VSCode not able to discover tests

[rpk2006]

My test file is as below:

from unittest import TestCase
import pytest

class TestNumbers(TestCase):
    def TestSum(self):
        self.assertEqual(AddNum(5,6), 11)
        
    def TestDiff(self):
        self.assertEqual(SubtractNum(6,3),3)

obj = TestNumbers()
obj.TestSum()
obj.TestDiff()

# if __name__ == "__main__":
#     unittest.main()

I have set VSCode configuration. Enabled test framework and set it to use pyTest. But each time I start VSCode, it shows message to select Test Framework as no tests were discovered.

In the code given above, the last two lines I have commented. Even when these were enabled, VSCode was not able to discover the tests.