conditional_argparse.py

import argparse
import sys
parser = argparse.ArgumentParser()
action1 = parser.add_argument('--user', help="username")
action2 = parser.add_argument('--bar',
                              help="Only use this if --user is set")
action3 = parser.add_argument('--bar2',
                              help="Only use this if --user is set")

argv = set(sys.argv)

def second_depends_on_first(action1, action2):
  ''' the second action depends on the first action being specified '''  
  # Inspired by: http://stackoverflow.com/a/11455440/992834
  if ((argv & set(action2.option_strings)) and not (argv & set(action1.option_strings))):  # set intersection
       parser.error(' or '.join(action1.option_strings) + ' must be given with ' +
                    ' or '.join(action2.option_strings))

second_depends_on_first(action1, action2)
second_depends_on_first(action1, action3)

parser.parse_args()