From 75638764adf4135d8ee889ea28e627ca4c30a94c Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Wed, 7 Mar 2018 18:43:32 -0800
Subject: [PATCH 1/9] implement add/sub for period_dtype other

---
 doc/source/whatsnew/v0.23.0.txt               |  1 +
 pandas/core/indexes/datetimelike.py           | 45 +++++++++++++-
 .../indexes/datetimes/test_arithmetic.py      | 11 ++++
 .../tests/indexes/period/test_arithmetic.py   | 62 +++++++++++++------
 .../indexes/timedeltas/test_arithmetic.py     | 12 ++++
 5 files changed, 111 insertions(+), 20 deletions(-)

diff --git a/doc/source/whatsnew/v0.23.0.txt b/doc/source/whatsnew/v0.23.0.txt
index 233816600ec0f..451974a8006d2 100644
--- a/doc/source/whatsnew/v0.23.0.txt
+++ b/doc/source/whatsnew/v0.23.0.txt
@@ -710,6 +710,7 @@ Other API Changes
 - ``Categorical.fillna`` now validates its ``value`` and ``method`` keyword arguments. It now raises when both or none are specified, matching the behavior of :meth:`Series.fillna` (:issue:`19682`)
 - ``pd.to_datetime('today')`` now returns a datetime, consistent with ``pd.Timestamp('today')``; previously ``pd.to_datetime('today')`` returned a ``.normalized()`` datetime (:issue:`19935`)
 - :func:`Series.str.replace` now takes an optional `regex` keyword which, when set to ``False``, uses literal string replacement rather than regex replacement (:issue:`16808`)
+- :class:`PeriodIndex` subtraction of another ``PeriodIndex`` will now return a numeric ``Index`` instead of raising a ``TypeErrorr`` (:issue:`?????`)
 
 .. _whatsnew_0230.deprecations:
 
diff --git a/pandas/core/indexes/datetimelike.py b/pandas/core/indexes/datetimelike.py
index e673bfe411cb4..f3ad87d798921 100644
--- a/pandas/core/indexes/datetimelike.py
+++ b/pandas/core/indexes/datetimelike.py
@@ -13,7 +13,8 @@
 import numpy as np
 
 from pandas._libs import lib, iNaT, NaT
-from pandas._libs.tslibs.period import Period
+from pandas._libs.tslibs.period import (Period, IncompatibleFrequency,
+                                        _DIFFERENT_FREQ_INDEX)
 from pandas._libs.tslibs.timedeltas import delta_to_nanoseconds
 from pandas._libs.tslibs.timestamps import round_ns
 
@@ -667,6 +668,43 @@ def _sub_nat(self):
     def _sub_period(self, other):
         return NotImplemented
 
+    def _sub_period_array(self, other):
+        """
+        Subtract one PeriodIndex from another.  This is only valid if they
+        have the same frequency.
+
+        Parameters
+        ----------
+        other : PeriodIndex
+
+        Returns
+        -------
+        result : np.ndarray
+            dtype is np.int64 if there are no NaTs in self or other,
+            otherwise np.float64
+        """
+        if not is_period_dtype(self):
+            raise TypeError("cannot subtract {dtype}-dtype to {cls}"
+                            .format(dtype=other.dtype,
+                                    cls=type(self).__name__))
+
+        if not len(self) == len(other):
+            raise ValueError("cannot add indices of unequal length")
+        if self.freq != other.freq:
+            msg = _DIFFERENT_FREQ_INDEX.format(self.freqstr, other.freqstr)
+            raise IncompatibleFrequency(msg)
+
+        new_values = checked_add_with_arr(self.asi8, -other.asi8,
+                                          arr_mask=self._isnan,
+                                          b_mask=other._isnan)
+        new_values = new_values
+        if self.hasnans or other.hasnans:
+            # we have to convert to float to represent NaNs
+            mask = (self._isnan) | (other._isnan)
+            new_values = new_values.astype(np.float64)
+            new_values[mask] = np.nan
+        return new_values
+
     def _add_offset(self, offset):
         raise com.AbstractMethodError(self)
 
