fix(python): make the assert_not_equal failure message describe the failure

src/fable-library-py/fable_library/util.py

@@ -338,7 +338,9 @@ def assert_equal(actual: Any, expected: Any, msg: str | None = None) -> None:
 
 def assert_not_equal[T](actual: T, expected: T, msg: str | None = None) -> None:
     if equals(actual, expected):
-        raise Exception(msg or f"Expected: {expected} - Actual: {actual}")
+        # "Expected: x - Actual: x" would describe a *passing* assertion, so say what was
+        # actually expected: a value other than this one. Mirrors xUnit's "Expected: Not x".
+        raise Exception(msg or f"Expected not equal to: {expected} - Actual: {actual}")
 
 
 MAX_LOCKS = 1024

src/fable-library-py/tests/test_assert.py

@@ -0,0 +1,33 @@
+import pytest
+from fable_library.util import assert_equal, assert_not_equal
+
+
+def test_assert_equal_passes_on_equal_values():
+    assert_equal(1, 1)
+    assert_equal([1, 2], [1, 2])
+
+
+def test_assert_equal_raises_on_mismatch():
+    with pytest.raises(Exception, match=r"^Expected: 1 - Actual: 2$"):
+        assert_equal(2, 1)
+
+
+def test_assert_equal_uses_the_custom_message():
+    with pytest.raises(Exception, match=r"^values differ$"):
+        assert_equal(2, 1, "values differ")
+
+
+def test_assert_not_equal_passes_on_different_values():
+    assert_not_equal(1, 2)
+
+
+def test_assert_not_equal_raises_when_equal():
+    # The failure has to describe what was expected -- some value *other* than 1. Rendering it as
+    # "Expected: 1 - Actual: 1" reads as a passing assertion, which is what it used to say.
+    with pytest.raises(Exception, match=r"^Expected not equal to: 1 - Actual: 1$"):
+        assert_not_equal(1, 1)
+
+
+def test_assert_not_equal_uses_the_custom_message():
+    with pytest.raises(Exception, match=r"^values are equal$"):
+        assert_not_equal(1, 1, "values are equal")