Require either of two arguments using argparse

I think you are searching for something like mutual exclusion (at least for the second part of your question).

This way, only foo or bar will be accepted, not both.

import argparse

parser = argparse.ArgumentParser()
group = parser.add_mutually_exclusive_group(required=True)
group.add_argument('--foo',action=.....)
group.add_argument('--bar',action=.....)
args = parser.parse_args()

BTW, just found another question referring to the same kind of issue.