@@ -740,7 +778,7 @@ def __add__(self, other):
             elif is_integer_dtype(other) and self.freq is None:
                 # GH#19123
                 raise NullFrequencyError("Cannot shift with no freq")
-            elif is_float_dtype(other):
+            elif is_float_dtype(other) or is_period_dtype(other):
                 # Explicitly catch invalid dtypes
                 raise TypeError("cannot add {dtype}-dtype to {cls}"
                                 .format(dtype=other.dtype,
@@ -799,6 +837,9 @@ def __sub__(self, other):
             elif is_datetime64_dtype(other) or is_datetime64tz_dtype(other):
                 # DatetimeIndex, ndarray[datetime64]
                 result = self._sub_datelike(other)
+            elif is_period_dtype(other):
+                # PeriodIndex
+                result = self._sub_period_array(other)
             elif isinstance(other, Index):
                 raise TypeError("cannot subtract {cls} and {typ}"
                                 .format(cls=type(self).__name__,
diff --git a/pandas/tests/indexes/datetimes/test_arithmetic.py b/pandas/tests/indexes/datetimes/test_arithmetic.py
index 8f259a7e78897..f1ac595e76a1a 100644
--- a/pandas/tests/indexes/datetimes/test_arithmetic.py
+++ b/pandas/tests/indexes/datetimes/test_arithmetic.py
@@ -696,6 +696,17 @@ def test_sub_period(self, freq):
         with pytest.raises(TypeError):
             p - idx
 
+    @pytest.mark.parametrize('op', [operator.add, ops.radd,
+                                    operator.sub, ops.rsub])
+    @pytest.mark.parametrize('pi_freq', ['D', 'W', 'Q', 'H'])
+    @pytest.mark.parametrize('dti_freq', [None, 'D'])
+    def test_dti_sub_pi(self, dti_freq, pi_freq, op):
+        # subtracting PeriodIndex should raise TypeError
+        dti = pd.DatetimeIndex(['2011-01-01', '2011-01-02'], freq=dti_freq)
+        pi = dti.to_period(pi_freq)
+        with pytest.raises(TypeError):
+            op(dti, pi)
+
     def test_ufunc_coercions(self):
         idx = date_range('2011-01-01', periods=3, freq='2D', name='x')
 
diff --git a/pandas/tests/indexes/period/test_arithmetic.py b/pandas/tests/indexes/period/test_arithmetic.py
index c75fdd35a974c..57170dc328d80 100644
--- a/pandas/tests/indexes/period/test_arithmetic.py
+++ b/pandas/tests/indexes/period/test_arithmetic.py
@@ -258,6 +258,42 @@ def test_comp_nat(self, dtype):
 
 
 class TestPeriodIndexArithmetic(object):
+    # ---------------------------------------------------------------
+    # __add__/__sub__ with PeriodIndex
+
+    def test_pi_add_iadd_pi_raises(self):
+        rng = pd.period_range('1/1/2000', freq='D', periods=5)
+        other = pd.period_range('1/6/2000', freq='D', periods=5)
+
+        # previously performed setop union, now raises TypeError (GH14164)
+        with pytest.raises(TypeError):
+            rng + other
+
+        with pytest.raises(TypeError):
+            rng += other
+
+    def test_pi_sub_pi(self):
+        rng = pd.period_range('1/1/2000', freq='D', periods=5)
+        other = pd.period_range('1/6/2000', freq='D', periods=5)
+
+        result = rng - other
+        expected = pd.Int64Index([-5, -5, -5, -5, -5])
+        tm.assert_index_equal(result, expected)
+
+    def test_pi_sub_pi_with_nat(self):
+        rng = pd.period_range('1/1/2000', freq='D', periods=5)
+        other = rng[1:].insert(0, pd.NaT)
+        assert other[1:].equals(rng[1:])
+
+        result = rng - other
+        expected = pd.Float64Index([np.nan, 0, 0, 0, 0])
+        tm.assert_index_equal(result, expected)
+
+    def test_pi_sub_pi_mismatched_freq(self):
+        rng = pd.period_range('1/1/2000', freq='D', periods=5)
+        other = pd.period_range('1/6/2000', freq='H', periods=5)
+        with pytest.raises(period.IncompatibleFrequency):
+            rng - other
 
     # -------------------------------------------------------------
     # Invalid Operations
@@ -379,17 +415,6 @@ def test_pi_sub_offset_array(self, box):
             with tm.assert_produces_warning(PerformanceWarning):
                 anchored - pi
 
