fix: Give better error message on non-existent device

src/blueapi/core/context.py

@@ -260,9 +260,13 @@ def __get_pydantic_core_schema__(
                 ) -> CoreSchema:
                     def valid(value):
                         val = self.find_device(value)
-                        if not val or not is_compatible(
-                            val, cls.origin or target, cls.args
-                        ):
+                        if not val:
+                            device_names = list(self.devices.keys())
+                            raise ValueError(
+                                f"Device {value} cannot be found, "
+                                f"available devices are: {device_names}"
+                            )
+                        elif not is_compatible(val, cls.origin or target, cls.args):
                             required = qualified_generic_name(target)
                             raise ValueError(
                                 f"Device {value} is not of type {required}"

tests/unit_tests/worker/test_task_worker.py

@@ -636,12 +636,13 @@ def injected_device_plan(
     assert params["dev"] == fake_device
 
 
-def test_missing_injected_devices_fail_early(
+def test_missing_injected_devices_fail_early_and_give_list_of_known_devices(
     context: BlueskyContext,
 ):
     def missing_injection(dev: FakeDevice = inject("does_not_exist")) -> MsgGenerator:
         yield from ()
 
     context.register_plan(missing_injection)
-    with pytest.raises(ValueError):
+    with pytest.raises(ValueError) as excinfo:
         Task(name="missing_injection").prepare_params(context)
+        assert "fake_device" in str(excinfo.value)