From dc2280fcbbaa7ad53d12424007edb3c1135abffb Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Tue, 16 Oct 2018 19:05:34 -0700
Subject: [PATCH 01/30] Make arithmetic code dispatch less redundant, fix
 datetime64 addition, add missing tests

---
 pandas/core/arrays/datetimelike.py          | 55 ++++++++++--
 pandas/core/arrays/datetimes.py             | 98 +++++++--------------
 pandas/core/arrays/period.py                | 40 ++-------
 pandas/core/arrays/timedeltas.py            | 70 +++++----------
 pandas/tests/arithmetic/test_period.py      | 15 ++++
 pandas/tests/arithmetic/test_timedelta64.py | 12 +++
 6 files changed, 134 insertions(+), 156 deletions(-)

diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index a98b0b3bf35f9..79978c6c4ea62 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -347,21 +347,59 @@ def _validate_frequency(cls, index, freq, **kwargs):
     # Arithmetic Methods
 
     def _add_datelike(self, other):
+        # Overriden by TimedeltaArray
         raise TypeError("cannot add {cls} and {typ}"
                         .format(cls=type(self).__name__,
                                 typ=type(other).__name__))
 
+    _add_datelike_dti = _add_datelike
+
     def _sub_datelike(self, other):
-        raise com.AbstractMethodError(self)
+        # Overridden by DatetimeArray
+        assert other is not NaT
+        raise TypeError("cannot subtract a datelike from a {cls}"
+                        .format(cls=type(self).__name__))
+
+    _sub_datelike_dti = _sub_datelike
 
     def _sub_period(self, other):
-        return NotImplemented
+        # Overriden by PeriodArray
+        raise TypeError("cannot subtract Period from a {cls}"
+                        .format(cls=type(self).__name__))
 
     def _add_offset(self, offset):
         raise com.AbstractMethodError(self)
 
     def _add_delta(self, other):
-        return NotImplemented
+        """
+        Add a timedelta-like, DateOffset, or TimedeltaIndex-like object
+        to self.
+
+        Parameters
+        ----------
+        delta : {timedelta, np.timedelta64, DateOffset,
+                 TimedeltaIndex, ndarray[timedelta64]}
+
+        Returns
+        -------
+        result : same type as self
+
+        Notes
+        -----
+        The result's name is set outside of _add_delta by the calling
+        method (__add__ or __sub__), if necessary (i.e. for Indexes).
+        """
+        # Note: The docstring here says the return type is the same type
+        #   as self, which is inaccurate.  Once wrapped by the inheriting
+        #   Array classes, this will be accurate.
+
+        if isinstance(other, (Tick, timedelta, np.timedelta64)):
+            new_values = self._add_delta_td(other)
+        elif is_timedelta64_dtype(other):
+            # ndarray[timedelta64] or TimedeltaArray/index
+            new_values = self._add_delta_tdi(other)
+
+        return new_values
 
     def _add_delta_td(self, other):
         """
@@ -371,8 +409,7 @@ def _add_delta_td(self, other):
         inc = delta_to_nanoseconds(other)
         new_values = checked_add_with_arr(self.asi8, inc,
                                           arr_mask=self._isnan).view('i8')
-        if self.hasnans:
-            new_values[self._isnan] = iNaT
+        new_values = self._maybe_mask_results(new_values, fill_value=iNaT)
         return new_values.view('i8')
 
     def _add_delta_tdi(self, other):
@@ -380,7 +417,7 @@ def _add_delta_tdi(self, other):
         Add a delta of a TimedeltaIndex
         return the i8 result view
         """
-        if not len(self) == len(other):
+        if len(self) != len(other):
             raise ValueError("cannot add indices of unequal length")
 
         if isinstance(other, np.ndarray):
@@ -441,7 +478,7 @@ def _sub_period_array(self, other):
                             .format(dtype=other.dtype,
                                     cls=type(self).__name__))
 
-        if not len(self) == len(other):
+        if len(self) != len(other):
             raise ValueError("cannot subtract arrays/indices of "
                              "unequal length")
         if self.freq != other.freq:
@@ -635,7 +672,7 @@ def __add__(self, other):
                 result = self._addsub_offset_array(other, operator.add)
             elif is_datetime64_dtype(other) or is_datetime64tz_dtype(other):
                 # DatetimeIndex, ndarray[datetime64]
-                return self._add_datelike(other)
+                return self._add_datelike_dti(other)
             elif is_integer_dtype(other):
                 result = self._addsub_int_array(other, operator.add)
             elif is_float_dtype(other):
@@ -695,7 +732,7 @@ def __sub__(self, other):
                 result = self._addsub_offset_array(other, operator.sub)
             elif is_datetime64_dtype(other) or is_datetime64tz_dtype(other):
                 # DatetimeIndex, ndarray[datetime64]
-                result = self._sub_datelike(other)
+                result = self._sub_datelike_dti(other)
             elif is_period_dtype(other):
                 # PeriodIndex
                 result = self._sub_period_array(other)
diff --git a/pandas/core/arrays/datetimes.py b/pandas/core/arrays/datetimes.py
index 4cc33d7afd6c8..0f6b8a4f62178 100644
--- a/pandas/core/arrays/datetimes.py
+++ b/pandas/core/arrays/datetimes.py
@@ -1,5 +1,5 @@
 # -*- coding: utf-8 -*-
-from datetime import datetime, timedelta, time
+from datetime import datetime, time
 import warnings
 
 import numpy as np
@@ -12,7 +12,7 @@
     conversion, fields, timezones,
     resolution as libresolution)
 
-from pandas.util._decorators import cache_readonly
+from pandas.util._decorators import cache_readonly, Appender
 from pandas.errors import PerformanceWarning
 from pandas import compat
 
@@ -21,7 +21,6 @@
     is_object_dtype,
     is_datetime64tz_dtype,
     is_datetime64_dtype,
-    is_timedelta64_dtype,
     ensure_int64)
 from pandas.core.dtypes.dtypes import DatetimeTZDtype
 from pandas.core.dtypes.missing import isna
@@ -425,10 +424,20 @@ def _assert_tzawareness_compat(self, other):
     # Arithmetic Methods
 
     def _sub_datelike_dti(self, other):
-        """subtraction of two DatetimeIndexes"""
-        if not len(self) == len(other):
+        """subtract DatetimeArray/Index or ndarray[datetime64]"""
+        if len(self) != len(other):
             raise ValueError("cannot add indices of unequal length")
 
+        if isinstance(other, np.ndarray):
+            # if other is an ndarray, we assume it is datetime64-dtype
+            other = type(self)(other)
+
+        if not self._has_same_tz(other):
+            # require tz compat
+            raise TypeError("{cls} subtraction must have the same "
+                            "timezones or no timezones"
+                            .format(cls=type(self).__name__))
+
         self_i8 = self.asi8
         other_i8 = other.asi8
         new_values = checked_add_with_arr(self_i8, -other_i8,
@@ -458,72 +467,27 @@ def _add_offset(self, offset):
 
     def _sub_datelike(self, other):
         # subtract a datetime from myself, yielding a ndarray[timedelta64[ns]]
-        if isinstance(other, (DatetimeArrayMixin, np.ndarray)):
-            if isinstance(other, np.ndarray):
-                # if other is an ndarray, we assume it is datetime64-dtype
-                other = type(self)(other)
-            if not self._has_same_tz(other):
-                # require tz compat
-                raise TypeError("{cls} subtraction must have the same "
-                                "timezones or no timezones"
-                                .format(cls=type(self).__name__))
-            result = self._sub_datelike_dti(other)
-        elif isinstance(other, (datetime, np.datetime64)):
-            assert other is not NaT
-            other = Timestamp(other)
-            if other is NaT:
-                return self - NaT
+        assert isinstance(other, (datetime, np.datetime64))
+        assert other is not NaT
+        other = Timestamp(other)
+        if other is NaT:
+            return self - NaT
+
+        if not self._has_same_tz(other):
             # require tz compat
-            elif not self._has_same_tz(other):
-                raise TypeError("Timestamp subtraction must have the same "
-                                "timezones or no timezones")
-            else:
-                i8 = self.asi8
-                result = checked_add_with_arr(i8, -other.value,
-                                              arr_mask=self._isnan)
-                result = self._maybe_mask_results(result,
-                                                  fill_value=iNaT)
-        else:
-            raise TypeError("cannot subtract {cls} and {typ}"
-                            .format(cls=type(self).__name__,
-                                    typ=type(other).__name__))
+            raise TypeError("Timestamp subtraction must have the same "
+                            "timezones or no timezones")
+
+        i8 = self.asi8
+        result = checked_add_with_arr(i8, -other.value,
+                                      arr_mask=self._isnan)
+        result = self._maybe_mask_results(result, fill_value=iNaT)
         return result.view('timedelta64[ns]')
 
+    @Appender(dtl.DatetimeLikeArrayMixin._add_delta.__doc__)
     def _add_delta(self, delta):
-        """
-        Add a timedelta-like, DateOffset, or TimedeltaIndex-like object
-        to self.
-
-        Parameters
-        ----------
-        delta : {timedelta, np.timedelta64, DateOffset,
-                 TimedeltaIndex, ndarray[timedelta64]}
-
-        Returns
-        -------
-        result : same type as self
-
-        Notes
-        -----
-        The result's name is set outside of _add_delta by the calling
-        method (__add__ or __sub__)
-        """
-        from pandas.core.arrays.timedeltas import TimedeltaArrayMixin
-
-        if isinstance(delta, (Tick, timedelta, np.timedelta64)):
-            new_values = self._add_delta_td(delta)
-        elif is_timedelta64_dtype(delta):
-            if not isinstance(delta, TimedeltaArrayMixin):
-                delta = TimedeltaArrayMixin(delta)
-            new_values = self._add_delta_tdi(delta)
-        else:
-            new_values = self.astype('O') + delta
-
-        tz = 'UTC' if self.tz is not None else None
-        result = type(self)(new_values, tz=tz, freq='infer')
-        if self.tz is not None and self.tz is not utc:
-            result = result.tz_convert(self.tz)
-        return result
+        new_values = dtl.DatetimeLikeArrayMixin._add_delta(self, delta)
+        return type(self)(new_values, tz=self.tz, freq='infer')
 
     # -----------------------------------------------------------------
     # Timezone Conversion and Localization Methods
diff --git a/pandas/core/arrays/period.py b/pandas/core/arrays/period.py
index 8624ddd8965e8..79671ee8aedbd 100644
--- a/pandas/core/arrays/period.py
+++ b/pandas/core/arrays/period.py
@@ -15,10 +15,10 @@
 from pandas._libs.tslibs.fields import isleapyear_arr
 
 from pandas import compat
-from pandas.util._decorators import (cache_readonly, deprecate_kwarg)
+from pandas.util._decorators import cache_readonly, deprecate_kwarg, Appender
 
 from pandas.core.dtypes.common import (
-    is_integer_dtype, is_float_dtype, is_period_dtype, is_timedelta64_dtype,
+    is_integer_dtype, is_float_dtype, is_period_dtype,
     is_datetime64_dtype, _TD_DTYPE)
 from pandas.core.dtypes.dtypes import PeriodDtype
 from pandas.core.dtypes.generic import ABCSeries
@@ -334,10 +334,6 @@ def to_timestamp(self, freq=None, how='start'):
 
     _create_comparison_method = classmethod(_period_array_cmp)
 
-    def _sub_datelike(self, other):
-        assert other is not NaT
-        return NotImplemented
-
     def _sub_period(self, other):
         # If the operation is well-defined, we return an object-Index
         # of DateOffsets.  Null entries are filled with pd.NaT
@@ -349,9 +345,7 @@ def _sub_period(self, other):
         new_data = asi8 - other.ordinal
         new_data = np.array([self.freq * x for x in new_data])
 