-    def test_pi_add_iadd_pi_raises(self):
-        rng = pd.period_range('1/1/2000', freq='D', periods=5)
-        other = pd.period_range('1/6/2000', freq='D', periods=5)
-
-        # previously performed setop union, now raises TypeError (GH14164)
-        with pytest.raises(TypeError):
-            rng + other
-
-        with pytest.raises(TypeError):
-            rng += other
-
     def test_pi_add_iadd_int(self, one):
         # Variants of `one` for #19012
         rng = pd.period_range('2000-01-01 09:00', freq='H', periods=10)
@@ -419,17 +444,18 @@ def test_pi_sub_intlike(self, five):
         exp = rng + (-five)
         tm.assert_index_equal(result, exp)
 
-    def test_pi_sub_isub_pi_raises(self):
-        # previously performed setop, now raises TypeError (GH14164)
-        # TODO needs to wait on #13077 for decision on result type
+    def test_pi_sub_isub_pi(self):
+        # previously raised TypeError (GH#14164), before that
+        # performed set operation.  See discussion in GH#13077
         rng = pd.period_range('1/1/2000', freq='D', periods=5)
         other = pd.period_range('1/6/2000', freq='D', periods=5)
 
-        with pytest.raises(TypeError):
-            rng - other
+        expected = pd.Int64Index([-5, -5, -5, -5, -5])
+        result = rng - other
+        tm.assert_index_equal(result, expected)
 
-        with pytest.raises(TypeError):
-            rng -= other
+        rng -= other
+        tm.assert_index_equal(rng, expected)
 
     def test_pi_sub_isub_offset(self):
         # offset
diff --git a/pandas/tests/indexes/timedeltas/test_arithmetic.py b/pandas/tests/indexes/timedeltas/test_arithmetic.py
index 9035434046ccb..8da9f31d1d3f8 100644
--- a/pandas/tests/indexes/timedeltas/test_arithmetic.py
+++ b/pandas/tests/indexes/timedeltas/test_arithmetic.py
@@ -305,6 +305,18 @@ def test_tdi_sub_period(self, freq):
         with pytest.raises(TypeError):
             p - idx
 
+    @pytest.mark.parametrize('op', [operator.add, ops.radd,
+                                    operator.sub, ops.rsub])
+    @pytest.mark.parametrize('pi_freq', ['D', 'W', 'Q', 'H'])
+    @pytest.mark.parametrize('tdi_freq', [None, 'H'])
+    def test_dti_sub_pi(self, tdi_freq, pi_freq, op):
+        # subtracting PeriodIndex should raise TypeError
+        tdi = pd.TimedeltaIndex(['1 hours', '2 hours'], freq=tdi_freq)
+        dti = pd.Timestamp('2018-03-07 17:16:40') + tdi
+        pi = dti.to_period(pi_freq)
+        with pytest.raises(TypeError):
+            op(dti, pi)
+
     # -------------------------------------------------------------
     # TimedeltaIndex.shift is used by __add__/__sub__
 

From 14f8d86f1b319f4a35a8c89fec5fccfda3190d43 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Wed, 7 Mar 2018 18:46:20 -0800
Subject: [PATCH 2/9] add GH reference

---
 doc/source/whatsnew/v0.23.0.txt                    | 2 +-
 pandas/tests/indexes/datetimes/test_arithmetic.py  | 2 +-
 pandas/tests/indexes/period/test_arithmetic.py     | 1 +
 pandas/tests/indexes/timedeltas/test_arithmetic.py | 2 +-
 4 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/doc/source/whatsnew/v0.23.0.txt b/doc/source/whatsnew/v0.23.0.txt
index 451974a8006d2..7dbc5239fa3df 100644
--- a/doc/source/whatsnew/v0.23.0.txt
+++ b/doc/source/whatsnew/v0.23.0.txt
@@ -710,7 +710,7 @@ Other API Changes
 - ``Categorical.fillna`` now validates its ``value`` and ``method`` keyword arguments. It now raises when both or none are specified, matching the behavior of :meth:`Series.fillna` (:issue:`19682`)
 - ``pd.to_datetime('today')`` now returns a datetime, consistent with ``pd.Timestamp('today')``; previously ``pd.to_datetime('today')`` returned a ``.normalized()`` datetime (:issue:`19935`)
 - :func:`Series.str.replace` now takes an optional `regex` keyword which, when set to ``False``, uses literal string replacement rather than regex replacement (:issue:`16808`)
-- :class:`PeriodIndex` subtraction of another ``PeriodIndex`` will now return a numeric ``Index`` instead of raising a ``TypeErrorr`` (:issue:`?????`)
+- :class:`PeriodIndex` subtraction of another ``PeriodIndex`` will now return a numeric ``Index`` instead of raising a ``TypeErrorr`` (:issue:`20049`)
 
