BUG: handle multi-dimensional grouping better (pandas-dev#36605)

Author: dimithras

pandas/tests/groupby/test_grouping.py

@@ -124,6 +124,20 @@ def test_getitem_single_column(self):
 
         tm.assert_series_equal(result, expected)
 
+    def test_indices_grouped_by_tuple_with_lambda(self):
+        # GH 36158
+        df = DataFrame(
+            {"Tuples": ((x, y) for x in [0, 1] for y in np.random.randint(3, 5, 5))}
+        )
+
+        gb = df.groupby("Tuples")
+        gb_lambda = df.groupby(lambda x: df.iloc[x, 0])
+
+        expected = gb.indices
+        result = gb_lambda.indices
+
+        tm.assert_dict_equal(result, expected)
+
 
 # grouping
 # --------------------------------
@@ -773,6 +787,20 @@ def test_get_group_grouped_by_tuple(self):
         expected = DataFrame({"ids": [(dt[0],), (dt[0],)]}, index=[0, 2])
         tm.assert_frame_equal(result, expected)
 
+    def test_get_group_grouped_by_tuple_with_lambda(self):
+        # GH 36158
+        df = DataFrame(
+            {"Tuples": ((x, y) for x in [0, 1] for y in np.random.randint(3, 5, 5))}
+        )
+
+        gb = df.groupby("Tuples")
+        gb_lambda = df.groupby(lambda x: df.iloc[x, 0])
+
+        expected = gb.get_group(list(gb.groups.keys())[0])
+        result = gb_lambda.get_group(list(gb_lambda.groups.keys())[0])
+
+        tm.assert_frame_equal(result, expected)
+
     def test_groupby_with_empty(self):
         index = pd.DatetimeIndex(())
         data = ()