fix this mutable class var

pioreactor/background_jobs/od_reading.py

@@ -617,10 +617,10 @@ def pop_reference_reading(self, batched_readings: PdChannelToVoltage) -> pt.Volt
 
 class CalibrationTransformer(LoggerMixin):
     _logger_name = "calibration_transformer"
-    models: dict[pt.PdChannel, Callable] = {}
 
     def __init__(self) -> None:
         super().__init__()
+        self.models: dict[pt.PdChannel, Callable] = {}
 
     def __call__(self, batched_readings: PdChannelToVoltage) -> PdChannelToVoltage:
         return batched_readings
@@ -629,6 +629,7 @@ def __call__(self, batched_readings: PdChannelToVoltage) -> PdChannelToVoltage:
 class NullCalibrationTransformer(CalibrationTransformer):
     def __init__(self) -> None:
         super().__init__()
+        self.models: dict[pt.PdChannel, Callable] = {}
 
     def hydate_models(self, calibration_data: structs.ODCalibration | None) -> None:
         return
@@ -637,6 +638,7 @@ def hydate_models(self, calibration_data: structs.ODCalibration | None) -> None:
 class CachedCalibrationTransformer(CalibrationTransformer):
     def __init__(self) -> None:
         super().__init__()
+        self.models: dict[pt.PdChannel, Callable] = {}
         self.has_logged_warning = False
 
     def hydate_models(self, calibration_data: structs.ODCalibration | None) -> None:

pioreactor/tests/test_od_reading.py

@@ -614,6 +614,7 @@ def test_calibration_not_present() -> None:
         c.pop("od")
 
     cal = load_active_calibration("od")
+    assert cal is None
 
     with start_od_reading("90", "REF", interval=None, fake_data=True, calibration=cal) as od:
         assert isinstance(od.calibration_transformer, NullCalibrationTransformer)