 .. _whatsnew_0230.deprecations:
 
diff --git a/pandas/tests/indexes/datetimes/test_arithmetic.py b/pandas/tests/indexes/datetimes/test_arithmetic.py
index f1ac595e76a1a..db7d0c7b58eac 100644
--- a/pandas/tests/indexes/datetimes/test_arithmetic.py
+++ b/pandas/tests/indexes/datetimes/test_arithmetic.py
@@ -701,7 +701,7 @@ def test_sub_period(self, freq):
     @pytest.mark.parametrize('pi_freq', ['D', 'W', 'Q', 'H'])
     @pytest.mark.parametrize('dti_freq', [None, 'D'])
     def test_dti_sub_pi(self, dti_freq, pi_freq, op):
-        # subtracting PeriodIndex should raise TypeError
+        # GH#20049subtracting PeriodIndex should raise TypeError
         dti = pd.DatetimeIndex(['2011-01-01', '2011-01-02'], freq=dti_freq)
         pi = dti.to_period(pi_freq)
         with pytest.raises(TypeError):
diff --git a/pandas/tests/indexes/period/test_arithmetic.py b/pandas/tests/indexes/period/test_arithmetic.py
index 57170dc328d80..75b3c2d3f9547 100644
--- a/pandas/tests/indexes/period/test_arithmetic.py
+++ b/pandas/tests/indexes/period/test_arithmetic.py
@@ -445,6 +445,7 @@ def test_pi_sub_intlike(self, five):
         tm.assert_index_equal(result, exp)
 
     def test_pi_sub_isub_pi(self):
+        # GH#20049
         # previously raised TypeError (GH#14164), before that
         # performed set operation.  See discussion in GH#13077
         rng = pd.period_range('1/1/2000', freq='D', periods=5)
diff --git a/pandas/tests/indexes/timedeltas/test_arithmetic.py b/pandas/tests/indexes/timedeltas/test_arithmetic.py
index 8da9f31d1d3f8..ac0a55a4e512b 100644
--- a/pandas/tests/indexes/timedeltas/test_arithmetic.py
+++ b/pandas/tests/indexes/timedeltas/test_arithmetic.py
@@ -310,7 +310,7 @@ def test_tdi_sub_period(self, freq):
     @pytest.mark.parametrize('pi_freq', ['D', 'W', 'Q', 'H'])
     @pytest.mark.parametrize('tdi_freq', [None, 'H'])
     def test_dti_sub_pi(self, tdi_freq, pi_freq, op):
-        # subtracting PeriodIndex should raise TypeError
+        # GH#20049 subtracting PeriodIndex should raise TypeError
         tdi = pd.TimedeltaIndex(['1 hours', '2 hours'], freq=tdi_freq)
         dti = pd.Timestamp('2018-03-07 17:16:40') + tdi
         pi = dti.to_period(pi_freq)

From f2dac592010a501b275d28924e942d44dfa1ed5e Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Mon, 4 Jun 2018 10:09:18 -0700
Subject: [PATCH 3/9] change return type from Int64Index to Index[DateOffset]

---
 pandas/core/indexes/datetimelike.py            | 12 +++++-------
 pandas/tests/indexes/period/test_arithmetic.py |  9 ++++++---
 2 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/pandas/core/indexes/datetimelike.py b/pandas/core/indexes/datetimelike.py
index 7b40a56a78375..023b875f6ee3c 100644
--- a/pandas/core/indexes/datetimelike.py
+++ b/pandas/core/indexes/datetimelike.py
@@ -796,9 +796,8 @@ def _sub_period_array(self, other):
 
