TYP: Add FrameORSeries to NDFrame

Author: topper-123

pandas/core/base.py

@@ -8,6 +8,7 @@
 import numpy as np
 
 import pandas._libs.lib as lib
+from pandas._typing import T
 from pandas.compat import PYPY
 from pandas.compat.numpy import function as nv
 from pandas.errors import AbstractMethodError
@@ -87,6 +88,14 @@ def __sizeof__(self):
         # object's 'sizeof'
         return super().__sizeof__()
 
+    def _ensure_type(self: T, obj) -> T:
+        """Ensure that an object has same type as self.
+
+        Used by type checkers.
+        """
+        assert isinstance(obj, type(self)), type(obj)
+        return obj
+
 
 class NoNewAttributesMixin:
     """Mixin which prevents adding new attributes.