-        if self.hasnans:
-            new_data[self._isnan] = NaT
-
+        new_data = self._maybe_mask_results(new_data, fill_value=NaT)
         return new_data
 
     def _add_offset(self, other):
@@ -379,20 +373,8 @@ def _add_delta_tdi(self, other):
         delta = self._check_timedeltalike_freq_compat(other)
         return self._addsub_int_array(delta, operator.add)
 
+    @Appender(DatetimeLikeArrayMixin._add_delta.__doc__)
     def _add_delta(self, other):
-        """
-        Add a timedelta-like, Tick, or TimedeltaIndex-like object
-        to self.
-
-        Parameters
-        ----------
-        other : {timedelta, np.timedelta64, Tick,
-                 TimedeltaIndex, ndarray[timedelta64]}
-
-        Returns
-        -------
-        result : same type as self
-        """
         if not isinstance(self.freq, Tick):
             # We cannot add timedelta-like to non-tick PeriodArray
             raise IncompatibleFrequency("Input has different freq from "
@@ -400,17 +382,9 @@ def _add_delta(self, other):
                                         .format(cls=type(self).__name__,
                                                 freqstr=self.freqstr))
 
-        # TODO: standardize across datetimelike subclasses whether to return
-        #  i8 view or _shallow_copy
-        if isinstance(other, (Tick, timedelta, np.timedelta64)):
-            new_values = self._add_delta_td(other)
-            return self._shallow_copy(new_values)
-        elif is_timedelta64_dtype(other):
-            # ndarray[timedelta64] or TimedeltaArray/index
-            new_values = self._add_delta_tdi(other)
-            return self._shallow_copy(new_values)
-        else:  # pragma: no cover
-            raise TypeError(type(other).__name__)
+        new_values = DatetimeLikeArrayMixin._add_delta(self, other)
+
+        return self._shallow_copy(new_values)
 
     @deprecate_kwarg(old_arg_name='n', new_arg_name='periods')
     def shift(self, periods):
diff --git a/pandas/core/arrays/timedeltas.py b/pandas/core/arrays/timedeltas.py
index 4904a90ab7b2b..a774c3fc2fb14 100644
--- a/pandas/core/arrays/timedeltas.py
+++ b/pandas/core/arrays/timedeltas.py
@@ -8,6 +8,7 @@
 from pandas._libs.tslibs.fields import get_timedelta_field
 from pandas._libs.tslibs.timedeltas import array_to_timedelta64
 
+from pandas.util._decorators import Appender
 from pandas import compat
 
 from pandas.core.dtypes.common import (
@@ -191,60 +192,35 @@ def _add_offset(self, other):
                         .format(typ=type(other).__name__,
                                 cls=type(self).__name__))
 
-    def _sub_datelike(self, other):
-        assert other is not NaT
-        raise TypeError("cannot subtract a datelike from a {cls}"
-                        .format(cls=type(self).__name__))
-
+    @Appender(dtl.DatetimeLikeArrayMixin._add_delta.__doc__)
     def _add_delta(self, delta):
-        """
-        Add a timedelta-like, Tick, or TimedeltaIndex-like object
-        to self.
-
-        Parameters
-        ----------
-        delta : timedelta, np.timedelta64, Tick, TimedeltaArray, TimedeltaIndex
-
-        Returns
-        -------
-        result : same type as self
+        new_values = dtl.DatetimeLikeArrayMixin._add_delta(self, delta)
+        return type(self)(new_values, freq='infer')
 
-        Notes
-        -----
-        The result's name is set outside of _add_delta by the calling
-        method (__add__ or __sub__)
-        """
-        if isinstance(delta, (Tick, timedelta, np.timedelta64)):
-            new_values = self._add_delta_td(delta)
-        elif isinstance(delta, TimedeltaArrayMixin):
-            new_values = self._add_delta_tdi(delta)
-        elif is_timedelta64_dtype(delta):
-            # ndarray[timedelta64] --> wrap in TimedeltaArray/Index
-            delta = type(self)(delta)
-            new_values = self._add_delta_tdi(delta)
-        else:
-            raise TypeError("cannot add the type {0} to a TimedeltaIndex"
-                            .format(type(delta)))
+    def _add_datelike_dti(self, other):
+        """Add DatetimeArray/Index or ndarray[datetime64] to TimedeltaArray"""
+        if isinstance(other, np.ndarray):
+            # At this point we have already checked that dtype is datetime64
+            from pandas.core.arrays import DatetimeArrayMixin
+            other = DatetimeArrayMixin(other)
 
-        return type(self)(new_values, freq='infer')
+        # defer to implementation in DatetimeArray
+        return other + self
 
     def _add_datelike(self, other):
         # adding a timedeltaindex to a datetimelike
         from pandas.core.arrays import DatetimeArrayMixin
-        if isinstance(other, (DatetimeArrayMixin, np.ndarray)):
-            # if other is an ndarray, we assume it is datetime64-dtype
-            # defer to implementation in DatetimeIndex
-            if not isinstance(other, DatetimeArrayMixin):
-                other = DatetimeArrayMixin(other)
-            return other + self
-        else:
-            assert other is not NaT
-            other = Timestamp(other)
-            i8 = self.asi8
-            result = checked_add_with_arr(i8, other.value,
-                                          arr_mask=self._isnan)
-            result = self._maybe_mask_results(result, fill_value=iNaT)
-            return DatetimeArrayMixin(result)
+
+        assert other is not NaT
+        other = Timestamp(other)
+        if other is NaT:
+            return self + NaT
+
+        i8 = self.asi8
+        result = checked_add_with_arr(i8, other.value,
+                                      arr_mask=self._isnan)
+        result = self._maybe_mask_results(result, fill_value=iNaT)
+        return DatetimeArrayMixin(result)
 
     def _addsub_offset_array(self, other, op):
         # Add or subtract Array-like of DateOffset objects
diff --git a/pandas/tests/arithmetic/test_period.py b/pandas/tests/arithmetic/test_period.py
index fe98b74499983..d564b29fdff80 100644
--- a/pandas/tests/arithmetic/test_period.py
+++ b/pandas/tests/arithmetic/test_period.py
@@ -411,6 +411,21 @@ def test_pi_add_sub_float(self, op, other):
         with pytest.raises(TypeError):
             op(pi, other)
 
+    @pytest.mark.parametrize('other', [pd.Timestamp.now(),
+                                       pd.Timestamp.now().to_pydatetime(),
+                                       pd.Timestamp.now().to_datetime64()])
+    def test_pi_add_sub_datetime(self, other):
+        rng = pd.period_range('1/1/2000', freq='D', periods=3)
+
+        with pytest.raises(TypeError):
+            rng + other
+        with pytest.raises(TypeError):
+            other + rng
+        with pytest.raises(TypeError):
+            rng - other
+        with pytest.raises(TypeError):
+            other - rng
+
     # -----------------------------------------------------------------
     # __add__/__sub__ with ndarray[datetime64] and ndarray[timedelta64]
 
diff --git a/pandas/tests/arithmetic/test_timedelta64.py b/pandas/tests/arithmetic/test_timedelta64.py
index fa1a2d9df9a58..aae7ae4b7490b 100644
--- a/pandas/tests/arithmetic/test_timedelta64.py
+++ b/pandas/tests/arithmetic/test_timedelta64.py
@@ -460,6 +460,18 @@ def test_td64arr_add_sub_timestamp(self, box):
         with pytest.raises(TypeError):
             tdser - ts
 
+    def test_td64arr_add_datetime64_nat(self, box):
+        other = np.datetime64('NaT')
+
+        tdi = timedelta_range('1 day', periods=3)
+        expected = pd.DatetimeIndex(["NaT", "NaT", "NaT"])
+
+        tdser = tm.box_expected(tdi, box)
+        expected = tm.box_expected(expected, box)
+
+        tm.assert_equal(other + tdser, expected)
+        tm.assert_equal(tdser + other, expected)
+
     def test_tdi_sub_dt64_array(self, box_df_broadcast_failure):
         box = box_df_broadcast_failure
 

From 37728ff55ed9a380bc70ec76c3a1c4db9f3879e2 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Tue, 16 Oct 2018 19:13:47 -0700
Subject: [PATCH 02/30] remove unnecessary nat_new

---
 pandas/core/arrays/datetimelike.py          | 25 +++------------------
 pandas/tests/indexes/datetimes/test_ops.py  | 10 ---------
 pandas/tests/indexes/period/test_ops.py     | 11 ---------
 pandas/tests/indexes/timedeltas/test_ops.py | 11 ---------
 4 files changed, 3 insertions(+), 54 deletions(-)

diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index 79978c6c4ea62..00f7ff9e4f0cf 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -246,27 +246,6 @@ def _maybe_mask_results(self, result, fill_value=None, convert=None):
             result[self._isnan] = fill_value
         return result
 
-    def _nat_new(self, box=True):
-        """
-        Return Array/Index or ndarray filled with NaT which has the same
-        length as the caller.
-
-        Parameters
-        ----------
-        box : boolean, default True
-            - If True returns a Array/Index as the same as caller.
-            - If False returns ndarray of np.int64.
-        """
-        result = np.zeros(len(self), dtype=np.int64)
-        result.fill(iNaT)
-        if not box:
-            return result
-
-        attribs = self._get_attributes_dict()
-        if not is_period_dtype(self):
-            attribs['freq'] = None
-        return self._simple_new(result, **attribs)
-
     # ------------------------------------------------------------------
     # Frequency Properties/Methods
 
@@ -444,7 +423,9 @@ def _add_nat(self):
 
         # GH#19124 pd.NaT is treated like a timedelta for both timedelta
         # and datetime dtypes
-        return self._nat_new(box=True)
+        result = np.zeros(len(self), dtype=np.int64)
+        result.fill(iNaT)
+        return self._shallow_copy(result, freq=None)
 
     def _sub_nat(self):
         """Subtract pd.NaT from self"""
diff --git a/pandas/tests/indexes/datetimes/test_ops.py b/pandas/tests/indexes/datetimes/test_ops.py
index 9ce77326d37b7..771165eafec7a 100644
--- a/pandas/tests/indexes/datetimes/test_ops.py
+++ b/pandas/tests/indexes/datetimes/test_ops.py
@@ -337,16 +337,6 @@ def test_infer_freq(self, freq):
         tm.assert_index_equal(idx, result)
         assert result.freq == freq
 
-    def test_nat_new(self):
-        idx = pd.date_range('2011-01-01', freq='D', periods=5, name='x')
-        result = idx._nat_new()
-        exp = pd.DatetimeIndex([pd.NaT] * 5, name='x')
-        tm.assert_index_equal(result, exp)
-
-        result = idx._nat_new(box=False)
-        exp = np.array([tslib.iNaT] * 5, dtype=np.int64)
-        tm.assert_numpy_array_equal(result, exp)
-
     def test_nat(self, tz_naive_fixture):
         tz = tz_naive_fixture
         assert pd.DatetimeIndex._na_value is pd.NaT
diff --git a/pandas/tests/indexes/period/test_ops.py b/pandas/tests/indexes/period/test_ops.py
index a59efe57f83c4..5e95fcfdd6c3e 100644
--- a/pandas/tests/indexes/period/test_ops.py
+++ b/pandas/tests/indexes/period/test_ops.py
@@ -312,17 +312,6 @@ def test_order(self):
             tm.assert_numpy_array_equal(indexer, exp, check_dtype=False)
             assert ordered.freq == 'D'
 
