ENH: allow attrs to be propagated via pd.concat

pandas/core/generic.py

@@ -5455,6 +5455,16 @@ 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
+            for obj in other.objs[1:]:
+                if obj.attrs != attrs:
+                    break
+            else:
+                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 +347,6 @@
     (pd.Series, ([1, 2],), operator.methodcaller("convert_dtypes")),
     pytest.param(
         (pd.DataFrame, frame_data, operator.methodcaller("convert_dtypes")),
-        marks=not_implemented_mark,
     ),
     (pd.Series, ([1, None, 3],), operator.methodcaller("interpolate")),
     (pd.DataFrame, ({"A": [1, None, 3]},), operator.methodcaller("interpolate")),
@@ -762,7 +761,7 @@ 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.apply(lambda y: y), Fixed with #42252
         lambda x: x.agg("std"),
         lambda x: x.agg("var"),
         lambda x: x.agg("sem"),

pandas/tests/reshape/concat/test_concat.py

@@ -653,3 +653,30 @@ 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():
+    ''' Retain the attrs during concat
+
+    Only retain the attrs when the attrs are the same across all dataframes.'''
+    d = {'col1': [1, 2], 'col2': [3, 4]}
+    df1 = pd.DataFrame(data=d)
+    df1.attrs = {1: 1}
+    df2 = pd.DataFrame(data=d)
+    df2.attrs = {1: 1}
+    df = pd.concat([df1, df2])
+    assert df.attrs == {1: 1}
+
+
+def test_concat_drop_attrs():
+    '''Discard attrs when they don't match.
+
+    Drop the attrs when the attrs when the attrs are different across
+    all dataframes.'''
+    d = {'col1': [1, 2], 'col2': [3, 4]}
+    df1 = pd.DataFrame(data=d)
+    df1.attrs = {1: 1}
+    df2 = pd.DataFrame(data=d)
+    df2.attrs = {1: 2}
+    df = pd.concat([df1, df2])
+    assert df.attrs == {}