adjust imports, docs and move whatsnew

Author: mroeschke

doc/source/whatsnew/v0.21.1.txt

@@ -39,7 +39,7 @@ Deprecations
 Performance Improvements
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
-- Added a keyword argument, `cache`, to :func:`to_datetime` that improved the performance of converting duplicate datetime arguments (:issue: `11665`)
+- 
 -
 -
 

doc/source/whatsnew/v0.22.0.txt

@@ -70,7 +70,7 @@ Performance Improvements
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
 - Indexers on ``Series`` or ``DataFrame`` no longer create a reference cycle (:issue:`17956`)
--
+- Added a keyword argument, ``cache``, to :func:`to_datetime` that improved the performance of converting duplicate datetime arguments (:issue:`11665`)
 -
 
 .. _whatsnew_0220.docs:

pandas/core/tools/datetimes.py

@@ -53,12 +53,10 @@ def _maybe_cache(arg, format, cache, tz, _convert_listlike):
 
 def _convert_and_box_cache(arg, cache_array, box, errors, tz, name=None):
     """Convert array of dates with a cache and box the result"""
-    from pandas import Series
-    from pandas.core.indexes.datetimes import DatetimeIndex
+    from pandas import Series, DatetimeIndex, Index
     result = Series(arg).map(cache_array)
     if box:
         if errors == 'ignore':
-            from pandas import Index
             result = Index(result)
         else:
             result = DatetimeIndex(result, tz=tz, name=name)
@@ -142,11 +140,11 @@ def to_datetime(arg, errors='raise', dayfirst=False, yearfirst=False,
 
         .. versionadded: 0.20.0
     cache : boolean, default False
-        If False, use a cache of unique, converted dates to apply the datetime
+        If True, use a cache of unique, converted dates to apply the datetime
         conversion. May produce sigificant speed-up when parsing duplicate date
         strings, especially ones with timezone offsets.
 
-        .. versionadded: 0.21.1
+        .. versionadded: 0.22.0
     Returns
     -------
     ret : datetime if parsing succeeded.