ENH: allow attrs to be propagated via pd.concat

pandas/core/generic.py

@@ -5522,6 +5522,14 @@ def __finalize__(
                 object.__setattr__(self, name, getattr(other, name, None))
 
         if method == "concat":
+            # Issue #41828, retain the attrs only if all NDFrame have the same
+            # attrs.
+            attrs = other.objs[0].attrs
+            check_attrs = all(objs.attrs == attrs for objs in other.objs[1:])
+            if check_attrs:
+                for name in attrs:
+                    self.attrs[name] = attrs[name]
+
             allows_duplicate_labels = all(
                 x.flags.allows_duplicate_labels for x in other.objs
             )

pandas/tests/generic/test_finalize.py

@@ -347,7 +347,9 @@
         operator.methodcaller("infer_objects"),
     ),
     (pd.Series, ([1, 2],), operator.methodcaller("convert_dtypes")),
-    (pd.DataFrame, frame_data, operator.methodcaller("convert_dtypes")),
+    pytest.param(
+        (pd.DataFrame, frame_data, operator.methodcaller("convert_dtypes")),
+    ),
     (pd.Series, ([1, None, 3],), operator.methodcaller("interpolate")),
     (pd.DataFrame, ({"A": [1, None, 3]},), operator.methodcaller("interpolate")),
     (pd.Series, ([1, 2],), operator.methodcaller("clip", lower=1)),
@@ -759,7 +761,6 @@ def test_groupby_finalize(obj, method):
     [
         lambda x: x.agg(["sum", "count"]),
         lambda x: x.transform(lambda y: y),
-        lambda x: x.apply(lambda y: y),
         lambda x: x.agg("std"),
         lambda x: x.agg("var"),
         lambda x: x.agg("sem"),

pandas/tests/reshape/concat/test_concat.py

@@ -698,3 +698,36 @@ def test_concat_posargs_deprecation():
         result = concat([df, df2], 0)
     expected = DataFrame([[1, 2, 3], [4, 5, 6]], index=["a", "b"])
     tm.assert_frame_equal(result, expected)
+
+
+def test_concat_retain_attrs_df():
+    # GH#41828
+    d = {"col1": [1, 2], "col2": [3, 4]}
+    df1 = DataFrame(data=d)
+    df1.attrs = {1: 1}
+    df2 = DataFrame(data=d)
+    df2.attrs = {1: 1}
+    df = concat([df1, df2])
+    assert df.attrs[1] == 1
+
+
+def test_concat_retain_attrs_series():
+    # GH#41828
+    d = [1, 2]
+    df1 = Series(data=d)
+    df1.attrs = {1: 1}
+    df2 = Series(data=d)
+    df2.attrs = {1: 1}
+    df = concat([df1, df2])
+    assert df.attrs[1] == 1
+
+
+def test_concat_drop_attrs():
+    # GH#41828
+    d = {"col1": [1, 2], "col2": [3, 4]}
+    df1 = DataFrame(data=d)
+    df1.attrs = {1: 1}
+    df2 = DataFrame(data=d)
+    df2.attrs = {1: 2}
+    df = concat([df1, df2])
+    assert len(df.attrs) == 0