Implement test for GH pandas-dev#21340

Author: Jason Mok

pandas/tests/groupby/test_grouping.py

@@ -823,6 +823,39 @@ def test_groupby_multiindex_level_empty(self):
         tm.assert_frame_equal(result, expected)
 
 
+def test_groupby_tuple_keys_handle_multiindex():
+    # https://github.com/pandas-dev/pandas/issues/21340
+    df = DataFrame(
+        {
+            "num1": [0, 8, 9, 4, 3, 3, 5, 9, 3, 6],
+            "num2": [3, 8, 6, 4, 9, 2, 1, 7, 0, 9],
+            "num3": [6, 5, 7, 8, 5, 1, 1, 10, 7, 8],
+            "category_tuple": [
+                (0, 1),
+                (0, 1),
+                (0, 1),
+                (0, 4),
+                (2, 3),
+                (2, 3),
+                (2, 3),
+                (2, 3),
+                (5,),
+                (6,),
+            ],
+            "category_string": list("aaabbbbcde"),
+        }
+    )
+    df = df[["category_tuple", "category_string", "num1", "num2", "num3"]]
+    expected = df.sort_values(by=["category_tuple", "num1"])
+
+    msg = "DataFrameGroupBy.apply operated on the grouping columns"
+    with tm.assert_produces_warning(DeprecationWarning, match=msg):
+        result = df.groupby("category_tuple").apply(lambda x: x.sort_values(by="num1"))
+    expected = expected[result.columns]
+
+    tm.assert_frame_equal(result.reset_index(drop=True), expected)
+
+
 # get_group
 # --------------------------------