-    def test_nat_new(self):
-
-        idx = pd.period_range('2011-01', freq='M', periods=5, name='x')
-        result = idx._nat_new()
-        exp = pd.PeriodIndex([pd.NaT] * 5, freq='M', name='x')
-        tm.assert_index_equal(result, exp)
-
-        result = idx._nat_new(box=False)
-        exp = np.array([tslib.iNaT] * 5, dtype=np.int64)
-        tm.assert_numpy_array_equal(result, exp)
-
     def test_shift(self):
         # This is tested in test_arithmetic
         pass
diff --git a/pandas/tests/indexes/timedeltas/test_ops.py b/pandas/tests/indexes/timedeltas/test_ops.py
index 9f8a3e893c3de..8af32f1f9421b 100644
--- a/pandas/tests/indexes/timedeltas/test_ops.py
+++ b/pandas/tests/indexes/timedeltas/test_ops.py
@@ -236,17 +236,6 @@ def test_infer_freq(self, freq):
         tm.assert_index_equal(idx, result)
         assert result.freq == freq
 
-    def test_nat_new(self):
-
-        idx = pd.timedelta_range('1', freq='D', periods=5, name='x')
-        result = idx._nat_new()
-        exp = pd.TimedeltaIndex([pd.NaT] * 5, name='x')
-        tm.assert_index_equal(result, exp)
-
-        result = idx._nat_new(box=False)
-        exp = np.array([iNaT] * 5, dtype=np.int64)
-        tm.assert_numpy_array_equal(result, exp)
-
     def test_shift(self):
         pass  # handled in test_arithmetic.py
 

From 15ad0a62e178039b8c687ebb0d60e99db39e15ed Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Tue, 16 Oct 2018 20:38:40 -0700
Subject: [PATCH 03/30] Fix interpretation of NaT

---
 pandas/core/arrays/timedeltas.py            | 5 ++++-
 pandas/tests/arithmetic/test_timedelta64.py | 2 +-
 pandas/util/testing.py                      | 1 +
 3 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/pandas/core/arrays/timedeltas.py b/pandas/core/arrays/timedeltas.py
index a774c3fc2fb14..45c6e94f742be 100644
--- a/pandas/core/arrays/timedeltas.py
+++ b/pandas/core/arrays/timedeltas.py
@@ -214,7 +214,10 @@ def _add_datelike(self, other):
         assert other is not NaT
         other = Timestamp(other)
         if other is NaT:
-            return self + NaT
+            # In this case we specifically interpret NaT as a datetime, not
+            # the timedelta interpretation we would get by returning self + NaT
+            result = self.asi8.view('m8[ms]') + NaT.to_datetime64()
+            return DatetimeArrayMixin(result)
 
         i8 = self.asi8
         result = checked_add_with_arr(i8, other.value,
diff --git a/pandas/tests/arithmetic/test_timedelta64.py b/pandas/tests/arithmetic/test_timedelta64.py
index aae7ae4b7490b..5bba98890b917 100644
--- a/pandas/tests/arithmetic/test_timedelta64.py
+++ b/pandas/tests/arithmetic/test_timedelta64.py
@@ -469,8 +469,8 @@ def test_td64arr_add_datetime64_nat(self, box):
         tdser = tm.box_expected(tdi, box)
         expected = tm.box_expected(expected, box)
 
-        tm.assert_equal(other + tdser, expected)
         tm.assert_equal(tdser + other, expected)
+        tm.assert_equal(other + tdser, expected)
 
     def test_tdi_sub_dt64_array(self, box_df_broadcast_failure):
         box = box_df_broadcast_failure
diff --git a/pandas/util/testing.py b/pandas/util/testing.py
index 1bd9043f42634..82be5edc73b55 100644
--- a/pandas/util/testing.py
+++ b/pandas/util/testing.py
@@ -934,6 +934,7 @@ def assert_attr_equal(attr, left, right, obj='Attributes'):
         Specify object name being compared, internally used to show appropriate
         assertion message
     """
+    __tracebackhide__ = True
 
     left_attr = getattr(left, attr)
     right_attr = getattr(right, attr)

From 94f174582e49424e20d6b105583aa024248fe8c4 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Wed, 17 Oct 2018 15:29:02 -0700
Subject: [PATCH 04/30] move test

---
 pandas/tests/arithmetic/test_timedelta64.py | 24 ++++++++++-----------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/pandas/tests/arithmetic/test_timedelta64.py b/pandas/tests/arithmetic/test_timedelta64.py
index 5bba98890b917..5fcddd7661f5c 100644
--- a/pandas/tests/arithmetic/test_timedelta64.py
+++ b/pandas/tests/arithmetic/test_timedelta64.py
@@ -460,18 +460,6 @@ def test_td64arr_add_sub_timestamp(self, box):
         with pytest.raises(TypeError):
             tdser - ts
 
-    def test_td64arr_add_datetime64_nat(self, box):
-        other = np.datetime64('NaT')
-
-        tdi = timedelta_range('1 day', periods=3)
-        expected = pd.DatetimeIndex(["NaT", "NaT", "NaT"])
-
-        tdser = tm.box_expected(tdi, box)
-        expected = tm.box_expected(expected, box)
-
-        tm.assert_equal(tdser + other, expected)
-        tm.assert_equal(other + tdser, expected)
-
     def test_tdi_sub_dt64_array(self, box_df_broadcast_failure):
         box = box_df_broadcast_failure
 
@@ -783,6 +771,18 @@ def test_td64arr_sub_timedeltalike(self, two_hours, box):
         result = rng - two_hours
         tm.assert_equal(result, expected)
 
+    def test_td64arr_add_datetime64_nat(self, box):
+        other = np.datetime64('NaT')
+
+        tdi = timedelta_range('1 day', periods=3)
+        expected = pd.DatetimeIndex(["NaT", "NaT", "NaT"])
+
+        tdser = tm.box_expected(tdi, box)
+        expected = tm.box_expected(expected, box)
+
+        tm.assert_equal(tdser + other, expected)
+        tm.assert_equal(other + tdser, expected)
+
     # ------------------------------------------------------------------
     # __add__/__sub__ with DateOffsets and arrays of DateOffsets
 

From 3bdf1047fd328d4a3c638972531d9a6f474535b6 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Wed, 17 Oct 2018 16:02:08 -0700
Subject: [PATCH 05/30] whatsnew, fix dropped timezone

---
 doc/source/whatsnew/v0.24.0.txt             |  3 +-
 pandas/core/arrays/timedeltas.py            |  2 +-
 pandas/tests/arithmetic/test_timedelta64.py | 45 +++++++++------------
 3 files changed, 23 insertions(+), 27 deletions(-)

diff --git a/doc/source/whatsnew/v0.24.0.txt b/doc/source/whatsnew/v0.24.0.txt
index 16f0b9ee99909..5f99434658b7c 100644
--- a/doc/source/whatsnew/v0.24.0.txt
+++ b/doc/source/whatsnew/v0.24.0.txt
@@ -825,7 +825,8 @@ Timedelta
 - Bug in :class:`TimedeltaIndex` incorrectly allowing indexing with ``Timestamp`` object (:issue:`20464`)
 - Fixed bug where subtracting :class:`Timedelta` from an object-dtyped array would raise ``TypeError`` (:issue:`21980`)
 - Fixed bug in adding a :class:`DataFrame` with all-`timedelta64[ns]` dtypes to a :class:`DataFrame` with all-integer dtypes returning incorrect results instead of raising ``TypeError`` (:issue:`22696`)
--
+- Bug in :class:`TimedeltaIndex` where adding a timezone-aware datetime scalar incorrectly returned a timezone-naive :class:`DatetimeIndex` (:issue:`?????`)
+- Bug in :class:`TimedeltaIndex` where adding ``np.timedelta64('NaT')`` incorrectly returned an all-`NaT` :class:`DatetimeIndex` instead of an all-`NaT` :class:`TimedeltaIndex` (:issue:`?????`)
 
 Timezones
 ^^^^^^^^^
diff --git a/pandas/core/arrays/timedeltas.py b/pandas/core/arrays/timedeltas.py
index 45c6e94f742be..b38cf017da9a5 100644
--- a/pandas/core/arrays/timedeltas.py
+++ b/pandas/core/arrays/timedeltas.py
@@ -223,7 +223,7 @@ def _add_datelike(self, other):
         result = checked_add_with_arr(i8, other.value,
                                       arr_mask=self._isnan)
         result = self._maybe_mask_results(result, fill_value=iNaT)
-        return DatetimeArrayMixin(result)
+        return DatetimeArrayMixin(result, tz=other.tz)  # FIXME: what about timezone?
 
     def _addsub_offset_array(self, other, op):
         # Add or subtract Array-like of DateOffset objects
diff --git a/pandas/tests/arithmetic/test_timedelta64.py b/pandas/tests/arithmetic/test_timedelta64.py
index 5fcddd7661f5c..acc434a5a2176 100644
--- a/pandas/tests/arithmetic/test_timedelta64.py
+++ b/pandas/tests/arithmetic/test_timedelta64.py
@@ -415,25 +415,20 @@ def test_td64arr_sub_timestamp_raises(self, box):
         with tm.assert_raises_regex(TypeError, msg):
             idx - Timestamp('2011-01-01')
 
-    def test_td64arr_add_timestamp(self, box):
-        idx = TimedeltaIndex(['1 day', '2 day'])
-        expected = DatetimeIndex(['2011-01-02', '2011-01-03'])
-
-        idx = tm.box_expected(idx, box)
-        expected = tm.box_expected(expected, box)
-
-        result = idx + Timestamp('2011-01-01')
-        tm.assert_equal(result, expected)
+    def test_td64arr_add_timestamp(self, box, tz_naive_fixture):
+        # TODO: parametrize over scalar datetime types?
+        tz = tz_naive_fixture
+        other = Timestamp('2011-01-01', tz=tz)
 
-    def test_td64_radd_timestamp(self, box):
         idx = TimedeltaIndex(['1 day', '2 day'])
-        expected = DatetimeIndex(['2011-01-02', '2011-01-03'])
+        expected = DatetimeIndex(['2011-01-02', '2011-01-03'], tz=tz)
 
         idx = tm.box_expected(idx, box)
         expected = tm.box_expected(expected, box)
 
-        # TODO: parametrize over scalar datetime types?
-        result = Timestamp('2011-01-01') + idx
+        result = idx + other
+        tm.assert_equal(result, expected)
+        result = other + idx
         tm.assert_equal(result, expected)
 
     def test_td64arr_add_sub_timestamp(self, box):
@@ -494,6 +489,18 @@ def test_tdi_add_dt64_array(self, box_df_broadcast_failure):
         result = dtarr + tdi
         tm.assert_equal(result, expected)
 
+    def test_td64arr_add_datetime64_nat(self, box):
+        other = np.datetime64('NaT')
+
+        tdi = timedelta_range('1 day', periods=3)
+        expected = pd.DatetimeIndex(["NaT", "NaT", "NaT"])
+
+        tdser = tm.box_expected(tdi, box)
+        expected = tm.box_expected(expected, box)
+
+        tm.assert_equal(tdser + other, expected)
+        tm.assert_equal(other + tdser, expected)
+
     # ------------------------------------------------------------------
     # Operations with int-like others
 
@@ -771,18 +778,6 @@ def test_td64arr_sub_timedeltalike(self, two_hours, box):
         result = rng - two_hours
         tm.assert_equal(result, expected)
 
-    def test_td64arr_add_datetime64_nat(self, box):
-        other = np.datetime64('NaT')
-
-        tdi = timedelta_range('1 day', periods=3)
-        expected = pd.DatetimeIndex(["NaT", "NaT", "NaT"])
-
-        tdser = tm.box_expected(tdi, box)
-        expected = tm.box_expected(expected, box)
-
-        tm.assert_equal(tdser + other, expected)
-        tm.assert_equal(other + tdser, expected)
-
     # ------------------------------------------------------------------
     # __add__/__sub__ with DateOffsets and arrays of DateOffsets
 

