Improve dictionary map performance on category series, fixes pandas-dev#23785

Author: rtlee9

asv_bench/benchmarks/series_methods.py

@@ -145,16 +145,25 @@ def time_searchsorted(self, dtype):
 
 class Map(object):
 
-    params = ['dict', 'Series']
+    params = (['dict', 'Series', 'lambda'], ['object', 'category', 'int'])
     param_names = 'mapper'
 
-    def setup(self, mapper):
+    def setup(self, mapper, dtype):
         map_size = 1000
-        map_data = Series(map_size - np.arange(map_size))
-        self.map_data = map_data if mapper == 'Series' else map_data.to_dict()
-        self.s = Series(np.random.randint(0, map_size, 10000))
+        map_data = Series(map_size - np.arange(map_size), dtype=dtype)
 
-    def time_map(self, mapper):
+        # construct mapper
+        if mapper == 'Series':
+            self.map_data = map_data
+        elif mapper == 'lambda':
+            self.map_data = map_data.to_dict()
+        else:
+            map_dict = map_data.to_dict()
+            self.map_data = lambda x: map_dict[x]
+
+        self.s = Series(np.random.randint(0, map_size, 10000), dtype=dtype)
+
+    def time_map(self, mapper, *args, **kwargs):
         self.s.map(self.map_data)
 
 

pandas/core/base.py

@@ -19,7 +19,7 @@
 from pandas.core.dtypes.common import (
     is_datetime64_ns_dtype, is_datetime64tz_dtype, is_datetimelike,
     is_extension_array_dtype, is_extension_type, is_list_like, is_object_dtype,
-    is_scalar, is_timedelta64_ns_dtype)
+    is_scalar, is_timedelta64_ns_dtype, is_categorical_dtype)
 from pandas.core.dtypes.generic import ABCDataFrame, ABCIndexClass, ABCSeries
 from pandas.core.dtypes.missing import isna
 
@@ -1201,6 +1201,10 @@ def _map_values(self, mapper, na_action=None):
             else:
                 values = self.values
 
+            # only need to map the categories not all values
+            if is_categorical_dtype(values):
+                return values.map(mapper)
+
             indexer = mapper.index.get_indexer(values)
             new_values = algorithms.take_1d(mapper._values, indexer)