Band a continuous SAP score on its published rating 🟥

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

datatypes/epc/domain/epc.py

@@ -32,6 +32,10 @@ class Epc(Enum):
             return cls.F
         return cls.G
 
+    @classmethod
+    def from_sap_continuous(cls, score: float) -> "Epc":
+        raise NotImplementedError
+
     def sap_lower_bound(self) -> int:
         """The minimum SAP rating in this band — the inverse of
         `from_sap_score` (A → 92, B → 81, C → 69, D → 55, E → 39, F → 21,

tests/datatypes/epc/domain/test_epc.py

@@ -28,6 +28,20 @@ def test_band_floor_round_trips_through_from_sap_score(band: Epc) -> None:
     assert Epc.from_sap_score(band.sap_lower_bound()) is band
 
 
+def test_a_continuous_score_bands_on_its_published_rating() -> None:
+    # Arrange — callers holding a continuous SAP want the band the dwelling
+    # would be *published* in, which means rounding half up. Rounding half to
+    # even (Python's `round`) drops an exact x.5 at a band floor into the band
+    # below: a continuous 68.5 is published SAP 69 and so band C, not band D.
+
+    # Act / Assert — the half point carries the score into the band above.
+    assert Epc.from_sap_continuous(68.5) is Epc.C
+    assert Epc.from_sap_continuous(54.5) is Epc.D
+    # And either side of a floor still bands the obvious way.
+    assert Epc.from_sap_continuous(68.49) is Epc.D
+    assert Epc.from_sap_continuous(69.4) is Epc.C
+
+
 @pytest.mark.parametrize("band", list(Epc))
 def test_continuous_band_floor_is_where_the_published_rating_enters_the_band(
     band: Epc,