From 982ea3081094f10dc982714307db13a56dd050e3 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Wed, 17 Oct 2018 16:07:23 -0700
Subject: [PATCH 06/30] remove comment

---
 pandas/core/arrays/timedeltas.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/core/arrays/timedeltas.py b/pandas/core/arrays/timedeltas.py
index b38cf017da9a5..3824945b2988f 100644
--- a/pandas/core/arrays/timedeltas.py
+++ b/pandas/core/arrays/timedeltas.py
@@ -223,7 +223,7 @@ def _add_datelike(self, other):
         result = checked_add_with_arr(i8, other.value,
                                       arr_mask=self._isnan)
         result = self._maybe_mask_results(result, fill_value=iNaT)
-        return DatetimeArrayMixin(result, tz=other.tz)  # FIXME: what about timezone?
+        return DatetimeArrayMixin(result, tz=other.tz)
 
     def _addsub_offset_array(self, other, op):
         # Add or subtract Array-like of DateOffset objects

From d046038b40f9a72131f9830e2e823ee7b6e606d3 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Wed, 17 Oct 2018 16:11:46 -0700
Subject: [PATCH 07/30] Add GH references

---
 doc/source/whatsnew/v0.24.0.txt             | 4 ++--
 pandas/tests/arithmetic/test_period.py      | 1 +
 pandas/tests/arithmetic/test_timedelta64.py | 2 ++
 3 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/doc/source/whatsnew/v0.24.0.txt b/doc/source/whatsnew/v0.24.0.txt
index 5f99434658b7c..94bc075265e45 100644
--- a/doc/source/whatsnew/v0.24.0.txt
+++ b/doc/source/whatsnew/v0.24.0.txt
@@ -825,8 +825,8 @@ Timedelta
 - Bug in :class:`TimedeltaIndex` incorrectly allowing indexing with ``Timestamp`` object (:issue:`20464`)
 - Fixed bug where subtracting :class:`Timedelta` from an object-dtyped array would raise ``TypeError`` (:issue:`21980`)
 - Fixed bug in adding a :class:`DataFrame` with all-`timedelta64[ns]` dtypes to a :class:`DataFrame` with all-integer dtypes returning incorrect results instead of raising ``TypeError`` (:issue:`22696`)
-- Bug in :class:`TimedeltaIndex` where adding a timezone-aware datetime scalar incorrectly returned a timezone-naive :class:`DatetimeIndex` (:issue:`?????`)
-- Bug in :class:`TimedeltaIndex` where adding ``np.timedelta64('NaT')`` incorrectly returned an all-`NaT` :class:`DatetimeIndex` instead of an all-`NaT` :class:`TimedeltaIndex` (:issue:`?????`)
+- Bug in :class:`TimedeltaIndex` where adding a timezone-aware datetime scalar incorrectly returned a timezone-naive :class:`DatetimeIndex` (:issue:`23215`)
+- Bug in :class:`TimedeltaIndex` where adding ``np.timedelta64('NaT')`` incorrectly returned an all-`NaT` :class:`DatetimeIndex` instead of an all-`NaT` :class:`TimedeltaIndex` (:issue:`23215`)
 
 Timezones
 ^^^^^^^^^
diff --git a/pandas/tests/arithmetic/test_period.py b/pandas/tests/arithmetic/test_period.py
index d564b29fdff80..ba9c73795bc27 100644
--- a/pandas/tests/arithmetic/test_period.py
+++ b/pandas/tests/arithmetic/test_period.py
@@ -415,6 +415,7 @@ def test_pi_add_sub_float(self, op, other):
                                        pd.Timestamp.now().to_pydatetime(),
                                        pd.Timestamp.now().to_datetime64()])
     def test_pi_add_sub_datetime(self, other):
+        # GH#23215
         rng = pd.period_range('1/1/2000', freq='D', periods=3)
 
         with pytest.raises(TypeError):
diff --git a/pandas/tests/arithmetic/test_timedelta64.py b/pandas/tests/arithmetic/test_timedelta64.py
index acc434a5a2176..795f44cdc91d0 100644
--- a/pandas/tests/arithmetic/test_timedelta64.py
+++ b/pandas/tests/arithmetic/test_timedelta64.py
@@ -416,6 +416,7 @@ def test_td64arr_sub_timestamp_raises(self, box):
             idx - Timestamp('2011-01-01')
 
     def test_td64arr_add_timestamp(self, box, tz_naive_fixture):
+        # GH#23215
         # TODO: parametrize over scalar datetime types?
         tz = tz_naive_fixture
         other = Timestamp('2011-01-01', tz=tz)
@@ -490,6 +491,7 @@ def test_tdi_add_dt64_array(self, box_df_broadcast_failure):
         tm.assert_equal(result, expected)
 
     def test_td64arr_add_datetime64_nat(self, box):
+        # GH#23215
         other = np.datetime64('NaT')
 
         tdi = timedelta_range('1 day', periods=3)

From a0c1a851391b5d756435e3be49ed62514620b396 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Wed, 17 Oct 2018 18:28:27 -0700
Subject: [PATCH 08/30] remove unused imports

---
 pandas/tests/indexes/datetimes/test_ops.py  | 1 -
 pandas/tests/indexes/timedeltas/test_ops.py | 1 -
 2 files changed, 2 deletions(-)

diff --git a/pandas/tests/indexes/datetimes/test_ops.py b/pandas/tests/indexes/datetimes/test_ops.py
index 771165eafec7a..c27207388fd6b 100644
--- a/pandas/tests/indexes/datetimes/test_ops.py
+++ b/pandas/tests/indexes/datetimes/test_ops.py
@@ -4,7 +4,6 @@
 from datetime import datetime
 
 import pandas as pd
-import pandas._libs.tslib as tslib
 import pandas.util.testing as tm
 from pandas import (DatetimeIndex, PeriodIndex, Series, Timestamp,
                     date_range, bdate_range, Index)
diff --git a/pandas/tests/indexes/timedeltas/test_ops.py b/pandas/tests/indexes/timedeltas/test_ops.py
index 8af32f1f9421b..16d70d79c217e 100644
--- a/pandas/tests/indexes/timedeltas/test_ops.py
+++ b/pandas/tests/indexes/timedeltas/test_ops.py
@@ -7,7 +7,6 @@
 import pandas.util.testing as tm
 from pandas import (Series, Timedelta, Timestamp, TimedeltaIndex,
                     timedelta_range, to_timedelta)
-from pandas._libs.tslib import iNaT
 from pandas.tests.test_base import Ops
 from pandas.tseries.offsets import Day, Hour
 from pandas.core.dtypes.generic import ABCDateOffset

From b932121998f208776b3171b8d1ce713b65b6b074 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Thu, 18 Oct 2018 15:20:29 -0700
Subject: [PATCH 09/30] dummy commit to force CI


From 9f3b18d118d549a9343759d1ad59992207437aaf Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Thu, 18 Oct 2018 19:23:36 -0700
Subject: [PATCH 10/30] Fix bug in adding DateOffset to PeriodIndex, Series,
 Frame

---
 doc/source/whatsnew/v0.24.0.txt        |  3 ++-
 pandas/_libs/tslibs/offsets.pyx        |  4 ++--
 pandas/core/arrays/period.py           |  7 +++++-
 pandas/tests/arithmetic/test_period.py | 33 ++++++++++++++++++++++++++
 4 files changed, 43 insertions(+), 4 deletions(-)

diff --git a/doc/source/whatsnew/v0.24.0.txt b/doc/source/whatsnew/v0.24.0.txt
index 2dd6076f67b89..55243fb6e049f 100644
--- a/doc/source/whatsnew/v0.24.0.txt
+++ b/doc/source/whatsnew/v0.24.0.txt
@@ -821,6 +821,7 @@ Datetimelike
 - Bug in rounding methods of :class:`DatetimeIndex` (:meth:`~DatetimeIndex.round`, :meth:`~DatetimeIndex.ceil`, :meth:`~DatetimeIndex.floor`) and :class:`Timestamp` (:meth:`~Timestamp.round`, :meth:`~Timestamp.ceil`, :meth:`~Timestamp.floor`) could give rise to loss of precision (:issue:`22591`)
 - Bug in :func:`to_datetime` with an :class:`Index` argument that would drop the ``name`` from the result (:issue:`21697`)
 - Bug in :class:`PeriodIndex` where adding or subtracting a :class:`timedelta` or :class:`Tick` object produced incorrect results (:issue:`22988`)
+- Bug in :class:`PeriodIndex` with attribute ``freq.n`` greater than 1 where adding a :class:`DateOffset` object would return incorrect results (:issue:`23215`)
 
 Timedelta
 ^^^^^^^^^
@@ -863,7 +864,7 @@ Offsets
 
 - Bug in :class:`FY5253` where date offsets could incorrectly raise an ``AssertionError`` in arithmetic operatons (:issue:`14774`)
 - Bug in :class:`DateOffset` where keyword arguments ``week`` and ``milliseconds`` were accepted and ignored.  Passing these will now raise ``ValueError`` (:issue:`19398`)
--
+- Bug in adding :class:`DateOffset` with :class:`DataFrame` or :class:`PeriodIndex` incorrectly raising ``TypeError`` (:issue:`23215`)
 
 Numeric
 ^^^^^^^
diff --git a/pandas/_libs/tslibs/offsets.pyx b/pandas/_libs/tslibs/offsets.pyx
index 393c2cdba8568..5baacfe5f725f 100644
--- a/pandas/_libs/tslibs/offsets.pyx
+++ b/pandas/_libs/tslibs/offsets.pyx
@@ -344,8 +344,8 @@ class _BaseOffset(object):
         return {name: kwds[name] for name in kwds if kwds[name] is not None}
 
     def __add__(self, other):
-        if getattr(other, "_typ", None) in ["datetimeindex",
-                                            "series", "period"]:
+        if getattr(other, "_typ", None) in ["datetimeindex", "periodindex",
+                                            "series", "period", "dataframe"]:
             # defer to the other class's implementation
             return other + self
         try:
diff --git a/pandas/core/arrays/period.py b/pandas/core/arrays/period.py
index 8f840a82c8c8f..1d7b57801dad2 100644
--- a/pandas/core/arrays/period.py
+++ b/pandas/core/arrays/period.py
@@ -354,7 +354,12 @@ def _add_offset(self, other):
         if base != self.freq.rule_code:
             msg = DIFFERENT_FREQ_INDEX.format(self.freqstr, other.freqstr)
             raise IncompatibleFrequency(msg)
-        return self._time_shift(other.n)
+
+        # Note: when calling parent class's _add_delta_td, it will call
+        #  delta_to_nanoseconds(delta).  Because delta here is an integer,
+        #  delta_to_nanoseconds will return it unchanged.
+        result = DatetimeLikeArrayMixin._add_delta_td(self, other.n)
+        return self._shallow_copy(result)
 
     def _add_delta_td(self, other):
         assert isinstance(self.freq, Tick)  # checked by calling function
diff --git a/pandas/tests/arithmetic/test_period.py b/pandas/tests/arithmetic/test_period.py
index ba9c73795bc27..516386c9f92d6 100644
--- a/pandas/tests/arithmetic/test_period.py
+++ b/pandas/tests/arithmetic/test_period.py
@@ -16,6 +16,7 @@
 import pandas.core.indexes.period as period
 from pandas.core import ops
 from pandas import Period, PeriodIndex, period_range, Series
+from pandas.tseries.frequencies import to_offset
 
