TST: test groupby.apply() with user-defined function returning an empty chunk

Author: toobaz

pandas/tests/groupby/test_apply.py

@@ -518,6 +518,19 @@ def test_func(x):
     tm.assert_frame_equal(result2, expected2)
 
 
+def test_groupby_apply_return_empty_chunk():
+    # GH 22221: apply filter which returns some empty groups
+    df = pd.DataFrame(dict(value=[0, 1], group=['filled', 'empty']))
+    groups = df.groupby('group')
+    result = groups.apply(lambda group: group[group.value != 1]['value'])
+    expected = pd.Series([0], name='value',
+                         index=MultiIndex.from_product([['empty', 'filled'],
+                                                        [0]],
+                                                       names=['group', None]
+                                                       ).drop('empty'))
+    tm.assert_series_equal(result, expected)
+
+
 def test_apply_with_mixed_types():
     # gh-20949
     df = pd.DataFrame({'A': 'a a b'.split(), 'B': [1, 2, 3], 'C': [4, 6, 5]})