         Returns
         -------
-        result : np.ndarray
-            dtype is np.int64 if there are no NaTs in self or other,
-            otherwise np.float64
+        result : np.ndarray[object]
+            Array of DateOffset objects; nulls represented by NaT
         """
         if not is_period_dtype(self):
             raise TypeError("cannot subtract {dtype}-dtype to {cls}"
@@ -814,12 +813,11 @@ def _sub_period_array(self, other):
         new_values = checked_add_with_arr(self.asi8, -other.asi8,
                                           arr_mask=self._isnan,
                                           b_mask=other._isnan)
-        new_values = new_values
+
+        new_values = np.array([self.freq * x for x in new_values])
         if self.hasnans or other.hasnans:
-            # we have to convert to float to represent NaNs
             mask = (self._isnan) | (other._isnan)
-            new_values = new_values.astype(np.float64)
-            new_values[mask] = np.nan
+            new_values[mask] = NaT
         return new_values
 
     def _add_offset(self, offset):
diff --git a/pandas/tests/indexes/period/test_arithmetic.py b/pandas/tests/indexes/period/test_arithmetic.py
index 399eeed06f700..bab503451de56 100644
--- a/pandas/tests/indexes/period/test_arithmetic.py
+++ b/pandas/tests/indexes/period/test_arithmetic.py
@@ -277,7 +277,8 @@ def test_pi_sub_pi(self):
         other = pd.period_range('1/6/2000', freq='D', periods=5)
 
         result = rng - other
-        expected = pd.Int64Index([-5, -5, -5, -5, -5])
+        off = rng.freq
+        expected = pd.Index([-5 * off] * 5)
         tm.assert_index_equal(result, expected)
 
     def test_pi_sub_pi_with_nat(self):
@@ -286,7 +287,8 @@ def test_pi_sub_pi_with_nat(self):
         assert other[1:].equals(rng[1:])
 
         result = rng - other
-        expected = pd.Float64Index([np.nan, 0, 0, 0, 0])
+        off = rng.freq
+        expected = pd.Index([pd.NaT, 0 * off, 0 * off, 0 * off, 0 * off])
         tm.assert_index_equal(result, expected)
 
     def test_pi_sub_pi_mismatched_freq(self):
@@ -451,7 +453,8 @@ def test_pi_sub_isub_pi(self):
         rng = pd.period_range('1/1/2000', freq='D', periods=5)
         other = pd.period_range('1/6/2000', freq='D', periods=5)
 
-        expected = pd.Int64Index([-5, -5, -5, -5, -5])
+        off = rng.freq
+        expected = pd.Index([-5 * off] * 5)
         result = rng - other
         tm.assert_index_equal(result, expected)
 

From 0adf604f0b6565c3ad70f0fd0806991ad7505c64 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Tue, 5 Jun 2018 09:55:08 -0700
Subject: [PATCH 4/9] remove merge leftover

---
 doc/source/whatsnew/v0.23.0.txt | 1 -
 1 file changed, 1 deletion(-)

diff --git a/doc/source/whatsnew/v0.23.0.txt b/doc/source/whatsnew/v0.23.0.txt
index b8cc4d249e80f..a099fb40c35a7 100644
--- a/doc/source/whatsnew/v0.23.0.txt
+++ b/doc/source/whatsnew/v0.23.0.txt
@@ -996,7 +996,6 @@ Other API Changes
 - :func:`DataFrame.to_dict` with ``orient='index'`` no longer casts int columns to float for a DataFrame with only int and float columns (:issue:`18580`)
 - A user-defined-function that is passed to :func:`Series.rolling().aggregate() <pandas.core.window.Rolling.aggregate>`, :func:`DataFrame.rolling().aggregate() <pandas.core.window.Rolling.aggregate>`, or its expanding cousins, will now *always* be passed a ``Series``, rather than a ``np.array``; ``.apply()`` only has the ``raw`` keyword, see :ref:`here <whatsnew_0230.enhancements.window_raw>`. This is consistent with the signatures of ``.aggregate()`` across pandas (:issue:`20584`)
 - Rolling and Expanding types raise ``NotImplementedError`` upon iteration (:issue:`11704`).
->>>>>>> cbec58eacd8e9cd94b7f42351b8de4559c250909
 
 .. _whatsnew_0230.deprecations:
 

From eb6ec2a16630b2158b1434d44d65534267e33531 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Tue, 5 Jun 2018 18:36:00 -0700
Subject: [PATCH 5/9] flesh out comments

