Apply suggestions from code review by @bprobert97 - thanks!

Author: rwb27

src/labthings_fastapi/properties.py

@@ -679,12 +679,16 @@ def __set__(
 
         This sets the property's value, and notifies any observers.
 
+        If property validation is enabled by `.FEATURE_FLAGS.validate_properties_on_set`
+        this will validate the value against the property's model, and an error
+        will be raised if the value is not valid.
+
         :param obj: the `.Thing` to which we are attached.
         :param value: the new value for the property.
         :param emit_changed_event: whether to emit a changed event.
         """
-        property_info = self.descriptor_info(obj)
         if FEATURE_FLAGS.validate_properties_on_set:
+            property_info = self.descriptor_info(obj)
             obj.__dict__[self.name] = property_info.validate(value)
         else:
             obj.__dict__[self.name] = value
@@ -922,8 +926,8 @@ def __set__(self, obj: Owner, value: Value) -> None:
         """
         if self.fset is None:
             raise ReadOnlyPropertyError(f"Property {self.name} of {obj} has no setter.")
-        property_info = self.descriptor_info(obj)
         if FEATURE_FLAGS.validate_properties_on_set:
+            property_info = self.descriptor_info(obj)
             value = property_info.validate(value)
 
         self.fset(obj, value)