Improve performance of CategoricalIndex.is_unique (pandas-dev#21107)

(cherry picked from commit 9f95f7d)

doc/source/whatsnew/v0.23.1.txt

@@ -30,6 +30,7 @@ Performance Improvements
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
 - Improved performance of :meth:`CategoricalIndex.is_monotonic_increasing`, :meth:`CategoricalIndex.is_monotonic_decreasing` and :meth:`CategoricalIndex.is_monotonic` (:issue:`21025`)
+- Improved performance of :meth:`CategoricalIndex.is_unique` (:issue:`21107`)
 -
 -
 

pandas/core/indexes/category.py

@@ -378,7 +378,7 @@ def _engine(self):
     # introspection
     @cache_readonly
     def is_unique(self):
-        return not self.duplicated().any()
+        return self._engine.is_unique
 
     @property
     def is_monotonic_increasing(self):

pandas/tests/indexes/test_category.py

@@ -581,6 +581,15 @@ def test_is_monotonic(self, data, non_lexsorted_data):
         assert c.is_monotonic_increasing
         assert not c.is_monotonic_decreasing
 
+    @pytest.mark.parametrize('values, expected', [
+        ([1, 2, 3], True),
+        ([1, 3, 1], False),
+        (list('abc'), True),
+        (list('aba'), False)])
+    def test_is_unique(self, values, expected):
+        ci = CategoricalIndex(values)
+        assert ci.is_unique is expected
+
     def test_duplicates(self):
 
         idx = CategoricalIndex([0, 0, 0], name='foo')