From 6313b2d9c6e6fc40408bf624d8fd7b1d8a2eb2ee Mon Sep 17 00:00:00 2001
From: jbrockmendel <jbrockmendel@gmail.com>
Date: Sat, 25 Apr 2020 19:07:49 -0700
Subject: [PATCH] BUG: incorrect freq in PeriodIndex-Period

---
 pandas/core/indexes/datetimelike.py    | 10 ++++++----
 pandas/tests/arithmetic/test_period.py |  9 +++++++--
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/pandas/core/indexes/datetimelike.py b/pandas/core/indexes/datetimelike.py
index 4ddac12ae8155..f41044db2b49c 100644
--- a/pandas/core/indexes/datetimelike.py
+++ b/pandas/core/indexes/datetimelike.py
@@ -88,7 +88,7 @@ def wrapped(self, other):
         if result is NotImplemented:
             return NotImplemented
 
-        new_freq = self._get_addsub_freq(other)
+        new_freq = self._get_addsub_freq(other, result)
         result._freq = new_freq
         return result
 
@@ -451,14 +451,16 @@ def _partial_date_slice(
     # --------------------------------------------------------------------
     # Arithmetic Methods
 
-    def _get_addsub_freq(self, other) -> Optional[DateOffset]:
+    def _get_addsub_freq(self, other, result) -> Optional[DateOffset]:
         """
         Find the freq we expect the result of an addition/subtraction operation
         to have.
         """
         if is_period_dtype(self.dtype):
-            # Only used for ops that stay PeriodDtype
-            return self.freq
+            if is_period_dtype(result.dtype):
+                # Only used for ops that stay PeriodDtype
+                return self.freq
+            return None
         elif self.freq is None:
             return None
         elif lib.is_scalar(other) and isna(other):
diff --git a/pandas/tests/arithmetic/test_period.py b/pandas/tests/arithmetic/test_period.py
index 4cf1988a33de1..55a547b361eb3 100644
--- a/pandas/tests/arithmetic/test_period.py
+++ b/pandas/tests/arithmetic/test_period.py
@@ -1444,8 +1444,13 @@ def test_pi_sub_period(self):
         tm.assert_index_equal(result, exp)
 
         exp = pd.TimedeltaIndex([np.nan, np.nan, np.nan, np.nan], name="idx")
-        tm.assert_index_equal(idx - pd.Period("NaT", freq="M"), exp)
-        tm.assert_index_equal(pd.Period("NaT", freq="M") - idx, exp)
+        result = idx - pd.Period("NaT", freq="M")
+        tm.assert_index_equal(result, exp)
+        assert result.freq == exp.freq
+
+        result = pd.Period("NaT", freq="M") - idx
+        tm.assert_index_equal(result, exp)
+        assert result.freq == exp.freq
 
     def test_pi_sub_pdnat(self):
         # GH#13071