foo 2 --foo 3

And also like this:

 tool a b c

I'd also like to support both at the same time:

 tool a b c --foo 1 --foo2 --foo 3

This works fine with:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('foo', nargs='*', action='append')
parser.add_argument('--foo', nargs='*', dest='foo', action='append')

The result list can be easily flattened out:

args = parser.parse_args('a b c --foo 1 --foo 2 --foo 3'.split())
args.foo = [el for elements in args.foo for el in elements]

yields:

>>> args
Namespace(foo=['a', 'b', 'c', '1', '2', '3'])

How do I add a default value in a way that the default is not being used as soon as one argument is specified by the user?

If adding just default=[['spam']] to one of the add_argument() calls, the default is always part of the result. I cannot get argparse to remove it by itself as soon as a user provides an argument herself.

I'm hoping that there's a solution with what argparse already provides itself.

Answer 1

I think this is a slightly more clean variation on the other answer (relying on the self.default attribute of custom actions):

import argparse
import sys

class Extender(argparse.Action):
    def __call__(self,parser,namespace,values,option_strings=None):
        #Need None here incase `argparse.SUPPRESS` was supplied for `dest`
        dest = getattr(namespace,self.dest,None) 
        #print dest,self.default,values,option_strings
        if(not hasattr(dest,'extend') or dest == self.default):
            dest = []
            setattr(namespace,self.dest,dest)
            #if default isn't set to None, this method might be called
            # with the default as `values` for other arguements which
            # share this destination.
            parser.set_defaults(**{self.dest:None}) 

        try:
            dest.extend(values)
        except ValueError:
            dest.append(values)

        #another option:
        #if not isinstance(values,basestring):
        #    dest.extend(values)
        #else:
        #    dest.append(values) #It's a string.  Oops.

def new_parser(args):
    parser = argparse.ArgumentParser()
    parser.add_argument('foo', nargs='*',action=Extender)
    parser.add_argument('--foo', nargs='*', dest='foo', action=Extender)
    parser.set_defaults(foo = [['spam']])
    return parser.parse_args(args.split())

tests = {'a b c --foo 1 --foo 2 --foo 3':['a','b','c','1','2','3'],
         '':[['spam']],
         'a b c --foo 1 2 3':['a','b','c','1','2','3'],
         '--foo 1':['1'],
         'a':['a']}

for s,r in tests.items():
    print ( "parsing: {0}".format(s) )
    args = new_parser(s)
    if(args.foo != r):
        print ("ERROR")
        print (args.foo)
        print (r)
        sys.exit(1)
    print ( args )
    print ('*'*80)

---

Also note that I've used parser.set_defaults(...) to set the default for the foo attribute.