python argparse if argument selected then another argument required =True - python-3.x

Question

Is there a way to make an argument required to be true if another specific argument choice is present otherwise argument required is false?
For example the following code if argument -act choice select is 'copy' then the argument dp required is true otherwise required is false:
import argparse
ap = argparse.ArgumentParser()
ap.add_argument("-act", "--act", required=True, choices=['tidy','copy'],type=str.lower,
help="Action Type")
ap.add_argument("-sp", "--sp", required=True,
help="Source Path")
args = vars(ap.parse_args())
if args["act"] == 'copy':
ap.add_argument("-dp", "--dp", required=True,
help="Destination Path")
else:
ap.add_argument("-dp", "--dp", required=False,
help="Destination Path")
args = vars(ap.parse_args())
### Tidy Function
def tidy():
print("tidy Function")
### Copy Function
def copy():
print("copy Function")
### Call Functions
if args["act"] == 'tidy':
tidy()
if args["act"] == 'copy':
copy()
I am currently getting an error unrecognized arguments: -dp with the above code.
The expected result would be to move on to call function. Thanks

Answer 1

I would use ArgumentParser.add_subparsers to define the action type {tidy, copy} and give the command specific arguments. Using a base parser with parents allows you to define arguments that are shared by both (or all) your sub-commands.
import argparse
parser = argparse.ArgumentParser(
prog='PROG',
epilog="See '<command> --help' to read about a specific sub-command."
)
base_parser = argparse.ArgumentParser(add_help=False)
base_parser.add_argument("--sp", required=True, help="source")
subparsers = parser.add_subparsers(dest='act', help='Sub-commands')
A_parser = subparsers.add_parser('copy', help='copy command', parents=[base_parser])
A_parser.add_argument('--dp', required=True, help="dest, required")
B_parser = subparsers.add_parser('tidy', help='tidy command', parents=[base_parser])
B_parser.add_argument('--dp', required=False, help="dest, optional")
args = parser.parse_args()
if args.act == 'copy':
pass
elif args.act == 'tidy':
pass
print(args)
Which produces the following help pages, note that instead of needing to use -act the command is given as a positional parameter.
~ python args.py -h
usage: PROG [-h] {tidy,copy} ...
positional arguments:
{tidy,copy} Sub-commands
tidy tidy command
copy copy command
optional arguments:
-h, --help show this help message and exit
See '<command> --help' to read about a specific sub-command.
~ python args.py copy -h
usage: PROG copy [-h] --sp SP [--dp DP]
optional arguments:
-h, --help show this help message and exit
--sp SP source
--dp DP dest, optional
~ python args.py tidy -h
usage: PROG tidy [-h] --sp SP --dp DP
optional arguments:
-h, --help show this help message and exit
--sp SP source
--dp DP dest, required

Answer 2

ap.add_argument("-dp", "--dp", help="Destination Path")
args = parser.parse_args()
if args.copy is in ['copy']:
if args.dp is None:
parser.error('With copy, a dp value is required')
Since a user can't set a value to None, is None is a good test for arguments that haven't been used.
The parser has a list of defined arguments, parser._actions. Each has a required attribute. During parsing it maintains a set of seen-actions. At the end of parsing it just checks this set against the actions for which required is True, and issues the error message if there are any.
I would argue that testing after parsing is simpler than trying to set the required parameter before hand.
An alternative is to provide dp with a reasonable default. Then you won't care whether the user provides a value or not.
I can imagine defining a custom Action class for copy that would set the required attribute of dp, but overall that would be more work.