Ensure failure when non int precision is given

Weird results can occur if a not integer is given as a precision.
Raise an exception if a floating point value is given.

coloraide/algebra.py

@@ -90,6 +90,9 @@ def order(x: float) -> int:
 def round_half_up(n: float, scale: int = 0) -> float:
     """Round half up."""
 
+    if not isinstance(scale, int):
+        raise ValueError("'float' object cannot be interpreted as an integer")
+
     mult = 10.0 ** scale
     return math.floor(n * mult + 0.5) / mult
 

tests/test_algebra.py

@@ -1113,6 +1113,8 @@ def test_round(self):
         self.assertEqual(alg.round_half_up(3.5), 4)
         self.assertEqual(alg.round_half_up(3.9), 4)
         self.assertEqual(alg.round_half_up(4), 4)
+        with self.assertRaises(ValueError):
+            alg.round_half_up(3.56, 3.4)
 
     def test_scale(self):
         """Test rounding."""