PERF: Override PeriodIndex.unique (#23083)

Author: jbrockmendel

asv_bench/benchmarks/period.py

@@ -119,3 +119,6 @@ def time_align(self):
 
     def time_intersection(self):
         self.index[:750].intersection(self.index[250:])
+
+    def time_unique(self):
+        self.index.unique()

doc/source/whatsnew/v0.24.0.txt

@@ -685,6 +685,7 @@ Performance Improvements
   (:issue:`21372`)
 - Improved the performance of :func:`pandas.get_dummies` with ``sparse=True`` (:issue:`21997`)
 - Improved performance of :func:`IndexEngine.get_indexer_non_unique` for sorted, non-unique indexes (:issue:`9466`)
+- Improved performance of :func:`PeriodIndex.unique` (:issue:`23083`)
 
 
 .. _whatsnew_0240.docs:

pandas/core/indexes/period.py

@@ -29,6 +29,7 @@
                                         DIFFERENT_FREQ_INDEX)
 from pandas._libs.tslibs import resolution, period
 
+from pandas.core.algorithms import unique1d
 from pandas.core.arrays import datetimelike as dtl
 from pandas.core.arrays.period import PeriodArrayMixin, dt64arr_to_periodarr
 from pandas.core.base import _shared_docs
@@ -539,6 +540,18 @@ def _get_unique_index(self, dropna=False):
             res = res.dropna()
         return res
 
+    @Appender(Index.unique.__doc__)
+    def unique(self, level=None):
+        # override the Index.unique method for performance GH#23083
+        if level is not None:
+            # this should never occur, but is retained to make the signature
+            # match Index.unique
+            self._validate_index_level(level)
+
+        values = self._ndarray_values
+        result = unique1d(values)
+        return self._shallow_copy(result)
+
     def get_loc(self, key, method=None, tolerance=None):
         """
         Get integer location for requested label