TST: Test Groupby with Categoricals consistent for Series and DataFrame results (pandas-dev#29343)

Author: keechongtan

pandas/tests/groupby/test_categorical.py

@@ -1193,6 +1193,13 @@ def test_seriesgroupby_observed_apply_dict(df_cat, observed, index, data):
     tm.assert_series_equal(result, expected)
 
 
+def test_groupby_categorical_series_dataframe_consistent(df_cat):
+    # GH 20416
+    expected = df_cat.groupby(["A", "B"])["C"].mean()
+    result = df_cat.groupby(["A", "B"]).mean()["C"]
+    tm.assert_series_equal(result, expected)
+
+
 @pytest.mark.parametrize("code", [([1, 0, 0]), ([0, 0, 0])])
 def test_groupby_categorical_axis_1(code):
     # GH 13420