---
 pandas/tests/indexes/datetimes/test_arithmetic.py |  2 +-
 pandas/tests/indexes/period/test_arithmetic.py    | 12 +++++++++---
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/pandas/tests/indexes/datetimes/test_arithmetic.py b/pandas/tests/indexes/datetimes/test_arithmetic.py
index 2744e49acbe34..a0c5e988e9407 100644
--- a/pandas/tests/indexes/datetimes/test_arithmetic.py
+++ b/pandas/tests/indexes/datetimes/test_arithmetic.py
@@ -745,7 +745,7 @@ def test_sub_period(self, freq):
     @pytest.mark.parametrize('pi_freq', ['D', 'W', 'Q', 'H'])
     @pytest.mark.parametrize('dti_freq', [None, 'D'])
     def test_dti_sub_pi(self, dti_freq, pi_freq, op):
-        # GH#20049subtracting PeriodIndex should raise TypeError
+        # GH#20049 subtracting PeriodIndex should raise TypeError
         dti = pd.DatetimeIndex(['2011-01-01', '2011-01-02'], freq=dti_freq)
         pi = dti.to_period(pi_freq)
         with pytest.raises(TypeError):
diff --git a/pandas/tests/indexes/period/test_arithmetic.py b/pandas/tests/indexes/period/test_arithmetic.py
index bab503451de56..8a62c44311a84 100644
--- a/pandas/tests/indexes/period/test_arithmetic.py
+++ b/pandas/tests/indexes/period/test_arithmetic.py
@@ -260,12 +260,17 @@ def test_comp_nat(self, dtype):
 class TestPeriodIndexArithmetic(object):
     # ---------------------------------------------------------------
     # __add__/__sub__ with PeriodIndex
+    # PeriodIndex + other is defined for integers and timedelta-like others
+    # PeriodIndex - other is defined for integers, timedelta-like others,
+    #   and PeriodIndex (with matching freq)
 
     def test_pi_add_iadd_pi_raises(self):
         rng = pd.period_range('1/1/2000', freq='D', periods=5)
         other = pd.period_range('1/6/2000', freq='D', periods=5)
 
-        # previously performed setop union, now raises TypeError (GH14164)
+        # An earlier implementation of PeriodIndex addition performed
+        # a set operation (union).  This has since been changed to
+        # raise a TypeError. See GH#14164 and GH#13077 for historical reference.
         with pytest.raises(TypeError):
             rng + other
 
@@ -448,8 +453,9 @@ def test_pi_sub_intlike(self, five):
 
     def test_pi_sub_isub_pi(self):
         # GH#20049
-        # previously raised TypeError (GH#14164), before that
-        # performed set operation.  See discussion in GH#13077
+        # For historical reference see GH#14164, GH#13077.
+        # PeriodIndex subtraction originally performed set difference,
+        # then changed to raise TypeError before being implemented in GH#20049
         rng = pd.period_range('1/1/2000', freq='D', periods=5)
         other = pd.period_range('1/6/2000', freq='D', periods=5)
 

From e6096319e9cbf70702cc1247a5ab9c16b8576962 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Tue, 5 Jun 2018 18:37:29 -0700
Subject: [PATCH 6/9] fixup too-long line

---
 pandas/tests/indexes/period/test_arithmetic.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/pandas/tests/indexes/period/test_arithmetic.py b/pandas/tests/indexes/period/test_arithmetic.py
index 8a62c44311a84..17c5e97ecef54 100644
--- a/pandas/tests/indexes/period/test_arithmetic.py
+++ b/pandas/tests/indexes/period/test_arithmetic.py
@@ -270,7 +270,8 @@ def test_pi_add_iadd_pi_raises(self):
 
         # An earlier implementation of PeriodIndex addition performed
         # a set operation (union).  This has since been changed to
-        # raise a TypeError. See GH#14164 and GH#13077 for historical reference.
+        # raise a TypeError. See GH#14164 and GH#13077 for historical
+        # reference.
         with pytest.raises(TypeError):
             rng + other
 

From 59b314f8c3f5ba2455759462165321246beb44e6 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Mon, 11 Jun 2018 09:08:53 -0700
Subject: [PATCH 7/9] Fixup typo add-->subtract

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

diff --git a/pandas/core/indexes/datetimelike.py b/pandas/core/indexes/datetimelike.py
index 023b875f6ee3c..70c87b0a231c1 100644
--- a/pandas/core/indexes/datetimelike.py
+++ b/pandas/core/indexes/datetimelike.py
@@ -805,7 +805,7 @@ def _sub_period_array(self, other):
                                     cls=type(self).__name__))
 
         if not len(self) == len(other):
-            raise ValueError("cannot add indices of unequal length")
+            raise ValueError("cannot subtract indices of unequal length")
         if self.freq != other.freq:
             msg = _DIFFERENT_FREQ_INDEX.format(self.freqstr, other.freqstr)
             raise IncompatibleFrequency(msg)

