Improve error when raw2derived argument has no type

src/ophyd_async/core/_derived_signal.py

@@ -209,6 +209,7 @@ def _make_factory(
     raw_devices_and_constants: dict[str, Device | Primitive] | None = None,
 ) -> DerivedSignalFactory:
     if raw_to_derived:
+        _get_first_arg_datatype(raw_to_derived)  # validate first arg type
 
         class DerivedTransform(Transform):
             def raw_to_derived(self, **kwargs) -> dict[str, SignalDatatypeT]:
@@ -278,11 +279,11 @@ def derived_signal_rw(
         The names of these arguments must match the arguments of raw_to_derived.
     """
     raw_to_derived_datatype = _get_return_datatype(raw_to_derived)
-    set_derived_datatype = _get_first_arg_datatype(set_derived)
-    if raw_to_derived_datatype != set_derived_datatype:
+    set_derived_arg_datatype = _get_first_arg_datatype(set_derived)
+    if raw_to_derived_datatype != set_derived_arg_datatype:
         msg = (
             f"{raw_to_derived} has datatype {raw_to_derived_datatype} "
-            f"!= {set_derived_datatype} datatype {set_derived_datatype}"
+            f"!= {set_derived_arg_datatype} datatype {set_derived_arg_datatype}"
         )
         raise TypeError(msg)
 

tests/unit_tests/core/test_single_derived_signal.py

@@ -181,6 +181,29 @@ async def _put(value: float) -> None:
     pass
 
 
+# function without type hint on first argument
+def _get_no_type(ts) -> float:
+    return ts
+
+
+async def test_derived_signal_rw_get_method_no_param_type():
+    signal_rw = soft_signal_rw(int, initial_value=4)
+    with pytest.raises(
+        TypeError,
+        match=" does not have a type hinted argument",
+    ):
+        derived_signal_rw(_get_no_type, _put, ts=signal_rw)
+
+
+async def test_derived_signal_r_get_method_no_param_type():
+    signal_rw = soft_signal_rw(int, initial_value=4)
+    with pytest.raises(
+        TypeError,
+        match=" does not have a type hinted argument",
+    ):
+        derived_signal_r(_get_no_type, ts=signal_rw)
+
+
 @pytest.fixture
 def derived_signal_backend() -> SignalBackend[SignalDatatype]:
     signal_rw = soft_signal_rw(int, initial_value=4)
@@ -264,14 +287,6 @@ async def test_set_derived_callback_already_set(derived_signal_backend: SignalBa
         derived_signal_backend.set_callback(mock_callback)
 
 
-@patch("ophyd_async.core._derived_signal.get_type_hints", return_value={})
-def test_get_return_datatype_no_type(movable_beamstop: MovableBeamstop):
-    with pytest.raises(
-        TypeError, match=re.escape("does not have a type hint for it's return value")
-    ):
-        derived_signal_r(movable_beamstop._get_position)
-
-
 @patch("ophyd_async.core._derived_signal.get_type_hints", return_value={})
 def test_get_first_arg_datatype_no_type(movable_beamstop: MovableBeamstop):
     with pytest.raises(