Update

[ghstack-poisoned]

tensordict/_lazy.py

@@ -2127,10 +2127,10 @@ def __getitem__(self, index: IndexType) -> Any:
             if index_key:
                 leaf = self._get_tuple(index_key, NO_DEFAULT)
                 if is_non_tensor(leaf):
-                    result = getattr(leaf, "data", NO_DEFAULT)
-                    if result is NO_DEFAULT:
+                    # Only lazy stacks of non tensors are actually tensordict instances
+                    if isinstance(leaf, TensorDictBase):
                         return leaf.tolist()
-                    return result
+                    return leaf.data
                 return leaf
         split_index = self._split_index(index)
         converted_idx = split_index["index_dict"]

tensordict/base.py

@@ -6268,10 +6268,10 @@ def _get_tuple(self, key, default): ...
     def _get_tuple_maybe_non_tensor(self, key, default):
         result = self._get_tuple(key, default)
         if is_non_tensor(result):
-            result_data = getattr(result, "data", NO_DEFAULT)
-            if result_data is NO_DEFAULT:
+            # Only lazy stacks of non tensors are actually tensordict instances
+            if isinstance(result, TensorDictBase):
                 return result.tolist()
-            return result_data
+            return result.data
         return result
 
     def get_at(

tensordict/tensorclass.py

@@ -3445,7 +3445,16 @@ def update_at_(
 
     @property
     def data(self):
-        raise AttributeError
+        """Attempts to return the unique value in the stack.
+
+        Raises a ValueError if there is more than one unique value.
+        """
+        try:
+            return NonTensorData._stack_non_tensor(
+                self.tensordicts, raise_if_non_unique=True
+            ).data
+        except ValueError:
+            raise AttributeError("Cannot get the non-unique data of a NonTensorStack. Use .tolist() instead.")
 
 
 _register_tensor_class(NonTensorStack)