Fix bug: delegate parameter doesn't call its source to get value

docs/changes/newsfragments/6671.improved

@@ -0,0 +1,2 @@
+Fix a regression introduced in 0.50.0 where a DelegateParameter initialized with a None source
+would not correctly call get/set on the source parameter when this has been set.

src/qcodes/parameters/parameter.py

@@ -245,7 +245,9 @@ def _set_manual_parameter(
             **kwargs,
         )
 
-        no_instrument_get = not self.gettable and (get_cmd is None or get_cmd is False)
+        no_instrument_get = not self._implements_get_raw and (
+            get_cmd is None or get_cmd is False
+        )
         # TODO: a matching check should be in ParameterBase but
         #   due to the current limited design the ParameterBase cannot
         #   know if this subclass will supply a get_cmd
@@ -261,13 +263,13 @@ def _set_manual_parameter(
         # in the scope of this class.
         # (previous call to `super().__init__` wraps existing
         # get_raw/set_raw into get/set methods)
-        if self.gettable and get_cmd not in (None, False):
+        if self._implements_get_raw and get_cmd not in (None, False):
             raise TypeError(
                 "Supplying a not None or False `get_cmd` to a Parameter"
                 " that already implements"
                 " get_raw is an error."
             )
-        elif not self.gettable and get_cmd is not False:
+        elif not self._implements_get_raw and get_cmd is not False:
             if get_cmd is None:
                 # ignore typeerror since mypy does not allow setting a method dynamically
                 self.get_raw = MethodType(_get_manual_parameter, self)  # type: ignore[method-assign]
@@ -293,13 +295,13 @@ def _set_manual_parameter(
             # this may be resolvable if Command above is correctly wrapped in MethodType
             self.get = self._wrap_get(self.get_raw)  # type: ignore[arg-type]
 
-        if self.settable and set_cmd not in (None, False):
+        if self._implements_set_raw and set_cmd not in (None, False):
             raise TypeError(
                 "Supplying a not None or False `set_cmd` to a Parameter"
                 " that already implements"
                 " set_raw is an error."
             )
-        elif not self.settable and set_cmd is not False:
+        elif not self._implements_set_raw and set_cmd is not False:
             if set_cmd is None:
                 # ignore typeerror since mypy does not allow setting a method dynamically
                 self.set_raw = MethodType(_set_manual_parameter, self)  # type: ignore[method-assign]

src/qcodes/parameters/parameter_base.py

@@ -263,11 +263,8 @@ def __init__(
         self.get_latest = GetLatest(self)
 
         self.get: Callable[..., ParamDataType]
-        implements_get_raw = hasattr(self, "get_raw") and not getattr(
-            self.get_raw, "__qcodes_is_abstract_method__", False
-        )
         self._gettable = False
-        if implements_get_raw:
+        if self._implements_get_raw:
             self.get = self._wrap_get(self.get_raw)
             self._gettable = True
         elif hasattr(self, "get"):
@@ -278,11 +275,8 @@ def __init__(
             )
 
         self.set: Callable[..., None]
-        implements_set_raw = hasattr(self, "set_raw") and not getattr(
-            self.set_raw, "__qcodes_is_abstract_method__", False
-        )
         self._settable: bool = False
-        if implements_set_raw:
+        if self._implements_set_raw:
             self.set = self._wrap_set(self.set_raw)
             self._settable = True
         elif hasattr(self, "set"):
@@ -347,6 +341,20 @@ def __init__(
 
             instrument.parameters[name] = self
 
+    @property
+    def _implements_get_raw(self) -> bool:
+        implements_get_raw = hasattr(self, "get_raw") and not getattr(
+            self.get_raw, "__qcodes_is_abstract_method__", False
+        )
+        return implements_get_raw
+
+    @property
+    def _implements_set_raw(self) -> bool:
+        implements_set_raw = hasattr(self, "set_raw") and not getattr(
+            self.set_raw, "__qcodes_is_abstract_method__", False
+        )
+        return implements_set_raw
+
     def _build__doc__(self) -> str | None:
         return self.__doc__
 

tests/parameter/test_delegate_parameter.py

@@ -271,6 +271,23 @@ def test_delegate_cache_pristine_if_not_set() -> None:
     assert gotten_delegate_cache is None
 
 
+def test_delegate_get_instrument_val(numeric_val: int) -> None:
+    """
+    Delegate should call its source to get value rather than just reading source cache
+    """
+    initial_value = numeric_val
+    t = ObservableParam("observable_parameter", initial_value=initial_value)
+    # delegate has no source initially to make sure it's not gettable on initialization
+    d = DelegateParameter("delegate", source=None)
+    d.source = t
+
+    new_instr_value = 3
+    # Update instrument value without changing parameter cache
+    t.instr_val = new_instr_value
+    # This check fails if delegate only reads source cache
+    assert d() == new_instr_value
+
+
 def test_delegate_get_updates_cache(
     make_observable_parameter: Callable[..., ObservableParam], numeric_val: int
 ) -> None: