REF: Refactor Datetimelike delegation

This is a generalization of PeriodIndex's dispatching to
PeriodArray, without any actual changes yet. This is split
from pandas-dev#24024, where DatetimeIndex and TimedeltaIndex will
implement and inherit from delgates similiar to PeriodDelegateMixin.

Author: TomAugspurger

pandas/core/indexes/datetimelike.py

@@ -2,6 +2,7 @@
 """
 Base and utility classes for tseries type pandas objects.
 """
+import operator
 import warnings
 
 import numpy as np
@@ -22,6 +23,7 @@
 from pandas.core.dtypes.missing import isna
 
 from pandas.core import algorithms, ops
+from pandas.core.accessor import PandasDelegate
 from pandas.core.arrays import PeriodArray
 from pandas.core.arrays.datetimelike import DatetimeLikeArrayMixin
 import pandas.core.indexes.base as ibase
@@ -855,3 +857,48 @@ def f(self):
     f.__name__ = fget.__name__
     f.__doc__ = fget.__doc__
     return property(f)
+
+
+class DatetimelikeDelegateMixin(PandasDelegate):
+    """
+    Delegation mechanism, specific for Datetime, Timedelta, and Period types.
+
+    Functionality is delegated from the Index class to an Array class. A
+    few things can be customized
+
+    * _delegate_class : type
+        The class being delegated to.
+    * _delegated_methods, delegated_properties : List
+        The list of property / method names being delagated.
+    * raw_methods : Set
+        The set of methods whose results should should *not* be
+        boxed in an index, after being returned from the array
+    * raw_properties : Set
+        The set of properties whose results should should *not* be
+        boxed in an index, after being returned from the array
+    """
+    # raw_methods : dispatch methods that shouldn't be boxed in an Index
+    _raw_methods = set()
+    # raw_properties : dispatch properties that shouldn't be boxed in an Index
+    _raw_properties = set()
+    name = None
+    _data = None
+
+    @property
+    def _delegate_class(self):
+        raise AbstractMethodError
+
+    def _delegate_property_get(self, name, *args, **kwargs):
+        result = getattr(self._data, name)
+        if name not in self._raw_properties:
+            result = Index(result, name=self.name)
+        return result
+
+    def _delegate_property_set(self, name, value, *args, **kwargs):
+        setattr(self._data, name, value)
+
+    def _delegate_method(self, name, *args, **kwargs):
+        result = operator.methodcaller(name, *args, **kwargs)(self._data)
+        if name not in self._raw_methods:
+            result = Index(result, name=self.name)
+        return result

pandas/core/indexes/period.py

@@ -1,6 +1,5 @@
 # pylint: disable=E1101,E1103,W0232
 from datetime import datetime, timedelta
-import operator
 import warnings
 
 import numpy as np
@@ -18,15 +17,16 @@
 
 from pandas import compat
 from pandas.core import common as com
-from pandas.core.accessor import PandasDelegate, delegate_names
+from pandas.core.accessor import delegate_names
 from pandas.core.algorithms import unique1d
 import pandas.core.arrays.datetimelike as dtl
 from pandas.core.arrays.period import PeriodArray, period_array
 from pandas.core.base import _shared_docs
 import pandas.core.indexes.base as ibase
 from pandas.core.indexes.base import _index_shared_docs, ensure_index
 from pandas.core.indexes.datetimelike import (
-    DatelikeOps, DatetimeIndexOpsMixin, wrap_arithmetic_op)
+    DatelikeOps, DatetimeIndexOpsMixin, DatetimelikeDelegateMixin,
+    wrap_arithmetic_op)
 from pandas.core.indexes.datetimes import DatetimeIndex, Index, Int64Index
 from pandas.core.missing import isna
 from pandas.core.ops import get_op_result_name
@@ -54,33 +54,26 @@ def _new_PeriodIndex(cls, **d):
         return cls(values, **d)
 
 
-class PeriodDelegateMixin(PandasDelegate):
+class PeriodDelegateMixin(DatetimelikeDelegateMixin):
     """
     Delegate from PeriodIndex to PeriodArray.
     """
-    def _delegate_property_get(self, name, *args, **kwargs):
-        result = getattr(self._data, name)
-        box_ops = (
-            set(PeriodArray._datetimelike_ops) - set(PeriodArray._bool_ops)
-        )
-        if name in box_ops:
-            result = Index(result, name=self.name)
-        return result
-
-    def _delegate_property_set(self, name, value, *args, **kwargs):
-        setattr(self._data, name, value)
-
-    def _delegate_method(self, name, *args, **kwargs):
-        result = operator.methodcaller(name, *args, **kwargs)(self._data)
-        return Index(result, name=self.name)
+    _delegate_class = PeriodArray
+    _delegated_properties = (
+        PeriodArray._datetimelike_ops + ['size', 'asi8', 'shape']
+    )
+    _delegated_methods = (
+        set(PeriodArray._datetimelike_methods) -
+        {'asfreq', 'to_timestamp'} | {'_addsub_int_array'}
+    )
+    _raw_properties = {'is_leap_year'}
 
 
 @delegate_names(PeriodArray,
-                PeriodArray._datetimelike_ops + ['size', 'asi8', 'shape'],
+                PeriodDelegateMixin._delegated_properties,
                 typ='property')
 @delegate_names(PeriodArray,
-                [x for x in PeriodArray._datetimelike_methods
-                 if x not in {"asfreq", "to_timestamp"}],
+                PeriodDelegateMixin._delegated_methods,
                 typ="method",
                 overwrite=True)
 class PeriodIndex(DatelikeOps, DatetimeIndexOpsMixin,