Extend unit test with code sample from pandas-dev#25167

Author: kpflugshaupt

pandas/tests/groupby/test_categorical.py

@@ -476,7 +476,16 @@ def test_dataframe_categorical_ordered_observed_sort(ordered, observed, sort):
     aggr = pd.Series(result.array)
     if not observed:
         aggr[aggr.isna()] = 'AWOL'
-    tm.assert_equal(label, aggr)
+    tm.assert_series_equal(label, aggr)
+
+    # GH 25167: Groupby with observed=True doesn't sort
+    df = pd.DataFrame({'A': pd.Categorical(['b', 'a']), 'B': [1, 2]})
+    result = df.groupby('A', observed=True).sum()
+    expected = pd.DataFrame({'B': [2, 1]},
+                            index=pd.CategoricalIndex(['b', 'a'], categories=['a', 'b'],
+                                                      ordered=False, name='A', dtype='category'),
+                            dtype='int')
+    tm.assert_frame_equal(result, expected)
 
 
 def test_datetime():