 
 # ------------------------------------------------------------------
@@ -591,6 +592,38 @@ def test_pi_sub_isub_offset(self):
         rng -= pd.offsets.MonthEnd(5)
         tm.assert_index_equal(rng, expected)
 
+    def test_pi_add_offset_n_gt1(self, box):
+        # GH#23215
+        # add offset to PeriodIndex with freq.n > 1
+        per = pd.Period('2016-01', freq='2M')
+        pi = pd.PeriodIndex([per])
+
+        expected = pd.PeriodIndex(['2016-03'], freq='2M')
+        pi = tm.box_expected(pi, box)
+        expected = tm.box_expected(expected, box)
+
+        result = pi + per.freq
+        tm.assert_equal(result, expected)
+
+        result = per.freq + pi
+        tm.assert_equal(result, expected)
+
+    def test_pi_add_offset_n_gt1_not_divisible(self, box):
+        # GH#23215
+        # PeriodIndex with freq.n > 1 add offset with offset.n % freq.n != 0
+
+        pi = pd.PeriodIndex(['2016-01'], freq='2M')
+        pi = tm.box_expected(pi, box)
+
+        expected = pd.PeriodIndex(['2016-04'], freq='2M')
+        expected = tm.box_expected(expected, box)
+
+        result = pi + to_offset('3M')
+        tm.assert_equal(result, expected)
+
+        result = to_offset('3M') + pi
+        tm.assert_equal(result, expected)
+
     # ---------------------------------------------------------------
     # __add__/__sub__ with integer arrays
 

From 7a8232e8ca0a441353194114498057e4e91bea8b Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Thu, 18 Oct 2018 22:09:01 -0700
Subject: [PATCH 11/30] Dummy commit to force CI

---
 pandas/core/arrays/datetimelike.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index 00f7ff9e4f0cf..ecfd91f4a9a97 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -351,12 +351,12 @@ def _add_offset(self, offset):
 
     def _add_delta(self, other):
         """
-        Add a timedelta-like, DateOffset, or TimedeltaIndex-like object
+        Add a timedelta-like, Tick or TimedeltaIndex-like object
         to self.
 
         Parameters
         ----------
-        delta : {timedelta, np.timedelta64, DateOffset,
+        delta : {timedelta, np.timedelta64, Tick,
                  TimedeltaIndex, ndarray[timedelta64]}
 
         Returns

From a743f74a4d11cc1713c323246229e96f0ca70697 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Fri, 19 Oct 2018 09:16:48 -0700
Subject: [PATCH 12/30] revert tracebackhide

---
 pandas/util/testing.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/pandas/util/testing.py b/pandas/util/testing.py
index 82be5edc73b55..1bd9043f42634 100644
--- a/pandas/util/testing.py
+++ b/pandas/util/testing.py
@@ -934,7 +934,6 @@ def assert_attr_equal(attr, left, right, obj='Attributes'):
         Specify object name being compared, internally used to show appropriate
         assertion message
     """
-    __tracebackhide__ = True
 
     left_attr = getattr(left, attr)
     right_attr = getattr(right, attr)

From af4872ea9b472684d76621e6779bd6b3104af8cc Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Fri, 19 Oct 2018 14:24:31 -0700
Subject: [PATCH 13/30] comment and reversion

---
 pandas/core/arrays/datetimelike.py |  2 +-
 pandas/core/arrays/datetimes.py    | 10 +++++++++-
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index ecfd91f4a9a97..0727bceaa6f7b 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -352,7 +352,7 @@ def _add_offset(self, offset):
     def _add_delta(self, other):
         """
         Add a timedelta-like, Tick or TimedeltaIndex-like object
-        to self.
+        to self, yielding another array of the same type
 
         Parameters
         ----------
diff --git a/pandas/core/arrays/datetimes.py b/pandas/core/arrays/datetimes.py
index 786354258aa6e..cf98d2043dfc7 100644
--- a/pandas/core/arrays/datetimes.py
+++ b/pandas/core/arrays/datetimes.py
@@ -487,7 +487,15 @@ def _sub_datelike(self, other):
     @Appender(dtl.DatetimeLikeArrayMixin._add_delta.__doc__)
     def _add_delta(self, delta):
         new_values = dtl.DatetimeLikeArrayMixin._add_delta(self, delta)
-        return type(self)(new_values, tz=self.tz, freq='infer')
+
+        # Note: this construction is _not_ equivalent to
+        #  type(self)(new_values, tz=self.tz, freq'infer'')
+        #  see GH#23215
+        tz = 'UTC' if self.tz is not None else None
+        result = type(self)(new_values, tz=tz, freq='infer')
+        if self.tz is not None and self.tz is not utc:
+            result = result.tz_convert(self.tz)
+        return result
 
     # -----------------------------------------------------------------
     # Timezone Conversion and Localization Methods

From 6707032f9773e4897a284725f5c76b1901dcf25a Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Sat, 20 Oct 2018 09:41:28 -0700
Subject: [PATCH 14/30] revert to simpler version

---
 pandas/core/arrays/datetimes.py | 10 +---------
 1 file changed, 1 insertion(+), 9 deletions(-)

diff --git a/pandas/core/arrays/datetimes.py b/pandas/core/arrays/datetimes.py
index cf98d2043dfc7..786354258aa6e 100644
--- a/pandas/core/arrays/datetimes.py
+++ b/pandas/core/arrays/datetimes.py
@@ -487,15 +487,7 @@ def _sub_datelike(self, other):
     @Appender(dtl.DatetimeLikeArrayMixin._add_delta.__doc__)
     def _add_delta(self, delta):
         new_values = dtl.DatetimeLikeArrayMixin._add_delta(self, delta)
-
-        # Note: this construction is _not_ equivalent to
-        #  type(self)(new_values, tz=self.tz, freq'infer'')
-        #  see GH#23215
-        tz = 'UTC' if self.tz is not None else None
-        result = type(self)(new_values, tz=tz, freq='infer')
-        if self.tz is not None and self.tz is not utc:
-            result = result.tz_convert(self.tz)
-        return result
+        return type(self)(new_values, tz=self.tz, freq='infer')
 
     # -----------------------------------------------------------------
     # Timezone Conversion and Localization Methods

From 18ef26d59282736515b8d246fe2ff534e011c8a4 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Sat, 20 Oct 2018 09:45:01 -0700
Subject: [PATCH 15/30] Move overriding of addsub_int_array to PeriodArray

---
 pandas/core/arrays/datetimelike.py | 14 +++-----------
 pandas/core/arrays/period.py       | 12 ++++++++++++
 2 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index 0727bceaa6f7b..fda33755f5afe 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -491,18 +491,10 @@ def _addsub_int_array(self, other, op):
         -------
         result : same class as self
         """
+        assert not is_period_dtype(self)  # overriden by PeriodArray
         assert op in [operator.add, operator.sub]
-        if is_period_dtype(self):
-            # easy case for PeriodIndex
-            if op is operator.sub:
-                other = -other
-            res_values = checked_add_with_arr(self.asi8, other,
-                                              arr_mask=self._isnan)
-            res_values = res_values.view('i8')
-            res_values[self._isnan] = iNaT
-            return self._from_ordinals(res_values, freq=self.freq)
-
-        elif self.freq is None:
+
+        if self.freq is None:
             # GH#19123
             raise NullFrequencyError("Cannot shift with no freq")
 
diff --git a/pandas/core/arrays/period.py b/pandas/core/arrays/period.py
index 1d7b57801dad2..e2c90c80aa480 100644
--- a/pandas/core/arrays/period.py
+++ b/pandas/core/arrays/period.py
@@ -24,6 +24,7 @@
 from pandas.core.dtypes.generic import ABCSeries
 
 import pandas.core.common as com
+from pandas.core.algorithms import checked_add_with_arr
 
 from pandas.tseries import frequencies
 from pandas.tseries.offsets import Tick, DateOffset
@@ -334,6 +335,17 @@ def to_timestamp(self, freq=None, how='start'):
 
     _create_comparison_method = classmethod(_period_array_cmp)
 
+    @Appender(DatetimeLikeArrayMixin._addsub_int_array.__doc__)
+    def _addsub_int_array(self, other, op):
+        assert op in [operator.add, operator.sub]
+        if op is operator.sub:
+            other = -other
+        res_values = checked_add_with_arr(self.asi8, other,
+                                          arr_mask=self._isnan)
+        res_values = res_values.view('i8')
+        res_values[self._isnan] = iNaT
+        return self._simple_new(res_values, freq=self.freq)
+
     def _sub_period(self, other):
         # If the operation is well-defined, we return an object-Index
         # of DateOffsets.  Null entries are filled with pd.NaT

From 937242390b042d56a6fcc73f942eff625b1f6e2e Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Mon, 22 Oct 2018 19:14:43 -0700
Subject: [PATCH 16/30] correct docstrings

---
 pandas/core/arrays/datetimelike.py | 6 +-----
 pandas/core/arrays/datetimes.py    | 3 ++-
 pandas/core/arrays/period.py       | 3 ++-
 pandas/core/arrays/timedeltas.py   | 3 ++-
 4 files changed, 7 insertions(+), 8 deletions(-)

diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index fda33755f5afe..ec982dee0c70d 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -361,17 +361,13 @@ def _add_delta(self, other):
 
         Returns
         -------
-        result : same type as self
+        result : ndarray[int64]
 
         Notes
         -----
         The result's name is set outside of _add_delta by the calling
         method (__add__ or __sub__), if necessary (i.e. for Indexes).
         """
-        # Note: The docstring here says the return type is the same type
-        #   as self, which is inaccurate.  Once wrapped by the inheriting
-        #   Array classes, this will be accurate.
-
         if isinstance(other, (Tick, timedelta, np.timedelta64)):
             new_values = self._add_delta_td(other)
         elif is_timedelta64_dtype(other):
diff --git a/pandas/core/arrays/datetimes.py b/pandas/core/arrays/datetimes.py
index 6e7fc10d529be..9b862ce472d21 100644
--- a/pandas/core/arrays/datetimes.py
+++ b/pandas/core/arrays/datetimes.py
@@ -484,7 +484,8 @@ def _sub_datelike(self, other):
         result = self._maybe_mask_results(result, fill_value=iNaT)
         return result.view('timedelta64[ns]')
 
-    @Appender(dtl.DatetimeLikeArrayMixin._add_delta.__doc__)
+    @Appender(dtl.DatetimeLikeArrayMixin._add_delta.__doc__.replace(
+        "ndarray[int64]", "same type as self"))
     def _add_delta(self, delta):
         new_values = dtl.DatetimeLikeArrayMixin._add_delta(self, delta)
         return type(self)(new_values, tz=self.tz, freq='infer')
diff --git a/pandas/core/arrays/period.py b/pandas/core/arrays/period.py
index e2c90c80aa480..f9d4396281c4d 100644
--- a/pandas/core/arrays/period.py
+++ b/pandas/core/arrays/period.py
@@ -390,7 +390,8 @@ def _add_delta_tdi(self, other):
         delta = self._check_timedeltalike_freq_compat(other)
         return self._addsub_int_array(delta, operator.add)
 
-    @Appender(DatetimeLikeArrayMixin._add_delta.__doc__)
+    @Appender(DatetimeLikeArrayMixin._add_delta.__doc__.replace(
+        "ndarray[int64]", "same type as self"))
     def _add_delta(self, other):
         if not isinstance(self.freq, Tick):
             # We cannot add timedelta-like to non-tick PeriodArray
diff --git a/pandas/core/arrays/timedeltas.py b/pandas/core/arrays/timedeltas.py
index 3824945b2988f..3a57f0a4d9fdb 100644
--- a/pandas/core/arrays/timedeltas.py
+++ b/pandas/core/arrays/timedeltas.py
@@ -192,7 +192,8 @@ def _add_offset(self, other):
                         .format(typ=type(other).__name__,
                                 cls=type(self).__name__))
 
-    @Appender(dtl.DatetimeLikeArrayMixin._add_delta.__doc__)
+    @Appender(dtl.DatetimeLikeArrayMixin._add_delta.__doc__.replace(
+        "ndarray[int64]", "same type as self"))
     def _add_delta(self, delta):
         new_values = dtl.DatetimeLikeArrayMixin._add_delta(self, delta)
         return type(self)(new_values, freq='infer')

From 2a6268e5834d0f72d659de919448927efe8d7319 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Mon, 22 Oct 2018 19:19:56 -0700
Subject: [PATCH 17/30] comments and assertions

---
 pandas/core/arrays/datetimelike.py | 3 ++-
 pandas/core/arrays/datetimes.py    | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index ec982dee0c70d..5dd4ca6145e6e 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -487,7 +487,8 @@ def _addsub_int_array(self, other, op):
         -------
         result : same class as self
         """
-        assert not is_period_dtype(self)  # overriden by PeriodArray
+        # _addsub_int_array is overriden by PeriodArray
+        assert not is_period_dtype(self)
         assert op in [operator.add, operator.sub]
 
         if self.freq is None:
diff --git a/pandas/core/arrays/datetimes.py b/pandas/core/arrays/datetimes.py
index 9b862ce472d21..fc4cdf631a9fb 100644
--- a/pandas/core/arrays/datetimes.py
+++ b/pandas/core/arrays/datetimes.py
@@ -429,7 +429,7 @@ def _sub_datelike_dti(self, other):
             raise ValueError("cannot add indices of unequal length")
 
         if isinstance(other, np.ndarray):
-            # if other is an ndarray, we assume it is datetime64-dtype
+            assert is_datetime64_dtype(other)
             other = type(self)(other)
 
         if not self._has_same_tz(other):

From 777f4d90d735248518f8024ab2d7cbd72dd7f039 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Mon, 22 Oct 2018 19:23:23 -0700
Subject: [PATCH 18/30] More explicit names for array/scalar add/sub methods

---
 pandas/core/arrays/datetimelike.py | 16 ++++++++--------
 pandas/core/arrays/datetimes.py    |  4 ++--
 pandas/core/arrays/timedeltas.py   |  4 ++--
 3 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index 5dd4ca6145e6e..d50f964d793a4 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -325,21 +325,21 @@ def _validate_frequency(cls, index, freq, **kwargs):
     # ------------------------------------------------------------------
     # Arithmetic Methods
 
-    def _add_datelike(self, other):
+    def _add_datetimelike_scalar(self, other):
         # Overriden by TimedeltaArray
         raise TypeError("cannot add {cls} and {typ}"
                         .format(cls=type(self).__name__,
                                 typ=type(other).__name__))
 
-    _add_datelike_dti = _add_datelike
+    _add_datetime_arraylike = _add_datetimelike_scalar
 
-    def _sub_datelike(self, other):
+    def _sub_datetimelike_scalar(self, other):
         # Overridden by DatetimeArray
         assert other is not NaT
         raise TypeError("cannot subtract a datelike from a {cls}"
                         .format(cls=type(self).__name__))
 
-    _sub_datelike_dti = _sub_datelike
+    _sub_datetime_arraylike = _sub_datetimelike_scalar
 
     def _sub_period(self, other):
         # Overriden by PeriodArray
@@ -627,7 +627,7 @@ def __add__(self, other):
                 # specifically _not_ a Tick
                 result = self._add_offset(other)
             elif isinstance(other, (datetime, np.datetime64)):
-                result = self._add_datelike(other)
+                result = self._add_datetimelike_scalar(other)
             elif lib.is_integer(other):
                 # This check must come after the check for np.timedelta64
                 # as is_integer returns True for these
@@ -642,7 +642,7 @@ def __add__(self, other):
                 result = self._addsub_offset_array(other, operator.add)
             elif is_datetime64_dtype(other) or is_datetime64tz_dtype(other):
                 # DatetimeIndex, ndarray[datetime64]
-                return self._add_datelike_dti(other)
+                return self._add_datetime_arraylike(other)
             elif is_integer_dtype(other):
                 result = self._addsub_int_array(other, operator.add)
             elif is_float_dtype(other):
@@ -685,7 +685,7 @@ def __sub__(self, other):
                 # specifically _not_ a Tick
                 result = self._add_offset(-other)
             elif isinstance(other, (datetime, np.datetime64)):
-                result = self._sub_datelike(other)
+                result = self._sub_datetimelike_scalar(other)
             elif lib.is_integer(other):
                 # This check must come after the check for np.timedelta64
                 # as is_integer returns True for these
@@ -702,7 +702,7 @@ def __sub__(self, other):
                 result = self._addsub_offset_array(other, operator.sub)
             elif is_datetime64_dtype(other) or is_datetime64tz_dtype(other):
                 # DatetimeIndex, ndarray[datetime64]
-                result = self._sub_datelike_dti(other)
+                result = self._sub_datetime_arraylike(other)
             elif is_period_dtype(other):
                 # PeriodIndex
                 result = self._sub_period_array(other)
diff --git a/pandas/core/arrays/datetimes.py b/pandas/core/arrays/datetimes.py
index fc4cdf631a9fb..fe372d475b7de 100644
--- a/pandas/core/arrays/datetimes.py
+++ b/pandas/core/arrays/datetimes.py
@@ -423,7 +423,7 @@ def _assert_tzawareness_compat(self, other):
     # -----------------------------------------------------------------
     # Arithmetic Methods
 
-    def _sub_datelike_dti(self, other):
+    def _sub_datetime_arraylike(self, other):
         """subtract DatetimeArray/Index or ndarray[datetime64]"""
         if len(self) != len(other):
             raise ValueError("cannot add indices of unequal length")
@@ -465,7 +465,7 @@ def _add_offset(self, offset):
 
         return type(self)(result, freq='infer')
 
-    def _sub_datelike(self, other):
+    def _sub_datetimelike_scalar(self, other):
         # subtract a datetime from myself, yielding a ndarray[timedelta64[ns]]
         assert isinstance(other, (datetime, np.datetime64))
         assert other is not NaT
diff --git a/pandas/core/arrays/timedeltas.py b/pandas/core/arrays/timedeltas.py
index 3a57f0a4d9fdb..a4b81d362b92b 100644
--- a/pandas/core/arrays/timedeltas.py
+++ b/pandas/core/arrays/timedeltas.py
@@ -198,7 +198,7 @@ def _add_delta(self, delta):
         new_values = dtl.DatetimeLikeArrayMixin._add_delta(self, delta)
         return type(self)(new_values, freq='infer')
 
-    def _add_datelike_dti(self, other):
+    def _add_datetime_arraylike(self, other):
         """Add DatetimeArray/Index or ndarray[datetime64] to TimedeltaArray"""
         if isinstance(other, np.ndarray):
             # At this point we have already checked that dtype is datetime64
@@ -208,7 +208,7 @@ def _add_datelike_dti(self, other):
         # defer to implementation in DatetimeArray
         return other + self
 
-    def _add_datelike(self, other):
+    def _add_datetimelike_scalar(self, other):
         # adding a timedeltaindex to a datetimelike
         from pandas.core.arrays import DatetimeArrayMixin
 

From 5f231b287a7aa692b05ccbf29bf9b37c20ebb5aa Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Mon, 22 Oct 2018 20:04:51 -0700
Subject: [PATCH 19/30] oo optimization fixup

---
 pandas/core/arrays/datetimes.py  | 2 +-
 pandas/core/arrays/period.py     | 2 +-
 pandas/core/arrays/timedeltas.py | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/pandas/core/arrays/datetimes.py b/pandas/core/arrays/datetimes.py
index 5fd40979c53b6..19cbbf53cec9d 100644
--- a/pandas/core/arrays/datetimes.py
+++ b/pandas/core/arrays/datetimes.py
@@ -484,7 +484,7 @@ def _sub_datetimelike_scalar(self, other):
         result = self._maybe_mask_results(result, fill_value=iNaT)
         return result.view('timedelta64[ns]')
 
-    @Appender(dtl.DatetimeLikeArrayMixin._add_delta.__doc__.replace(
+    @Appender((dtl.DatetimeLikeArrayMixin._add_delta.__doc__ or "").replace(
         "ndarray[int64]", "same type as self"))
     def _add_delta(self, delta):
         new_values = dtl.DatetimeLikeArrayMixin._add_delta(self, delta)
diff --git a/pandas/core/arrays/period.py b/pandas/core/arrays/period.py
index f9d4396281c4d..41184a58c2396 100644
--- a/pandas/core/arrays/period.py
+++ b/pandas/core/arrays/period.py
@@ -390,7 +390,7 @@ def _add_delta_tdi(self, other):
         delta = self._check_timedeltalike_freq_compat(other)
         return self._addsub_int_array(delta, operator.add)
 
-    @Appender(DatetimeLikeArrayMixin._add_delta.__doc__.replace(
+    @Appender((DatetimeLikeArrayMixin._add_delta.__doc__ or "").replace(
         "ndarray[int64]", "same type as self"))
     def _add_delta(self, other):
         if not isinstance(self.freq, Tick):
diff --git a/pandas/core/arrays/timedeltas.py b/pandas/core/arrays/timedeltas.py
index 49a67ca6cbe56..985e4e42f7fe9 100644
--- a/pandas/core/arrays/timedeltas.py
+++ b/pandas/core/arrays/timedeltas.py
@@ -192,7 +192,7 @@ def _add_offset(self, other):
                         .format(typ=type(other).__name__,
                                 cls=type(self).__name__))
 
-    @Appender(dtl.DatetimeLikeArrayMixin._add_delta.__doc__.replace(
+    @Appender((dtl.DatetimeLikeArrayMixin._add_delta.__doc__ or "").replace(
         "ndarray[int64]", "same type as self"))
     def _add_delta(self, delta):
         new_values = dtl.DatetimeLikeArrayMixin._add_delta(self, delta)

From d799d8e46257b4e7d9fdae1ad9da203ea6c474e8 Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Wed, 24 Oct 2018 22:15:28 -0700
Subject: [PATCH 20/30] Dummy commit to force CI

---
 pandas/tests/indexes/datetimes/test_ops.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/tests/indexes/datetimes/test_ops.py b/pandas/tests/indexes/datetimes/test_ops.py
index c27207388fd6b..4db5001b196d8 100644
--- a/pandas/tests/indexes/datetimes/test_ops.py
+++ b/pandas/tests/indexes/datetimes/test_ops.py
@@ -33,7 +33,7 @@ def test_ops_properties(self):
     def test_ops_properties_basic(self):
 
         # sanity check that the behavior didn't change
-        # GH7206
+        # GH#7206
         for op in ['year', 'day', 'second', 'weekday']:
             pytest.raises(TypeError, lambda x: getattr(self.dt_series, op))
 

From 8cf614b692edf54f03f6fea2ae5a6713bfe7c464 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Thu, 25 Oct 2018 08:34:42 -0700
Subject: [PATCH 21/30] change default fill_value for maybe_mask_results

---
 pandas/core/arrays/datetimelike.py |  4 ++--
 pandas/core/arrays/datetimes.py    | 11 ++++++-----
 pandas/core/arrays/timedeltas.py   | 10 ++++++----
 pandas/core/indexes/timedeltas.py  |  3 ++-
 4 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index 526796acd1dbf..f38f3b9b2c151 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -221,7 +221,7 @@ def hasnans(self):
         """ return if I have any nans; enables various perf speedups """
         return self._isnan.any()
 
-    def _maybe_mask_results(self, result, fill_value=None, convert=None):
+    def _maybe_mask_results(self, result, fill_value=iNaT, convert=None):
         """
         Parameters
         ----------
@@ -384,7 +384,7 @@ def _add_delta_td(self, other):
         inc = delta_to_nanoseconds(other)
         new_values = checked_add_with_arr(self.asi8, inc,
                                           arr_mask=self._isnan).view('i8')
-        new_values = self._maybe_mask_results(new_values, fill_value=iNaT)
+        new_values = self._maybe_mask_results(new_values)
         return new_values.view('i8')
 
     def _add_delta_tdi(self, other):
diff --git a/pandas/core/arrays/datetimes.py b/pandas/core/arrays/datetimes.py
index cc15aad749459..dff3465d7c97e 100644
--- a/pandas/core/arrays/datetimes.py
+++ b/pandas/core/arrays/datetimes.py
@@ -75,11 +75,12 @@ def f(self):
 
         if field in self._object_ops:
             result = fields.get_date_name_field(values, field)
-            result = self._maybe_mask_results(result)
+            result = self._maybe_mask_results(result, fill_value=None)
 
         else:
             result = fields.get_date_field(values, field)
-            result = self._maybe_mask_results(result, convert='float64')
+            result = self._maybe_mask_results(result, fill_value=None,
+                                              convert='float64')
 
         return result
 
@@ -481,7 +482,7 @@ def _sub_datetimelike_scalar(self, other):
         i8 = self.asi8
         result = checked_add_with_arr(i8, -other.value,
                                       arr_mask=self._isnan)
-        result = self._maybe_mask_results(result, fill_value=iNaT)
+        result = self._maybe_mask_results(result)
         return result.view('timedelta64[ns]')
 
     @Appender((dtl.DatetimeLikeArrayMixin._add_delta.__doc__ or "").replace(
@@ -869,7 +870,7 @@ def month_name(self, locale=None):
 
         result = fields.get_date_name_field(values, 'month_name',
                                             locale=locale)
-        result = self._maybe_mask_results(result)
+        result = self._maybe_mask_results(result, fill_value=None)
         return result
 
     def day_name(self, locale=None):
@@ -905,7 +906,7 @@ def day_name(self, locale=None):
 
         result = fields.get_date_name_field(values, 'day_name',
                                             locale=locale)
-        result = self._maybe_mask_results(result)
+        result = self._maybe_mask_results(result, fill_value=None)
         return result
 
     @property
diff --git a/pandas/core/arrays/timedeltas.py b/pandas/core/arrays/timedeltas.py
index c81a214e29d0f..611ada7fc2106 100644
--- a/pandas/core/arrays/timedeltas.py
+++ b/pandas/core/arrays/timedeltas.py
@@ -47,7 +47,8 @@ def f(self):
         values = self.asi8
         result = get_timedelta_field(values, alias)
         if self.hasnans:
-            result = self._maybe_mask_results(result, convert='float64')
+            result = self._maybe_mask_results(result, fill_value=None,
+                                              convert='float64')
 
         return result
 
@@ -220,7 +221,7 @@ def _add_datetimelike_scalar(self, other):
         i8 = self.asi8
         result = checked_add_with_arr(i8, other.value,
                                       arr_mask=self._isnan)
-        result = self._maybe_mask_results(result, fill_value=iNaT)
+        result = self._maybe_mask_results(result)
         return DatetimeArrayMixin(result, tz=other.tz)
 
     def _addsub_offset_array(self, other, op):
@@ -256,7 +257,8 @@ def _evaluate_with_timedelta_like(self, other, op):
                     result = op(left, right)
                 else:
                     result = op(left, np.float64(right))
-                result = self._maybe_mask_results(result, convert='float64')
+                result = self._maybe_mask_results(result, fill_value=None,
+                                                  convert='float64')
                 return result
 
         return NotImplemented
@@ -319,7 +321,7 @@ def total_seconds(self):
         Float64Index([0.0, 86400.0, 172800.0, 259200.00000000003, 345600.0],
                      dtype='float64')
         """
-        return self._maybe_mask_results(1e-9 * self.asi8)
+        return self._maybe_mask_results(1e-9 * self.asi8, fill_value=None)
 
     def to_pytimedelta(self):
         """
diff --git a/pandas/core/indexes/timedeltas.py b/pandas/core/indexes/timedeltas.py
index 1efa0a15d34d7..e5da21478d0a4 100644
--- a/pandas/core/indexes/timedeltas.py
+++ b/pandas/core/indexes/timedeltas.py
@@ -232,7 +232,8 @@ def astype(self, dtype, copy=True):
             # return an index (essentially this is division)
             result = self.values.astype(dtype, copy=copy)
             if self.hasnans:
-                values = self._maybe_mask_results(result, convert='float64')
+                values = self._maybe_mask_results(result, fill_value=None,
+                                                  convert='float64')
                 return Index(values, name=self.name)
             return Index(result.astype('i8'), name=self.name)
         return super(TimedeltaIndex, self).astype(dtype, copy=copy)

From fb007cba966f8dc7b425e9f38a3c01b03e9b6d2f Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Thu, 25 Oct 2018 10:31:10 -0700
Subject: [PATCH 22/30] Make docstring extra explicit

---
 pandas/core/arrays/datetimelike.py | 2 +-
 pandas/core/arrays/datetimes.py    | 3 ++-
 pandas/core/arrays/period.py       | 3 ++-
 pandas/core/arrays/timedeltas.py   | 5 +++--
 4 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index f38f3b9b2c151..4e71ffcf581c9 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -352,7 +352,7 @@ def _add_offset(self, offset):
     def _add_delta(self, other):
         """
         Add a timedelta-like, Tick or TimedeltaIndex-like object
-        to self, yielding another array of the same type
+        to self, yielding an int64 numpy array
 
         Parameters
         ----------
diff --git a/pandas/core/arrays/datetimes.py b/pandas/core/arrays/datetimes.py
index dff3465d7c97e..9fa7b5e63f1a1 100644
--- a/pandas/core/arrays/datetimes.py
+++ b/pandas/core/arrays/datetimes.py
@@ -486,7 +486,8 @@ def _sub_datetimelike_scalar(self, other):
         return result.view('timedelta64[ns]')
 
     @Appender((dtl.DatetimeLikeArrayMixin._add_delta.__doc__ or "").replace(
-        "ndarray[int64]", "same type as self"))
+        "ndarray[int64]", "same type as self").replace(
+        "an int64 numpy array", "another array of the same type as self"))
     def _add_delta(self, delta):
         new_values = dtl.DatetimeLikeArrayMixin._add_delta(self, delta)
         return type(self)(new_values, tz=self.tz, freq='infer')
diff --git a/pandas/core/arrays/period.py b/pandas/core/arrays/period.py
index 3445b35ca134b..3759451c7834f 100644
--- a/pandas/core/arrays/period.py
+++ b/pandas/core/arrays/period.py
@@ -393,7 +393,8 @@ def _add_delta_tdi(self, other):
         return self._addsub_int_array(delta, operator.add)
 
     @Appender((DatetimeLikeArrayMixin._add_delta.__doc__ or "").replace(
-        "ndarray[int64]", "same type as self"))
+        "ndarray[int64]", "same type as self").replace(
+        "an int64 numpy array", "another array of the same type as self"))
     def _add_delta(self, other):
         if not isinstance(self.freq, Tick):
             # We cannot add timedelta-like to non-tick PeriodArray
diff --git a/pandas/core/arrays/timedeltas.py b/pandas/core/arrays/timedeltas.py
index 611ada7fc2106..e11a1e260d0ee 100644
--- a/pandas/core/arrays/timedeltas.py
+++ b/pandas/core/arrays/timedeltas.py
@@ -4,7 +4,7 @@
 import numpy as np
 
 from pandas._libs import tslibs
-from pandas._libs.tslibs import Timedelta, Timestamp, NaT, iNaT
+from pandas._libs.tslibs import Timedelta, Timestamp, NaT
 from pandas._libs.tslibs.fields import get_timedelta_field
 from pandas._libs.tslibs.timedeltas import array_to_timedelta64
 
@@ -191,7 +191,8 @@ def _add_offset(self, other):
                                 cls=type(self).__name__))
 
     @Appender((dtl.DatetimeLikeArrayMixin._add_delta.__doc__ or "").replace(
-        "ndarray[int64]", "same type as self"))
+        "ndarray[int64]", "same type as self").replace(
+        "an int64 numpy array", "another array of the same type as self"))
     def _add_delta(self, delta):
         new_values = dtl.DatetimeLikeArrayMixin._add_delta(self, delta)
         return type(self)(new_values, freq='infer')

From aecfef7d7a560b35eb1f5a05056fd6d160516c8b Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Thu, 25 Oct 2018 16:13:26 -0700
Subject: [PATCH 23/30] prettify docstrings

---
 pandas/core/arrays/datetimes.py  | 18 ++++++++++++++----
 pandas/core/arrays/timedeltas.py | 17 +++++++++++++----
 2 files changed, 27 insertions(+), 8 deletions(-)

diff --git a/pandas/core/arrays/datetimes.py b/pandas/core/arrays/datetimes.py
index f4e3635699baf..83566d046f5cd 100644
--- a/pandas/core/arrays/datetimes.py
+++ b/pandas/core/arrays/datetimes.py
@@ -12,7 +12,7 @@
     conversion, fields, timezones,
     resolution as libresolution)
 
-from pandas.util._decorators import cache_readonly, Appender
+from pandas.util._decorators import cache_readonly
 from pandas.errors import PerformanceWarning
 from pandas import compat
 
@@ -485,10 +485,20 @@ def _sub_datetimelike_scalar(self, other):
         result = self._maybe_mask_results(result)
         return result.view('timedelta64[ns]')
 
-    @Appender((dtl.DatetimeLikeArrayMixin._add_delta.__doc__ or "").replace(
-        "ndarray[int64]", "same type as self").replace(
-        "an int64 numpy array", "another array of the same type as self"))
     def _add_delta(self, delta):
+        """
+        Add a timedelta-like, Tick, or TimedeltaIndex-like object
+        to self, yielding a new DatetimeArray
+
+        Parameters
+        ----------
+        other : {timedelta, np.timedelta64, Tick,
+                 TimedeltaIndex, ndarray[timedelta64]}
+
+        Returns
+        -------
+        result : DatetimeArray
+        """
         new_values = dtl.DatetimeLikeArrayMixin._add_delta(self, delta)
         return type(self)(new_values, tz=self.tz, freq='infer')
 
diff --git a/pandas/core/arrays/timedeltas.py b/pandas/core/arrays/timedeltas.py
index e11a1e260d0ee..397297c1b88d0 100644
--- a/pandas/core/arrays/timedeltas.py
+++ b/pandas/core/arrays/timedeltas.py
@@ -8,7 +8,6 @@
 from pandas._libs.tslibs.fields import get_timedelta_field
 from pandas._libs.tslibs.timedeltas import array_to_timedelta64
 
-from pandas.util._decorators import Appender
 from pandas import compat
 
 from pandas.core.dtypes.common import (
@@ -190,10 +189,20 @@ def _add_offset(self, other):
                         .format(typ=type(other).__name__,
                                 cls=type(self).__name__))
 
