adding test for #18451 (#35494)

Author: smithto1

pandas/tests/groupby/test_groupby.py

@@ -2055,3 +2055,17 @@ def test_groups_repr_truncates(max_seq_items, expected):
 
         result = df.groupby(np.array(df.a)).groups.__repr__()
         assert result == expected
+
+
+def test_group_on_two_row_multiindex_returns_one_tuple_key():
+    # GH 18451
+    df = pd.DataFrame([{"a": 1, "b": 2, "c": 99}, {"a": 1, "b": 2, "c": 88}])
+    df = df.set_index(["a", "b"])
+
+    grp = df.groupby(["a", "b"])
+    result = grp.indices
+    expected = {(1, 2): np.array([0, 1], dtype=np.int64)}
+
+    assert len(result) == 1
+    key = (1, 2)
+    assert (result[key] == expected[key]).all()