Fix pandas-devGH-29442 DataFrame.groupby doesn't preserve _metadata (pandas-dev#35688)

Author: Japanuspus

doc/source/whatsnew/v1.1.4.rst

@@ -27,7 +27,7 @@ Fixed regressions
 
 Bug fixes
 ~~~~~~~~~
--
+- Bug causing ``groupby(...).sum()`` and similar to not preserve metadata (:issue:`29442`)
 
 .. ---------------------------------------------------------------------------
 

pandas/core/groupby/groupby.py

@@ -1010,8 +1010,9 @@ def _agg_general(
     ):
         with group_selection_context(self):
             # try a cython aggregation if we can
+            result = None
             try:
-                return self._cython_agg_general(
+                result = self._cython_agg_general(
                     how=alias,
                     alt=npfunc,
                     numeric_only=numeric_only,
@@ -1030,8 +1031,9 @@ def _agg_general(
                     raise
 
             # apply a non-cython aggregation
-            result = self.aggregate(lambda x: npfunc(x, axis=self.axis))
-            return result
+            if result is None:
+                result = self.aggregate(lambda x: npfunc(x, axis=self.axis))
+            return result.__finalize__(self.obj, method="groupby")
 
     def _cython_agg_general(
         self, how: str, alt=None, numeric_only: bool = True, min_count: int = -1

pandas/tests/generic/test_finalize.py

@@ -762,13 +762,27 @@ def test_categorical_accessor(method):
     [
         operator.methodcaller("sum"),
         lambda x: x.agg("sum"),
+    ],
+)
+def test_groupby_finalize(obj, method):
+    obj.attrs = {"a": 1}
+    result = method(obj.groupby([0, 0]))
+    assert result.attrs == {"a": 1}
+
+
+@pytest.mark.parametrize(
+    "obj", [pd.Series([0, 0]), pd.DataFrame({"A": [0, 1], "B": [1, 2]})]
+)
+@pytest.mark.parametrize(
+    "method",
+    [
         lambda x: x.agg(["sum", "count"]),
         lambda x: x.transform(lambda y: y),
         lambda x: x.apply(lambda y: y),
     ],
 )
 @not_implemented_mark
-def test_groupby(obj, method):
+def test_groupby_finalize_not_implemented(obj, method):
     obj.attrs = {"a": 1}
     result = method(obj.groupby([0, 0]))
     assert result.attrs == {"a": 1}