Conditional passing of arguments to methods in python

I have many possible arguments from argparse that I want to pass to a function. If the variable hasn't been set, I want the method to use its default variable. However, handling which arguments have been set and which haven't is tedious:

import argparse
def my_func(a = 1, b = 2):
  return a+b

if __name__ == "__main__":
    parser = argparse.ArgumentParser(description='Get the numeric values.')
    parser.add_argument('-a', type=int)
    parser.add_argument('-b', type=int)
    args = parser.parse_args()

    if not args.a is None and not args.b is None:
        result = my_func(a = args.a, b = args.b)
    elif not args.a is None and args.b is None:
        result = my_func(a = args.a)
    elif not args.b is None and args.a is None:
        result = my_func(b = args.b)
    else:
        result = my_func()

It seems like I should be able to do something like this:

result = my_func(a = args.a if not args.a is None, b = args.b if not args.b is None)

But this gives a syntax error on the comma.

I could set default values in the argparser, but I want to use the defaults set in the method definition.