Argparse expected one argument

Question

I have the following

import argparse
parser = argparse.ArgumentParser(prog='macc', usage='macc [options] [address]')
parser.add_argument('-l', '--list', help='Lists MAC Addresses')
args = parser.parse_args()
print(args)
def list_macs():
 print("Found the following MAC Addresses")

I get an error when running with python macc.py -l it says that an argument was expected. Even when I change my code to parser.add_argument('-l', '--list', help='Lists MAC Addresses' default=1) I get the same error.

• Kalma: I am not an expert on argparse, but what it is saying is that there must be a value after "-l" option. If you run it using 'macc.py -l address_1' (or whatever value), it works!
• wjandrea: The problem is clearer if you don't override the usage: macc [-h] [-l LIST]

Answer 1

The default action for an argument is store, which sets the value of the attribute in the namespace returned by parser.parse_args using the next command line argument.

You don't want to store any particular value; you just want to acknowledge that -l was used. A quick hack would be to use the store_true action (which would set args.list to True).

parser = argparse.ArgumentParser(prog='macc')
parser.add_argument('-l', '--list', action='store_true', help='Lists MAC Addresses')
args = parser.parse_args()
if args.list:
 list_macs()

The store_true action implies type=bool and default=False as well.

---

However, a slightly cleaner approach would be to define a subcommand named list. With this approach, your invocation would be macc.py list rather than macc.py --list.

parser = argparse.ArgumentParser(prog='macc')
subparsers = parser.add_subparsers(dest='cmd_name')
subparsers.add_parser('list')
args = parser.parse_args()
if args.cmd_name == "list":
 list_macs()

Answer 2

If you use the argument -l on the cli you need to specify an argument, like:

python macc.py -l something

If you set default = 1 on the -l argument you can run your script without using it like this:

python macc.py

Answer 3

You could solve this by setting a default value and set nargs='?'.

If nothing is provided after -l, the default value is saved for the argument. It would look something like this:

parser.add_argument('-l', '--list', help='Lists MAC Addresses', default=1, nargs='?')

More info: https://docs.python.org/3/library/argparse.html#nargs