BUG: groupby agg fails silently with mixed dtypes

doc/source/whatsnew/v1.3.3.rst

@@ -17,7 +17,7 @@ Fixed regressions
 - Fixed regression in :class:`DataFrame` constructor failing to broadcast for defined :class:`Index` and len one list of :class:`Timestamp` (:issue:`42810`)
 - Performance regression in :meth:`core.window.ewm.ExponentialMovingWindow.mean` (:issue:`42333`)
 - Fixed regression in :meth:`.GroupBy.agg` incorrectly raising in some cases (:issue:`42390`)
--
+- Fixed regression in :meth:`.GroupBy.agg` where it was failing silently with mixed data types  (:issue:`43209`)
 
 .. ---------------------------------------------------------------------------
 

pandas/core/groupby/generic.py

@@ -41,6 +41,7 @@
     doc,
 )
 
+from pandas.core.dtypes.cast import find_common_type
 from pandas.core.dtypes.common import (
     ensure_int64,
     is_bool,
@@ -1597,7 +1598,8 @@ def _gotitem(self, key, ndim: int, subset=None):
     def _get_data_to_aggregate(self) -> Manager2D:
         obj = self._obj_with_exclusions
         if self.axis == 1:
-            return obj.T._mgr
+            transposed_dtype = find_common_type(obj.dtypes.tolist())
+            return obj.T.astype(transposed_dtype)._mgr
         else:
             return obj._mgr
 

pandas/tests/groupby/test_groupby.py

@@ -2419,3 +2419,17 @@ def test_rolling_wrong_param_min_period():
     result_error_msg = r"__init__\(\) got an unexpected keyword argument 'min_period'"
     with pytest.raises(TypeError, match=result_error_msg):
         test_df.groupby("name")["val"].rolling(window=2, min_period=1).sum()
+
+
+@pytest.mark.parametrize("dtype", [int, "Int64"])
+def test_multiindex_groupby(dtype):
+    # GH#43209
+    df = DataFrame(
+        [[1, 2, 3, 4, 5, 6]] * 3,
+        columns=MultiIndex.from_product([["a", "b"], ["i", "j", "k"]]),
+    ).astype({("a", "j"): dtype, ("b", "j"): dtype})
+    result = df.groupby(level=1, axis=1).sum()
+    expected = DataFrame(
+        [[5, 7, 9], [5, 7, 9], [5, 7, 9]], columns=["i", "j", "k"], dtype=dtype
+    )
+    tm.assert_frame_equal(result, expected)