Backport PR #37461 on branch 1.1.x: BUG: Metadata propagation for gro…

…upby iterator (#37628)

Co-authored-by: Janus <janus@insignificancegalore.net>

doc/source/whatsnew/v1.1.5.rst

@@ -23,7 +23,7 @@ Fixed regressions
 
 Bug fixes
 ~~~~~~~~~
--
+- Bug in metadata propagation for ``groupby`` iterator (:issue:`37343`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/core/groupby/ops.py

@@ -131,9 +131,16 @@ def get_iterator(self, data: FrameOrSeries, axis: int = 0):
         splitter = self._get_splitter(data, axis=axis)
         keys = self._get_group_keys()
         for key, (i, group) in zip(keys, splitter):
-            yield key, group
+            yield key, group.__finalize__(data, method="groupby")
 
     def _get_splitter(self, data: FrameOrSeries, axis: int = 0) -> "DataSplitter":
+        """
+        Returns
+        -------
+        Generator yielding subsetted objects
+
+        __finalize__ has not been called for the the subsetted objects returned.
+        """
         comp_ids, _, ngroups = self.group_info
         return get_splitter(data, comp_ids, ngroups, axis=axis)
 
@@ -955,7 +962,8 @@ class SeriesSplitter(DataSplitter):
     def _chop(self, sdata: Series, slice_obj: slice) -> Series:
         # fastpath equivalent to `sdata.iloc[slice_obj]`
         mgr = sdata._mgr.get_slice(slice_obj)
-        return type(sdata)(mgr, name=sdata.name, fastpath=True)
+        # __finalize__ not called here, must be applied by caller if applicable
+        return sdata._constructor(mgr, name=sdata.name, fastpath=True)
 
 
 class FrameSplitter(DataSplitter):
@@ -971,7 +979,8 @@ def _chop(self, sdata: DataFrame, slice_obj: slice) -> DataFrame:
         # else:
         #     return sdata.iloc[:, slice_obj]
         mgr = sdata._mgr.get_slice(slice_obj, axis=1 - self.axis)
-        return type(sdata)(mgr)
+        # __finalize__ not called here, must be applied by caller if applicable
+        return sdata._constructor(mgr)
 
 
 def get_splitter(

pandas/tests/groupby/test_groupby_subclass.py

@@ -51,6 +51,15 @@ def test_groupby_preserves_subclass(obj, groupby_func):
         tm.assert_series_equal(result1, result2)
 
 
+def test_groupby_preserves_metadata():
+    # GH-37343
+    custom_df = tm.SubclassedDataFrame({"a": [1, 2, 3], "b": [1, 1, 2], "c": [7, 8, 9]})
+    assert "testattr" in custom_df._metadata
+    custom_df.testattr = "hello"
+    for _, group_df in custom_df.groupby("c"):
+        assert group_df.testattr == "hello"
+
+
 @pytest.mark.parametrize(
     "obj", [DataFrame, tm.SubclassedDataFrame],
 )