fix: comments

Author: kitagry

gokart/task.py

@@ -236,16 +236,7 @@ def make_target(self, relative_file_path: str | None = None, use_unique_id: bool
             file_path=file_path, unique_id=unique_id, processor=processor, task_lock_params=task_lock_params, store_index_in_feather=self.store_index_in_feather
         )
 
-    def _create_processor_for_dataframe_type(self, file_path: str) -> FileProcessor | None:
-        """
-        Create a file processor with appropriate return_type based on task's type parameter.
-
-        Args:
-            file_path: Path to the file
-
-        Returns:
-            FileProcessor with return_type set, or None to use default processor
-        """
+    def _create_processor_for_dataframe_type(self, file_path: str) -> FileProcessor:
         df_type = get_dataframe_type_from_task(self)
         return make_file_processor(file_path, dataframe_type=df_type, store_index_in_feather=self.store_index_in_feather)
 

gokart/utils.py

@@ -118,15 +118,16 @@ def get_dataframe_type_from_task(task: Any) -> Literal['pandas', 'polars', 'pola
     """
     task_class = task if isinstance(task, type) else task.__class__
 
-    if not hasattr(task_class, '__orig_bases__'):
-        return 'pandas'
-
-    for base in task_class.__orig_bases__:
-        origin = get_origin(base)
-        # Check if this is a TaskOnKart subclass
-        if origin and hasattr(origin, '__name__') and origin.__name__ == 'TaskOnKart':
-            args = get_args(base)
-            if args:
+    # Walk the MRO to find TaskOnKart[...] even when defined on a parent class
+    mro = task_class.mro() if hasattr(task_class, 'mro') else [task_class]
+
+    for cls in mro:
+        for base in getattr(cls, '__orig_bases__', ()):
+            origin = get_origin(base)
+            if origin and hasattr(origin, '__name__') and origin.__name__ == 'TaskOnKart':
+                args = get_args(base)
+                if not args:
+                    continue
                 df_type = args[0]
                 module = getattr(df_type, '__module__', '')
 

test/test_utils.py

@@ -168,3 +168,21 @@ class _DerivedLazyTask(_BaseLazyTask):
 
         task = _DerivedLazyTask()
         self.assertEqual(get_dataframe_type_from_task(task), 'polars-lazy')
+
+    @pytest.mark.skipif(not HAS_POLARS, reason='polars not installed')
+    def test_nested_inheritance_polars_with_mixin(self):
+        """Derived class with multiple bases should still detect polars through MRO."""
+
+        class _Mixin:
+            pass
+
+        class _BasePolarsTask(TaskOnKart[pl.DataFrame]):
+            pass
+
+        # Multiple inheritance gives _DerivedTask its own __orig_bases__,
+        # which shadows the parent's and doesn't contain TaskOnKart[...].
+        class _DerivedTask(_BasePolarsTask, _Mixin):
+            pass
+
+        task = _DerivedTask()
+        self.assertEqual(get_dataframe_type_from_task(task), 'polars')