Fixed syntax

src/ophyd_async/plan_stubs/fly.py

@@ -32,9 +32,8 @@ def time_resolved_fly_and_collect_with_static_seq_table(
     stages/unstages the devices, and opens and closes the run.
 
     """
-    # Ensure there is at least on detector
-    if len(detectors) == 0:
-        raise ValueError
+    if not detectors:
+        raise ValueError("No detectors provided. There must be at least one.")
 
     # Set up scan and prepare trigger
     deadtime = max(det.controller.get_deadtime(exposure) for det in detectors)

tests/plan_stubs/test_fly.py

@@ -359,6 +359,8 @@ async def test_at_least_one_detector_in_fly_plan(
     exposure = 1
     shutter_time = 0.004
 
+    assert not detector_list
+
     def fly():
         yield from time_resolved_fly_and_collect_with_static_seq_table(
             stream_name="stream1",
@@ -369,5 +371,6 @@ def fly():
             shutter_time=shutter_time,
         )
 
-    with pytest.raises(ValueError):
+    with pytest.raises(ValueError) as exc:
         RE(fly())
+        assert(str(exc)=="No detectors provided. There must be at least one.")