ENH: add nunique function to count unique elements, GH #297

Author: wesm

pandas/core/series.py

@@ -627,6 +627,16 @@ def value_counts(self):
             counter[value] += 1
         return Series(counter).order(ascending=False)
 
+    def nunique(self):
+        """
+        Return count of unique elements in the Series
+
+        Returns
+        -------
+        nunique : int
+        """
+        return len(self.value_counts())
+
     def sum(self, axis=0, dtype=None, out=None, skipna=True, level=None):
         if level is not None:
             return self._agg_by_level('sum', level=level, skipna=skipna)

pandas/tests/test_series.py

@@ -900,12 +900,14 @@ def test_count(self):
 
         self.assertEqual(self.ts.count(), np.isfinite(self.ts).sum())
 
-    def test_value_counts(self):
+    def test_value_counts_nunique(self):
         s = Series(['a', 'b', 'b', 'b', 'b', 'a', 'c', 'd', 'd', 'a'])
         hist = s.value_counts()
         expected = Series([4, 3, 2, 1], index=['b', 'a', 'd', 'c'])
         assert_series_equal(hist, expected)
 
+        self.assertEquals(s.nunique(), 4)
+
         # handle NA's properly
         s[5:7] = np.nan
         hist = s.value_counts()