Module 11 testing: assertIsNone vs assertEqual(..., None) for the invalid-input test?

[dab00gieman]

python-study-Neo_1/curriculum/11_testing/exercises.py supplies:

def add_numbers(a, b):
    if not isinstance(a, (int, float)) or not isinstance(b, (int, float)):
        return None
    return a + b

# TODO: TestAddNumbers with test_positive_numbers, test_negative_numbers, test_invalid_input

For test_invalid_input ("assert equal add_numbers('2', 3) and None" per the TODO comment), is self.assertIsNone(...) the preferred assertion over assertEqual(..., None) — since assertEqual would also pass for objects that compare equal to None-ish values... and where should the solved TestCase live, answers/exercise11/answer.py?

[mgkvibez]

"Both work — assertEqual(add_numbers("2", 3), None) passes — but assertIsNone is the better choice, and here's why it's worth deviating from the TODO's literal wording:

• Intent: assertIsNone(result) reads as "this returns None", which is the test's meaning. assertEqual(result, None) reads as "this equals None", which happens to be the same thing only because None is a singleton. Reviewers and graders read the first as fluency.
• Failure messages: assertIsNone's failure output is purpose-built (None is not None plus the actual repr), while assertEqual's generic diff buries the point.
• The habit generalizes: assertTrue(x) vs assertIs(True, ...), assertIn, assertRaises — the specific assertions exist so tests say what they mean. General rule: use the most specific assertion that expresses the property you're checking.

So the full method:

def test_invalid_input(self):
    self.assertIsNone(add_numbers("2", 3))
    self.assertIsNone(add_numbers(3, None))

One bonus trap worth an extra test if you want to go past the exercise: booleans. isinstance(True, int) is True in Python (bool subclasses int), so add_numbers(True, 3) happily returns 4, not None. The function's docstring says "Returns None if inputs are not numbers" — and by that spec, True is arguably a number, so it's a specification question rather than a bug. But an assertIsNone(add_numbers(True, 3)) test would fail and make you decide the contract explicitly — which is the most educational test in the whole module.