BUG: Error upon Series.Groupby.nunique with empty Series (pandas-dev#12553)

Modified tests

Author: mroeschke

pandas/core/groupby.py

@@ -2898,6 +2898,10 @@ def true_and_notnull(x, *args, **kwargs):
     def nunique(self, dropna=True):
         """ Returns number of unique elements in the group """
         ids, _, _ = self.grouper.group_info
+
+        if len(ids) == 0:
+            return Series(index=self.dtype.index, name=self.name)
+
         val = self.obj.get_values()
 
         try:

pandas/tests/test_groupby.py

@@ -6754,6 +6754,17 @@ def test_nunique_with_object(self):
         expected = pd.Series([1] * 5, name='name', index=index)
         tm.assert_series_equal(result, expected)
 
+    def test_nunique_with_empty_series(self):
+        # GH 12553
+        empty_idxs = [pd.Index([]), pd.Int64Index([]), pd.CategoricalIndex([])
+                      pd.DatetimeIndex([]), pd.TimedeltaIndex([]),
+                      pd.PeriodIndex([], freq='1s')]
+        for empty_idx in empty_idx:
+            data = pd.Series(index=empty_idx, name='name')
+            result = data.groupby(level=0).nunique()
+            expected = pd.Series(index=empty_idx, name='name')
+            tm.assert_series_equal(result, expected)
+
     def test_transform_with_non_scalar_group(self):
         # GH 10165
         cols = pd.MultiIndex.from_tuples([