Python command line arguments check if default or given

Question

Here is my code portion:

parser = argparse.ArgumentParser()
parser.add_argument('-a', action='store', dest='xxx', default = 'ABC')
parser.add_argument('-b', action='store', dest='yyy')
parser.add_argument('-c', action='store', dest='zzz')
args = parser.parse_args()

I want the code to work like this:

If b and c are given, do command2. Otherwise, do command1

if -a argument is given, then adding -b or -c throws an error

I tried this way:

if args.xxx and (args.yyy or args.zzz):
   parser.print_help()
   sys.exit()

But it didn't worked, because '-a' always has a deafult value and i can't change it. How can i fix it?

Answer 1

Here's one way to do it:

# If option xxx is not the default, yyy and zzz should not be present.
if args.xxx != 'ABC' and (args.yyy or args.zzz):
   # Print help, exit.

# Options yyy and zzz should both be either present or None.
if (args.yyy is None) != (args.zzz is None):
   # Print help, exit.

# Earn our pay.
if args.yyy is None:
    command2()
else:
    command1()

You might also consider a usage pattern based on subcommands , as noted in the comment by user toine.

Answer 2

I would use:

parser = argparse.ArgumentParser()
parser.add_argument('-a', dest='xxx')
parser.add_argument('-b', dest='yyy')
parser.add_argument('-c', dest='zzz')
args = parser.parse_args()

if args.xxx is None:
    args.xxx = 'ABC'
else:
    if args.zzz is not None or args.yyy is not None:
        parser.error('cannot use "b" or "c" with "a"')
if args.zzz is not None and args.yyy is not None:
     command2()
else:
     command1()

Testing for None is the surest way of testing whether the argument was given or not (though the simpler truth test is nearly as good). Internally parse_args keeps a list of seen_actions , but that isn't available to the user. In http://bugs.python.org/issue11588 there's a proposal to provide a testing hook that would have access to this list.