TST: test groupby cumcount (pandas-dev#42656)

Author: saehuihwang

pandas/tests/groupby/transform/test_transform.py

@@ -1276,3 +1276,16 @@ def test_string_rank_grouping():
     result = df.groupby("A").transform("rank")
     expected = DataFrame({"B": [1.0, 2.0, 1.0]})
     tm.assert_frame_equal(result, expected)
+
+
+def test_transform_cumcount():
+    # GH 27472
+    df = DataFrame({"a": [0, 0, 0, 1, 1, 1], "b": range(6)})
+    grp = df.groupby(np.repeat([0, 1], 3))
+
+    result = grp.cumcount()
+    expected = Series([0, 1, 2, 0, 1, 2])
+    tm.assert_series_equal(result, expected)
+
+    result = grp.transform("cumcount")
+    tm.assert_series_equal(result, expected)