I do something such as:
p = argparse.ArgumentParser()
p.add_argument('-a', action='store_true')
p.add_argument('-b', action='store_true')
p.parse_args('-ab='.split())

thos code code jump in 1903, and the explicit_arg's value is: 'b='

1901   action_tuples.append((action, [], option_string))
1902   char = option_string[0]
1903   option_string = char + explicit_arg[0]

IMHO, we should judge this explicit_arg's value before jump this judgment statement.