Rename "active_detector" for background detecor. Improve thing selector coercion

tests/unit_tests/test_simulated_camera.py

@@ -186,4 +186,4 @@ def test_simulation_cam_calibration(camera):
     assert camera.calibration_required
     camera.full_auto_calibrate()
     assert not camera.calibration_required
-    assert camera.active_detector.ready
+    assert camera.background_detector.ready

tests/unit_tests/test_thing_selector.py

@@ -0,0 +1,59 @@
+"""Test selector coercion logic for Thing mappings."""
+
+import logging
+
+import pytest
+
+from openflexure_microscope_server.utilities import coerce_thing_selector
+
+
+@pytest.mark.parametrize(
+    ("selected", "default", "warning_count"),
+    [
+        ("a", "b", 1),
+        (None, "b", 0),
+        ("a", None, 1),
+        (None, None, 0),
+    ],
+)
+def test_coerce_with_empty_mapping(selected, default, warning_count, caplog):
+    """Test None always returned for empty mappings, check warnings when appropriate."""
+    with caplog.at_level(logging.WARNING):
+        output = coerce_thing_selector(
+            thing_mapping={}, selected=selected, default=default
+        )
+    assert output is None
+    assert len(caplog.records) == warning_count
+
+
+@pytest.mark.parametrize(
+    ("selected", "default", "returned", "warning_count"),
+    [
+        ("b", "b", "b", 0),
+        ("b", "a", "b", 0),
+        ("b", None, "b", 0),
+        ("b", "z", "b", 0),
+        (None, "b", "b", 0),
+        (None, "a", "a", 0),
+        (None, None, "a", 0),
+        (None, "z", "a", 1),
+        ("z", "b", "b", 1),
+        ("z", "a", "a", 1),
+        ("z", None, "a", 1),
+        ("z", "z", "a", 2),
+    ],
+)
+def test_coerce_with_populated_mapping(
+    selected, default, returned, warning_count, caplog
+):
+    """Test coercion of selected key for a populated thing mapping.
+
+    Check that warnings are raised when appropriate.
+    """
+    thing_mapping = {"a": "Foo", "b": "Bar", "c": "FooBar"}
+    with caplog.at_level(logging.WARNING):
+        output = coerce_thing_selector(
+            thing_mapping=thing_mapping, selected=selected, default=default
+        )
+    assert output == returned
+    assert len(caplog.records) == warning_count