I have this ArgumentParser class:

from argparse import ArgumentParser

class ArgumentHandler:

    def __init__(self):
        self.parser = ArgumentParser()
        self.parser.add_argument('--environment', help='Environment', choices=['TEST', 'DEV', 'PROD'], nargs=1, type=str.upper)
        self.parser.add_argument('--asofdate', help='As of date', nargs=1)

    def parse(self):
        parsed = {}

        args = dict(vars(self.parser.parse_args()))
        ...

        return parsed

I’m trying to test it using unittest:

def test_providing_incorrect_environment(self):
        print(f"RUNNING TEST CASE {inspect.currentframe().f_code.co_name}")

        app = sys.argv[0] if len(sys.argv) > 0 and sys.argv[0] is not None else __file__
        sys.argv = [app, '--environment=UAT' ]

        with self.assertRaises(ArgumentError):
            ArgumentHandler().parse()

But I’m getting this error:

argparse.ArgumentError: argument --environment: invalid choice: 'UAT' (choose from 'TEST', 'DEV', 'PROD')
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "C:SWDTOOLSPYTHON39libunittestcase.py", line 59, in testPartExecutor
    yield
  File "C:SWDTOOLSPYTHON39libunittestcase.py", line 593, in run
    self._callTestMethod(testMethod)
  File "C:SWDTOOLSPYTHON39libunittestcase.py", line 550, in _callTestMethod
    method()
  File "c:ParisNmdProcessortestscomponentshelperstest_argument_handler.py", line 57, in test_providing_incorrect_environment
    ArgumentHandler().parse()
  File "c:ParisNmdProcessorsrccomponentshelpersargument_handler.py", line 28, in parse
    args = dict(vars(self.parser.parse_args()))
  File "C:SWDTOOLSPYTHON39libargparse.py", line 1818, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "C:SWDTOOLSPYTHON39libargparse.py", line 1854, in parse_known_args
    self.error(str(err))
  File "C:SWDTOOLSPYTHON39libargparse.py", line 2575, in error
    self.exit(2, _('%(prog)s: error: %(message)sn') % args)
  File "C:SWDTOOLSPYTHON39libargparse.py", line 2562, in exit
    _sys.exit(status)
SystemExit: 2