BUG: Fix unintentionally added suffix in DataFrame.apply/agg and Series.apply/agg

doc/source/whatsnew/v1.1.3.rst

@@ -14,6 +14,7 @@ including other versions of pandas.
 
 Fixed regressions
 ~~~~~~~~~~~~~~~~~
+- Fixed regression in :meth:`DataFrame.agg`, :meth:`DataFrame.apply`, :meth:`Series.agg`, and :meth:`Series.apply` where internal suffix is exposed to the users when no relabelling is applied (:issue:`36189`)
 - Fixed regression in :class:`IntegerArray` unary plus and minus operations raising a ``TypeError`` (:issue:`36063`)
 - Fixed regression in :meth:`Series.__getitem__` incorrectly raising when the input was a tuple (:issue:`35534`)
 - Fixed regression in :meth:`Series.__getitem__` incorrectly raising when the input was a frozenset (:issue:`35747`)

pandas/core/aggregation.py

@@ -63,7 +63,7 @@ def reconstruct_func(
     Examples
     --------
     >>> reconstruct_func(None, **{"foo": ("col", "min")})
-    (True, defaultdict(None, {'col': ['min']}), ('foo',), array([0]))
+    (True, defaultdict(<class 'list'>, {'col': ['min']}), ('foo',), array([0]))
 
     >>> reconstruct_func("min")
     (False, 'min', None, None)
@@ -87,7 +87,6 @@ def reconstruct_func(
 
     if relabeling:
         func, columns, order = normalize_keyword_aggregation(kwargs)
-    func = maybe_mangle_lambdas(func)
 
     return relabeling, func, columns, order
 

pandas/core/groupby/generic.py

@@ -950,6 +950,7 @@ def aggregate(self, func=None, *args, engine=None, engine_kwargs=None, **kwargs)
             return self.obj._constructor(result, index=index, columns=data.columns)
 
         relabeling, func, columns, order = reconstruct_func(func, **kwargs)
+        func = maybe_mangle_lambdas(func)
 
         result, how = self._aggregate(func, *args, **kwargs)
         if how is None:

pandas/tests/frame/apply/test_frame_apply.py

@@ -1519,3 +1519,14 @@ def test_apply_empty_list_reduce():
     result = df.apply(lambda x: [], result_type="reduce")
     expected = pd.Series({"a": [], "b": []}, dtype=object)
     tm.assert_series_equal(result, expected)
+
+
+def test_apply_no_suffix_index():
+    # GH36189
+    pdf = pd.DataFrame([[4, 9]] * 3, columns=["A", "B"])
+    result = pdf.apply(["sum", lambda x: x.sum(), lambda x: x.sum()])
+    expected = pd.DataFrame(
+        {"A": [12, 12, 12], "B": [27, 27, 27]}, index=["sum", "<lambda>", "<lambda>"]
+    )
+
+    tm.assert_frame_equal(result, expected)

pandas/tests/groupby/aggregate/test_aggregate.py

@@ -1153,3 +1153,18 @@ def test_nonagg_agg():
     expected = g.agg("cumsum")
 
     tm.assert_frame_equal(result, expected)
+
+
+def test_agg_no_suffix_index():
+    # GH36189
+    df = pd.DataFrame([[4, 9]] * 3, columns=["A", "B"])
+    result = df.agg(["sum", lambda x: x.sum(), lambda x: x.sum()])
+    expected = pd.DataFrame(
+        {"A": [12, 12, 12], "B": [27, 27, 27]}, index=["sum", "<lambda>", "<lambda>"]
+    )
+    tm.assert_frame_equal(result, expected)
+
+    # test Series case
+    result = df["A"].agg(["sum", lambda x: x.sum(), lambda x: x.sum()])
+    expected = pd.Series([12, 12, 12], index=["sum", "<lambda>", "<lambda>"], name="A")
+    tm.assert_series_equal(result, expected)

pandas/tests/series/apply/test_series_apply.py

@@ -445,6 +445,14 @@ def test_agg_cython_table_raises(self, series, func, expected):
             # e.g. Series('a b'.split()).cumprod() will raise
             series.agg(func)
 
+    def test_series_apply_no_suffix_index(self):
+        # GH36189
+        s = pd.Series([4] * 3)
+        result = s.apply(["sum", lambda x: x.sum(), lambda x: x.sum()])
+        expected = pd.Series([12, 12, 12], index=["sum", "<lambda>", "<lambda>"])
+
+        tm.assert_series_equal(result, expected)
+
 
 class TestSeriesMap:
     def test_map(self, datetime_series):