fix: Give better error message on non-existent device

src/blueapi/core/context.py

@@ -432,12 +432,13 @@ 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:
+                            raise ValueError(f"Device {value} cannot be found")
+                        elif not is_compatible(val, cls.origin or target, cls.args):
+                            actual = qualified_name(type(val))
                             required = qualified_generic_name(target)
                             raise ValueError(
-                                f"Device {value} is not of type {required}"
+                                f"Device {value} ({actual}) is not of type {required}"
                             )
                         return val
 

tests/unit_tests/worker/test_task_worker.py

@@ -10,7 +10,7 @@
 
 import pydantic
 import pytest
-from bluesky.protocols import Movable, Status
+from bluesky.protocols import Movable, Readable, Status
 from bluesky.utils import MsgGenerator
 from dodal.common import inject
 from dodal.common.types import UpdatingPathProvider
@@ -681,10 +681,27 @@ def missing_injection(dev: FakeDevice = inject("does_not_exist")) -> MsgGenerato
         yield from ()
 
     context.register_plan(missing_injection)
-    with pytest.raises(ValueError):
+    with pytest.raises(ValueError, match="Device does_not_exist cannot be found"):
         Task(name="missing_injection").prepare_params(context)
 
 
+def test_invalid_injected_devices_fail_early(
+    context: BlueskyContext,
+):
+    def invalid_injection(dev: Readable = inject("fake_device")) -> MsgGenerator:
+        yield from ()
+
+    context.register_plan(invalid_injection)
+    with pytest.raises(
+        ValueError,
+        match=(
+            r"Device fake_device \(test_task_worker.FakeDevice\) "
+            "is not of type bluesky.protocols.Readable"
+        ),
+    ):
+        Task(name="invalid_injection").prepare_params(context)
+
+
 @patch("blueapi.worker.task_worker.plan_tag_filter_context")
 def test_worker_uses_plan_tag_filter_context(
     mock_context: Mock, inert_worker: TaskWorker