DelegateParameters inherit their source's settable/gettable/snapshot_value/unit/label

docs/changes/newsfragments/6648.improved

@@ -0,0 +1,2 @@
+DelegateParameter now reads delegated properties from source rather than setting them at source change.
+This resolves an issue when delegating from another delegate parameter may result in incorrect attributes.

src/qcodes/parameters/delegate_parameter.py

@@ -156,19 +156,6 @@ def __init__(
         if "bind_to_instrument" not in kwargs.keys():
             kwargs["bind_to_instrument"] = False
 
-        self._attr_inherit = {
-            "label": {"fixed": False, "value_when_without_source": name},
-            "unit": {"fixed": False, "value_when_without_source": ""},
-        }
-
-        for attr, attr_props in self._attr_inherit.items():
-            if attr in kwargs:
-                attr_props["fixed"] = True
-            else:
-                attr_props["fixed"] = False
-            source_attr = getattr(source, attr, attr_props["value_when_without_source"])
-            kwargs[attr] = kwargs.get(attr, source_attr)
-
         for cmd in ("set_cmd", "get_cmd"):
             if cmd in kwargs:
                 raise KeyError(
@@ -188,9 +175,14 @@ def __init__(
         initial_cache_value = kwargs.pop("initial_cache_value", None)
         self.source = source
         super().__init__(name, *args, **kwargs)
-        # explicitly set the source properties as
-        # init will overwrite the ones set when assigning source
-        self._set_properties_from_source(source)
+        self.label = kwargs.get("label", None)
+        self.unit = kwargs.get("unit", None)
+
+        # Hack While we inherit the settable status from the parent parameter
+        # we do allow param.set_to to temporary override _settable in a
+        # context. Here _settable should always be true except when set_to
+        # i.e. _SetParamContext overrides it
+        self._settable = True
 
         self.cache = self._DelegateCache(self)
         if initial_cache_value is not None:
@@ -209,25 +201,62 @@ def source(self) -> Parameter | None:
 
     @source.setter
     def source(self, source: Parameter | None) -> None:
-        self._set_properties_from_source(source)
         self._source: Parameter | None = source
 
-    def _set_properties_from_source(self, source: Parameter | None) -> None:
-        if source is None:
-            self._gettable = False
-            self._settable = False
-            self._snapshot_value = False
+    @property
+    def snapshot_value(self) -> bool:
+        if self.source is None:
+            return False
+        return self.source.snapshot_value
+
+    @property
+    def unit(self) -> str:
+        """
+        The unit of measure. Read from source if not explicitly overwritten.
+        Set to None to disable overwrite.
+        """
+        if self._unit_override is not None:
+            return self._unit_override
+        elif self.source is not None:
+            return self.source.unit
         else:
-            self._gettable = source.gettable
-            self._settable = source.settable
-            self._snapshot_value = source._snapshot_value
-
-        for attr, attr_props in self._attr_inherit.items():
-            if not attr_props["fixed"]:
-                attr_val = getattr(
-                    source, attr, attr_props["value_when_without_source"]
-                )
-                setattr(self, attr, attr_val)
+            return ""
+
+    @unit.setter
+    def unit(self, unit: str | None) -> None:
+        self._unit_override = unit
+
+    @property
+    def label(self) -> str:
+        """
+        Label of the data used for plots etc.
+        Read from source if not explicitly overwritten.
+        Set to None to disable overwrite.
+        """
+        if self._label_override is not None:
+            return self._label_override
+        elif self.source is not None:
+            return self.source.label
+        else:
+            return self.name
+
+    @label.setter
+    def label(self, label: str | None) -> None:
+        self._label_override = label
+
+    @property
+    def gettable(self) -> bool:
+        if self.source is None:
+            return False
+        return self.source.gettable
+
+    @property
+    def settable(self) -> bool:
+        if self._settable is False:
+            return False
+        if self.source is None:
+            return False
+        return self.source.settable
 
     def get_raw(self) -> Any:
         if self.source is None:

src/qcodes/parameters/parameter_base.py

@@ -537,7 +537,7 @@ def snapshot_base(
 
         state: dict[str, Any] = {"__class__": full_class(self), "full_name": str(self)}
 
-        if self._snapshot_value:
+        if self.snapshot_value:
             has_get = self.gettable
             allowed_to_call_get_when_snapshotting = (
                 self._snapshot_get and update is not False

tests/parameter/test_delegate_parameter.py

@@ -426,7 +426,7 @@ def test_gettable_settable_snapshotget_delegate_parameter(
     delegate_param = DelegateParameter("delegate", source=source_param)
     assert delegate_param.gettable is gettable
     assert delegate_param.settable is settable
-    assert delegate_param._snapshot_value is snapshot_value
+    assert delegate_param.snapshot_value is snapshot_value
 
 
 @pytest.mark.parametrize("snapshot_value", [True, False])
@@ -450,7 +450,7 @@ def test_gettable_settable_snapshotget_delegate_parameter_2(
     delegate_param.source = source_param
     assert delegate_param.gettable is gettable
     assert delegate_param.settable is settable
-    assert delegate_param._snapshot_value is snapshot_value
+    assert delegate_param.snapshot_value is snapshot_value
 
 
 def test_initial_value_and_none_source_raises() -> None:
@@ -624,3 +624,64 @@ def test_value_validation_with_offset_and_scale() -> None:
         delegate_param.validate(1)  # raw_value = 100
     with pytest.raises(ValueError):
         delegate_param.set(1)
+
+
+def test_delegate_of_delegate_updates_settable_gettable():
+    gettable_settable_source_param = Parameter(
+        "source", set_cmd=None, get_cmd=None, vals=vals.Numbers(-5, 5)
+    )
+    not_gettable_source_param = Parameter(
+        "source", set_cmd=None, get_cmd=False, vals=vals.Numbers(-5, 5)
+    )
+    not_settable_source_param = Parameter(
+        "source", set_cmd=False, get_cmd=None, vals=vals.Numbers(-5, 5)
+    )
+    delegate_param_inner = DelegateParameter(
+        "delegate_inner", source=None, vals=vals.Numbers(-10, 10)
+    )
+    delegate_param_outer = DelegateParameter(
+        "delegate_outer", source=None, vals=vals.Numbers(-10, 10)
+    )
+    delegate_param_outer.source = delegate_param_inner
+    delegate_param_inner.source = gettable_settable_source_param
+
+    assert delegate_param_outer.gettable
+    assert delegate_param_outer.settable
+
+    delegate_param_inner.source = not_gettable_source_param
+
+    assert not delegate_param_outer.gettable
+    assert delegate_param_outer.settable
+
+    delegate_param_inner.source = not_settable_source_param
+
+    assert delegate_param_outer.gettable
+    assert not delegate_param_outer.settable
+
+
+def test_delegate_parameter_context() -> None:
+    gettable_settable_source_param = Parameter(
+        "source", set_cmd=None, get_cmd=None, vals=vals.Numbers(-5, 5)
+    )
+
+    delegate_param = DelegateParameter(
+        "delegate_outer", source=None, vals=vals.Numbers(-10, 10)
+    )
+
+    delegate_param.source = gettable_settable_source_param
+
+    delegate_param(2)
+    assert delegate_param() == 2
+
+    with delegate_param.set_to(3):
+        assert delegate_param() == 3
+        with pytest.raises(NotImplementedError):
+            delegate_param(4)
+        assert delegate_param() == 3
+    assert delegate_param() == 2
+
+    with delegate_param.set_to(3, allow_changes=True):
+        assert delegate_param() == 3
+        delegate_param(4)
+        assert delegate_param() == 4
+    assert delegate_param() == 2