-    @Appender((dtl.DatetimeLikeArrayMixin._add_delta.__doc__ or "").replace(
-        "ndarray[int64]", "same type as self").replace(
-        "an int64 numpy array", "another array of the same type as self"))
     def _add_delta(self, delta):
+        """
+        Add a timedelta-like, Tick, or TimedeltaIndex-like object
+        to self, yielding a new TimedeltaArray
+
+        Parameters
+        ----------
+        other : {timedelta, np.timedelta64, Tick,
+                 TimedeltaIndex, ndarray[timedelta64]}
+
+        Returns
+        -------
+        result : TimedeltaArray
+        """
         new_values = dtl.DatetimeLikeArrayMixin._add_delta(self, delta)
         return type(self)(new_values, freq='infer')
 

From a6eb01cc0f967ebb5fc243d71439a6c3ab6f907e Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Thu, 25 Oct 2018 18:22:25 -0700
Subject: [PATCH 24/30] fixup super usage

---
 pandas/core/arrays/period.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pandas/core/arrays/period.py b/pandas/core/arrays/period.py
index 71a8ff6f920cb..9f593b35eef2d 100644
--- a/pandas/core/arrays/period.py
+++ b/pandas/core/arrays/period.py
@@ -739,7 +739,7 @@ def _add_offset(self, other):
         # Note: when calling parent class's _add_delta_td, it will call
         #  delta_to_nanoseconds(delta).  Because delta here is an integer,
         #  delta_to_nanoseconds will return it unchanged.
-        result = super(PeriodArray, self)._add_delta_td(self, other.n)
+        result = super(PeriodArray, self)._add_delta_td(other.n)
         return type(self)(result, freq=self.freq)
 
     def _add_delta_td(self, other):
@@ -781,7 +781,7 @@ def _add_delta(self, other):
                                         .format(cls=type(self).__name__,
                                                 freqstr=self.freqstr))
 
-        new_ordinals = super(PeriodArray, self)._add_delta(self, other)
+        new_ordinals = super(PeriodArray, self)._add_delta(other)
         return type(self)(new_ordinals, freq=self.freq)
 
     def _check_timedeltalike_freq_compat(self, other):

From acf1f74c5ea9ace096154754b9b4e4fa604746f0 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Thu, 25 Oct 2018 18:27:13 -0700
Subject: [PATCH 25/30] flesh out TODO comment

---
 pandas/core/arrays/period.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/pandas/core/arrays/period.py b/pandas/core/arrays/period.py
index 9f593b35eef2d..edc144e0508db 100644
--- a/pandas/core/arrays/period.py
+++ b/pandas/core/arrays/period.py
@@ -494,7 +494,8 @@ def _time_shift(self, n, freq=None):
             Frequency increment to shift by.
         """
         if freq is not None:
-            raise NotImplementedError  # TODO: ??
+            # TODO: don't silently ignore kwarg
+            raise NotImplementedError
         values = self.asi8 + n * self.freq.n
         if self.hasnans:
             values[self._isnan] = iNaT

From d306277163f724009cce355d0852b8c4fd05ee3e Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Thu, 25 Oct 2018 20:07:37 -0700
Subject: [PATCH 26/30] move test for moved method

---
 pandas/tests/arrays/test_period.py         | 14 --------------
 pandas/tests/indexes/period/test_period.py | 11 +++++++++++
 2 files changed, 11 insertions(+), 14 deletions(-)

diff --git a/pandas/tests/arrays/test_period.py b/pandas/tests/arrays/test_period.py
index 780df579d2778..dcbb0d4048b0f 100644
--- a/pandas/tests/arrays/test_period.py
+++ b/pandas/tests/arrays/test_period.py
@@ -190,17 +190,3 @@ def tet_sub_period():
     other = pd.Period("2000", freq="M")
     with tm.assert_raises_regex(IncompatibleFrequency, "freq"):
         arr - other
-
-
-# ----------------------------------------------------------------------------
-# other
-
-def test_maybe_convert_timedelta():
-    arr = period_array(['2000', '2001'], freq='D')
-    offset = pd.tseries.offsets.Day(2)
-    assert arr._maybe_convert_timedelta(offset) == 2
-    assert arr._maybe_convert_timedelta(2) == 2
-
-    offset = pd.tseries.offsets.BusinessDay()
-    with tm.assert_raises_regex(ValueError, 'freq'):
-        arr._maybe_convert_timedelta(offset)
diff --git a/pandas/tests/indexes/period/test_period.py b/pandas/tests/indexes/period/test_period.py
index 405edba83dc7a..7ce51181d1b0c 100644
--- a/pandas/tests/indexes/period/test_period.py
+++ b/pandas/tests/indexes/period/test_period.py
@@ -557,3 +557,14 @@ def test_insert(self):
         for na in (np.nan, pd.NaT, None):
             result = period_range('2017Q1', periods=4, freq='Q').insert(1, na)
             tm.assert_index_equal(result, expected)
+
+
+def test_maybe_convert_timedelta():
+    pi = PeriodIndex(['2000', '2001'], freq='D')
+    offset = offsets.Day(2)
+    assert pi._maybe_convert_timedelta(offset) == 2
+    assert pi._maybe_convert_timedelta(2) == 2
+
+    offset = BusinessDay()
+    with tm.assert_raises_regex(ValueError, 'freq'):
+        pi._maybe_convert_timedelta(offset)

From f6e4073a378a8d85957d8810b932440505840dee Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Thu, 25 Oct 2018 21:57:31 -0700
Subject: [PATCH 27/30] Fixup name

---
 pandas/tests/indexes/period/test_period.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/tests/indexes/period/test_period.py b/pandas/tests/indexes/period/test_period.py
index 7ce51181d1b0c..efa193859e86a 100644
--- a/pandas/tests/indexes/period/test_period.py
+++ b/pandas/tests/indexes/period/test_period.py
@@ -565,6 +565,6 @@ def test_maybe_convert_timedelta():
     assert pi._maybe_convert_timedelta(offset) == 2
     assert pi._maybe_convert_timedelta(2) == 2
 
-    offset = BusinessDay()
+    offset = offsets.BusinessDay()
     with tm.assert_raises_regex(ValueError, 'freq'):
         pi._maybe_convert_timedelta(offset)

From 4e4b9edaeb3f611f919122f9b990c4162ec5e76f Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Sat, 27 Oct 2018 07:38:43 -0700
Subject: [PATCH 28/30] change NotImplementedError to TypeError

---
 pandas/core/arrays/period.py | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/pandas/core/arrays/period.py b/pandas/core/arrays/period.py
index edc144e0508db..48a7d0f952fd5 100644
--- a/pandas/core/arrays/period.py
+++ b/pandas/core/arrays/period.py
@@ -494,8 +494,9 @@ def _time_shift(self, n, freq=None):
             Frequency increment to shift by.
         """
         if freq is not None:
-            # TODO: don't silently ignore kwarg
-            raise NotImplementedError
+            raise TypeError("`freq` argument is not supported for "
+                            "{cls}._time_shift"
+                            .format(cls=type(self).__name__))
         values = self.asi8 + n * self.freq.n
         if self.hasnans:
             values[self._isnan] = iNaT

From f35b3b68681f4453bdf53456769429f12e1c1af3 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Sat, 27 Oct 2018 07:54:32 -0700
Subject: [PATCH 29/30] Fix add_delta_tdi return type; docstrings

---
 pandas/core/arrays/period.py | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/pandas/core/arrays/period.py b/pandas/core/arrays/period.py
index 48a7d0f952fd5..feadf28150618 100644
--- a/pandas/core/arrays/period.py
+++ b/pandas/core/arrays/period.py
@@ -745,6 +745,15 @@ def _add_offset(self, other):
         return type(self)(result, freq=self.freq)
 
     def _add_delta_td(self, other):
+        """
+        Parameters
+        ----------
+        other : timedelta, Tick, np.timedelta64
+
+        Returns
+        -------
+        result : ndarray[int64]
+        """
         assert isinstance(self.freq, Tick)  # checked by calling function
         assert isinstance(other, (timedelta, np.timedelta64, Tick))
 
@@ -757,10 +766,19 @@ def _add_delta_td(self, other):
         return ordinals
 
     def _add_delta_tdi(self, other):
+        """
+        Parameters
+        ----------
+        other : TimedeltaArray or ndarray[timedelta64]
+
+        Returns
+        -------
+        result : ndarray[int64]
+        """
         assert isinstance(self.freq, Tick)  # checked by calling function
 
         delta = self._check_timedeltalike_freq_compat(other)
-        return self._addsub_int_array(delta, operator.add)
+        return self._addsub_int_array(delta, operator.add).asi8
 
     def _add_delta(self, other):
         """

From 0466b9c8efd2f38f61a79fdf90464ee71ae72750 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Sat, 27 Oct 2018 22:22:55 -0700
Subject: [PATCH 30/30] docstring edit, rename

---
 pandas/core/arrays/datetimelike.py |  5 +++--
 pandas/core/arrays/period.py       | 18 +++++++++---------
 2 files changed, 12 insertions(+), 11 deletions(-)

diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index 93325e80b7019..0247ce8dc6ac4 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -226,6 +226,7 @@ def _maybe_mask_results(self, result, fill_value=iNaT, convert=None):
         Parameters
         ----------
         result : a ndarray
+        fill_value : object, default iNaT
         convert : string/dtype or None
 
         Returns
@@ -369,14 +370,14 @@ def _add_delta(self, other):
         method (__add__ or __sub__), if necessary (i.e. for Indexes).
         """
         if isinstance(other, (Tick, timedelta, np.timedelta64)):
-            new_values = self._add_delta_td(other)
+            new_values = self._add_timedeltalike_scalar(other)
         elif is_timedelta64_dtype(other):
             # ndarray[timedelta64] or TimedeltaArray/index
             new_values = self._add_delta_tdi(other)
 
         return new_values
 
-    def _add_delta_td(self, other):
+    def _add_timedeltalike_scalar(self, other):
         """
         Add a delta of a timedeltalike
         return the i8 result view
diff --git a/pandas/core/arrays/period.py b/pandas/core/arrays/period.py
index feadf28150618..31bcac2f4f529 100644
--- a/pandas/core/arrays/period.py
+++ b/pandas/core/arrays/period.py
@@ -738,13 +738,13 @@ def _add_offset(self, other):
             msg = DIFFERENT_FREQ_INDEX.format(self.freqstr, other.freqstr)
             raise IncompatibleFrequency(msg)
 
-        # Note: when calling parent class's _add_delta_td, it will call
-        #  delta_to_nanoseconds(delta).  Because delta here is an integer,
-        #  delta_to_nanoseconds will return it unchanged.
-        result = super(PeriodArray, self)._add_delta_td(other.n)
+        # Note: when calling parent class's _add_timedeltalike_scalar,
+        #  it will call delta_to_nanoseconds(delta).  Because delta here
+        #  is an integer, delta_to_nanoseconds will return it unchanged.
+        result = super(PeriodArray, self)._add_timedeltalike_scalar(other.n)
         return type(self)(result, freq=self.freq)
 
-    def _add_delta_td(self, other):
+    def _add_timedeltalike_scalar(self, other):
         """
         Parameters
         ----------
@@ -759,10 +759,10 @@ def _add_delta_td(self, other):
 
         delta = self._check_timedeltalike_freq_compat(other)
 
-        # Note: when calling parent class's _add_delta_td, it will call
-        #  delta_to_nanoseconds(delta).  Because delta here is an integer,
-        #  delta_to_nanoseconds will return it unchanged.
-        ordinals = super(PeriodArray, self)._add_delta_td(delta)
+        # Note: when calling parent class's _add_timedeltalike_scalar,
+        #  it will call delta_to_nanoseconds(delta).  Because delta here
+        #  is an integer, delta_to_nanoseconds will return it unchanged.
+        ordinals = super(PeriodArray, self)._add_timedeltalike_scalar(delta)
         return ordinals
 
     def _add_delta_tdi(self, other):