DEPR: _metadata propagation

doc/source/whatsnew/v2.1.0.rst

@@ -236,6 +236,7 @@ Deprecations
 - Deprecated :func:`is_interval_dtype`, check ``isinstance(dtype, pd.IntervalDtype)`` instead (:issue:`52607`)
 - Deprecated :meth:`DataFrame.applymap`. Use the new :meth:`DataFrame.map` method instead (:issue:`52353`)
 - Deprecated :meth:`DataFrame.swapaxes` and :meth:`Series.swapaxes`, use :meth:`DataFrame.transpose` or :meth:`Series.transpose` instead (:issue:`51946`)
+- Deprecated ``_metadata`` propagation (:issue:`51280`)
 - Deprecated ``freq`` parameter in :class:`PeriodArray` constructor, pass ``dtype`` instead (:issue:`52462`)
 - Deprecated behavior of :func:`concat` when :class:`DataFrame` has columns that are all-NA, in a future version these will not be discarded when determining the resulting dtype (:issue:`40893`)
 - Deprecated behavior of :meth:`Series.dt.to_pydatetime`, in a future version this will return a :class:`Series` containing python ``datetime`` objects instead of an ``ndarray`` of datetimes; this matches the behavior of other :meth:`Series.dt` properties (:issue:`20306`)

pandas/core/generic.py

@@ -6057,6 +6057,14 @@ def __finalize__(self, other, method: str | None = None, **kwargs) -> Self:
             # For subclasses using _metadata.
             for name in set(self._metadata) & set(other._metadata):
                 assert isinstance(name, str)
+                if name != "_name":
+                    warnings.warn(
+                        "_metadata propagation is deprecated and will be removed "
+                        "in a future version. To retain the old behavior, "
+                        "override __finalize__ in a subclass.",
+                        FutureWarning,
+                        stacklevel=find_stack_level(),
+                    )
                 object.__setattr__(self, name, getattr(other, name, None))
 
         if method == "concat":
@@ -6111,6 +6119,12 @@ def __setattr__(self, name: str, value) -> None:
         if name in self._internal_names_set:
             object.__setattr__(self, name, value)
         elif name in self._metadata:
+            warnings.warn(
+                "_metadata handling is deprecated and will be removed in "
+                "a future version.",
+                FutureWarning,
+                stacklevel=find_stack_level(),
+            )
             object.__setattr__(self, name, value)
         else:
             try:

pandas/tests/frame/test_arithmetic.py

@@ -2069,12 +2069,18 @@ def _constructor(self):
         def _constructor_sliced(self):
             return SubclassedSeries
 
-    sdf = SubclassedDataFrame("some_data", {"A": [1, 2, 3], "B": [4, 5, 6]})
-    result = sdf * 2
-    expected = SubclassedDataFrame("some_data", {"A": [2, 4, 6], "B": [8, 10, 12]})
+    msg = "_metadata handling is deprecated"
+
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        sdf = SubclassedDataFrame("some_data", {"A": [1, 2, 3], "B": [4, 5, 6]})
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        result = sdf * 2
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        expected = SubclassedDataFrame("some_data", {"A": [2, 4, 6], "B": [8, 10, 12]})
     tm.assert_frame_equal(result, expected)
 
-    result = sdf + sdf
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        result = sdf + sdf
     tm.assert_frame_equal(result, expected)