From 0f6d67d208fcbc939b2e6cfe54a5701321bbc871 Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Mon, 11 Jun 2018 09:23:16 -0700
Subject: [PATCH 8/9] remove redundant test

---
 .../tests/indexes/period/test_arithmetic.py   | 27 +++++++------------
 1 file changed, 9 insertions(+), 18 deletions(-)

diff --git a/pandas/tests/indexes/period/test_arithmetic.py b/pandas/tests/indexes/period/test_arithmetic.py
index 17c5e97ecef54..79f08ab490494 100644
--- a/pandas/tests/indexes/period/test_arithmetic.py
+++ b/pandas/tests/indexes/period/test_arithmetic.py
@@ -278,15 +278,22 @@ def test_pi_add_iadd_pi_raises(self):
         with pytest.raises(TypeError):
             rng += other
 
-    def test_pi_sub_pi(self):
+    def test_pi_sub_isub_pi(self):
+        # GH#20049
+        # For historical reference see GH#14164, GH#13077.
+        # PeriodIndex subtraction originally performed set difference,
+        # then changed to raise TypeError before being implemented in GH#20049
         rng = pd.period_range('1/1/2000', freq='D', periods=5)
         other = pd.period_range('1/6/2000', freq='D', periods=5)
 
-        result = rng - other
         off = rng.freq
         expected = pd.Index([-5 * off] * 5)
+        result = rng - other
         tm.assert_index_equal(result, expected)
 
+        rng -= other
+        tm.assert_index_equal(rng, expected)
+
     def test_pi_sub_pi_with_nat(self):
         rng = pd.period_range('1/1/2000', freq='D', periods=5)
         other = rng[1:].insert(0, pd.NaT)
@@ -452,22 +459,6 @@ def test_pi_sub_intlike(self, five):
         exp = rng + (-five)
         tm.assert_index_equal(result, exp)
 
-    def test_pi_sub_isub_pi(self):
-        # GH#20049
-        # For historical reference see GH#14164, GH#13077.
-        # PeriodIndex subtraction originally performed set difference,
-        # then changed to raise TypeError before being implemented in GH#20049
-        rng = pd.period_range('1/1/2000', freq='D', periods=5)
-        other = pd.period_range('1/6/2000', freq='D', periods=5)
-
-        off = rng.freq
-        expected = pd.Index([-5 * off] * 5)
-        result = rng - other
-        tm.assert_index_equal(result, expected)
-
-        rng -= other
-        tm.assert_index_equal(rng, expected)
-
     def test_pi_sub_isub_offset(self):
         # offset
         # DateOffset

From 088691785e9d8cddf73a34ac57d48ed367c7530d Mon Sep 17 00:00:00 2001
From: Brock Mendel <jbrockmendel@gmail.com>
Date: Thu, 21 Jun 2018 20:50:56 -0700
Subject: [PATCH 9/9] fixup typo

---
 doc/source/whatsnew/v0.24.0.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v0.24.0.txt b/doc/source/whatsnew/v0.24.0.txt
index ed5b6a882e9f3..80709c098036b 100644
--- a/doc/source/whatsnew/v0.24.0.txt
+++ b/doc/source/whatsnew/v0.24.0.txt
@@ -68,7 +68,7 @@ Datetimelike API Changes
 
 - For :class:`DatetimeIndex` and :class:`TimedeltaIndex` with non-``None`` ``freq`` attribute, addition or subtraction of integer-dtyped array or ``Index`` will return an object of the same class (:issue:`19959`)
 - :class:`DateOffset` objects are now immutable. Attempting to alter one of these will now raise ``AttributeError`` (:issue:`21341`)
-- :class:`PeriodIndex` subtraction of another ``PeriodIndex`` will now return an object-dtype :class:`Index` of :class:`DateOffset` objects instead of raising a ``TypeErrorr`` (:issue:`20049`)
+- :class:`PeriodIndex` subtraction of another ``PeriodIndex`` will now return an object-dtype :class:`Index` of :class:`DateOffset` objects instead of raising a ``TypeError`` (:issue:`20049`)
 
 .. _whatsnew_0240.api.other: