TST: Groupby.filter dropna=False with empty group (#10780)

Author: mroeschke

pandas/tests/groupby/test_filters.py

@@ -596,6 +596,14 @@ def test_filter_non_bool_raises(self):
         with tm.assertRaisesRegexp(TypeError, 'filter function returned a.*'):
             df.groupby('a').filter(lambda g: g.c.mean())
 
+    def test_filter_dropna_false_with_empty_groups(self):
+        # GH 10780
+        data = pd.Series(np.random.rand(9), index=np.repeat([1, 2, 3], 3))
+        groupped = data.groupby(level=0)
+        result = groupped.filter(lambda x: x.mean() > 1, dropna=False)
+        expected = pd.Series([np.nan] * 9, index=np.repeat([1, 2, 3], 3))
+        tm.assert_series_equal(result, expected)
+
 
 def assert_fp_equal(a, b):
     assert (np.abs(a - b) < 1e-12).all()