import argparse

class FooAction(argparse.Action):
    def __call__(self, parser, namespace, values, option_string=None):
        if values=='Q':
            setattr(namespace, self.dest, {'type':values,'quick':True,'slow':False})
        else:
            setattr(namespace, self.dest, {'type':values,'quick':False,'slow':True})

def bar(string):
    return string.upper()[0]

parser = argparse.ArgumentParser()
parser.add_argument('-a','--algorithm', choices=['Q','S'],
    default='s', action=FooAction, type=bar)
parser.add_argument('foo', choices=['Q','S'],
    default='s', action=FooAction, nargs='?', type=bar)

print(parser.parse_args([]))
# Namespace(algorithm='S',
#           foo={'slow': True, 'type': 'S', 'quick': False})
print(parser.parse_args(['quick']))
# Namespace(algorithm='S',
#           foo={'type': 'Q', 'quick': True, 'slow': False})
print(parser.parse_args(['-a', 'quick']))
# Namespace(algorithm={'type': 'Q', 'quick': True, 'slow': False},
#           foo={'type': 'S', 'quick': False, 'slow': True})
print(parser.parse_args())