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

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)
+        # Note: use _gettable/_settable attributes in __init__ function.
+        # Avoid using gettable/settable properties here.
+        no_instrument_get = not self._gettable 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._gettable 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._gettable 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._settable 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._settable 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]

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: