Ensure the right values are set in SeriesGroupBy.nunique

We only need to use the group boundaries as the index for `res` so that
the dimensions match those of `out`. Fixes pandas-dev#13453

Author: Diego Fernandez

pandas/core/groupby.py

@@ -3032,7 +3032,7 @@ def nunique(self, dropna=True):
         # we might have duplications among the bins
         if len(res) != len(ri):
             res, out = np.zeros(len(ri), dtype=out.dtype), res
-            res[ids] = out
+            res[ids[idx]] = out
 
         return Series(res,
                       index=ri,

pandas/tests/groupby/test_groupby.py

@@ -4159,6 +4159,19 @@ def test_nunique_with_empty_series(self):
         expected = pd.Series(name='name', dtype='int64')
         tm.assert_series_equal(result, expected)
 
+    def test_nunique_with_timegrouper(self):
+        # GH 13453
+        test = pd.DataFrame({
+            'time': [Timestamp('2016-06-28 09:35:35'),
+                     Timestamp('2016-06-28 16:09:30'),
+                     Timestamp('2016-06-28 16:46:28')],
+            'data': ['1', '2', '3']}).set_index('time')
+        result = test.groupby(pd.TimeGrouper(freq='h'))['data'].nunique()
+        expected = test.groupby(
+            pd.TimeGrouper(freq='h')
+        )['data'].apply(pd.Series.nunique)
+        tm.assert_series_equal(result, expected)
+
     def test_numpy_compat(self):
         # see gh-12811
         df = pd.DataFrame({'A': [1, 2, 1], 'B': [1, 2, 3]})