Fix approx() raising InvalidOperation on Decimal("NaN") actual value

changelog/15335.bugfix.rst

@@ -0,0 +1 @@
+Fixed :func:`pytest.approx` raising :class:`decimal.InvalidOperation` when the actual value is a :class:`decimal.Decimal` NaN and the expected value is finite.

src/_pytest/approx.py

@@ -589,6 +589,9 @@ def __eq__(self, actual) -> bool:
         if _is_nan(abs(self.expected)):
             return self.nan_ok and _is_nan(abs(actual))
 
+        if _is_nan(abs(actual)):
+            return False
+
         # Infinity shouldn't be approximately equal to anything but itself, but
         # if there's a relative tolerance, it will be infinite and infinity
         # will seem approximately equal to everything.  The equal-to-itself

testing/python/approx.py

@@ -634,6 +634,16 @@ def test_decimal_outside_float_range(self):
         assert expected != approx(Decimal("Infinity"))
         assert Decimal("Infinity") != approx(expected)
 
+    def test_decimal_actual_nan(self):
+        """Comparing a Decimal('NaN') actual value against approx() compares unequal (#15335)."""
+        assert Decimal("NaN") != approx(Decimal("1.1"))
+        assert Decimal("NaN") != approx(Decimal("1.1"), nan_ok=True)
+        assert Decimal("NaN") != approx(1.1)
+        assert [Decimal("NaN")] != approx([Decimal("1.1")])
+        assert {"val": Decimal("NaN")} != approx({"val": Decimal("1.1")})
+        assert Decimal("NaN") == approx(Decimal("NaN"), nan_ok=True)
+        assert Decimal("NaN") != approx(Decimal("NaN"), nan_ok=False)
+
     def test_fraction(self):
         within_1e6 = [
             (1 + Fraction(1, 1000000), Fraction(1)),