Question

here is a simple example.

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('-a', action = 'append_const', dest = 'letter', const = 'a')
parser.add_argument('-b', action = 'append_const', dest = 'letter', const = 'b')
parser.set_defaults(letter = ['a', 'b'])
print(parser.parse_args('-a'.split()))
print(parser.parse_args('-b'.split()))
print(parser.parse_args(''.split()))

Results :

Namespace(letter=['a', 'b', 'a'])
Namespace(letter=['a', 'b', 'b'])
Namespace(letter=['a', 'b'])

Without the set_defaults line, results are :

Namespace(letter=['a'])
Namespace(letter=['b'])
Namespace(letter=None)

How is it possible to configure argparse for such results ?

Namespace(letter=['a'])
Namespace(letter=['b'])
Namespace(letter=['a', 'b'])

Answer 1

As others have mentioned, store_const almost does what you want, except that (as @Theodros Zelleke points out) -ab would not be parsed correctly. I think the simplest way to address that problem is to simply handle the case of no arguments after parse_args has been called:

import argparse
import sys

def parse_args(argv = sys.argv[1:]):
    parser = argparse.ArgumentParser()
    parser.add_argument('-a', action = 'append_const', dest = 'letter', const = 'a')
    parser.add_argument('-b', action = 'append_const', dest = 'letter', const = 'b')
    args = parser.parse_args(argv)
    if args.letter is None:
        args.letter = ['a','b']
    return args

print(parse_args('-a'.split()))
print(parse_args('-b'.split()))
print(parse_args('-ab'.split()))
print(parse_args(''.split()))

yields

Namespace(letter=['a'])
Namespace(letter=['b'])
Namespace(letter=['a', 'b'])
Namespace(letter=['a', 'b'])