TST: Group by callable test (#55926)

* add test

* lint

Author: quangngd

pandas/tests/groupby/test_grouping.py

@@ -297,6 +297,23 @@ def test_grouper_creation_bug3(self):
         expected = ser.groupby(level="one").sum()
         tm.assert_series_equal(result, expected)
 
+    @pytest.mark.parametrize("func", [False, True])
+    def test_grouper_returning_tuples(self, func):
+        # GH 22257 , both with dict and with callable
+        df = DataFrame({"X": ["A", "B", "A", "B"], "Y": [1, 4, 3, 2]})
+        mapping = dict(zip(range(4), [("C", 5), ("D", 6)] * 2))
+
+        if func:
+            gb = df.groupby(by=lambda idx: mapping[idx], sort=False)
+        else:
+            gb = df.groupby(by=mapping, sort=False)
+
+        name, expected = next(iter(gb))
+        assert name == ("C", 5)
+        result = gb.get_group(name)
+
+        tm.assert_frame_equal(result, expected)
+
     def test_grouper_column_and_index(self):
         # GH 14327