[core] set value of dict property not working

sdk/core/azure-core/tests/specs_sdk/modeltypes/modeltypes/_utils/model_base.py

@@ -360,9 +360,39 @@ def __contains__(self, key: typing.Any) -> bool:
         return key in self._data
 
     def __getitem__(self, key: str) -> typing.Any:
+        # If this key has been deserialized (for mutable types), we need to handle serialization
+        if hasattr(self, "_attr_to_rest_field"):
+            cache_attr = f"_deserialized_{key}"
+            if hasattr(self, cache_attr):
+                rf = _get_rest_field(self._attr_to_rest_field, key)
+                if rf:
+                    value = self._data.get(key)
+                    if isinstance(value, (dict, list, set)):
+                        # For mutable types, serialize and return
+                        # But also update _data with serialized form and clear flag
+                        # so mutations via this returned value affect _data
+                        serialized = _serialize(value, rf._format)
+                        # If serialized form is same type (no transformation needed),
+                        # return _data directly so mutations work
+                        if type(serialized) == type(value) and serialized == value:
+                            return self._data.get(key)
+                        # Otherwise return serialized copy and clear flag
+                        try:
+                            object.__delattr__(self, cache_attr)
+                        except AttributeError:
+                            pass
+                        # Store serialized form back
+                        self._data[key] = serialized
+                        return serialized
         return self._data.__getitem__(key)
 
     def __setitem__(self, key: str, value: typing.Any) -> None:
+        # Clear any cached deserialized value when setting through dictionary access
+        cache_attr = f"_deserialized_{key}"
+        try:
+            object.__delattr__(self, cache_attr)
+        except AttributeError:
+            pass
         self._data.__setitem__(key, value)
 
     def __delitem__(self, key: str) -> None:
@@ -1039,14 +1069,37 @@ def _rest_name(self) -> str:
     def __get__(self, obj: Model, type=None):  # pylint: disable=redefined-builtin
         # by this point, type and rest_name will have a value bc we default
         # them in __new__ of the Model class
-        item = obj.get(self._rest_name)
+        # Use _data.get() directly to avoid triggering __getitem__ which clears the cache
+        item = obj._data.get(self._rest_name)
         if item is None:
             return item
         if self._is_model:
             return item
-        return _deserialize(self._type, _serialize(item, self._format), rf=self)
+
+        # For mutable types, we want mutations to directly affect _data
+        # Check if we've already deserialized this value
+        cache_attr = f"_deserialized_{self._rest_name}"
+        if hasattr(obj, cache_attr):
+            # Return the value from _data directly (it's been deserialized in place)
+            return obj._data.get(self._rest_name)
+
+        deserialized = _deserialize(self._type, _serialize(item, self._format), rf=self)
+
+        # For mutable types, store the deserialized value back in _data
+        # so mutations directly affect _data
+        if isinstance(deserialized, (dict, list, set)):
+            obj._data[self._rest_name] = deserialized
+            object.__setattr__(obj, cache_attr, True)  # Mark as deserialized
+            return deserialized
+
+        return deserialized
 
     def __set__(self, obj: Model, value) -> None:
+        # Clear the cached deserialized object when setting a new value
+        cache_attr = f"_deserialized_{self._rest_name}"
+        if hasattr(obj, cache_attr):
+            object.__delattr__(obj, cache_attr)
+
         if value is None:
             # we want to wipe out entries if users set attr to None
             try: