STYLE/DEPR: catch PerformanceWarnings

closes #18989

Author: jreback

pandas/tests/indexes/datetimes/test_arithmetic.py

@@ -368,20 +368,25 @@ def test_dti_add_offset_array(self, tz, box):
         # GH#18849
         dti = pd.date_range('2017-01-01', periods=2, tz=tz)
         other = box([pd.offsets.MonthEnd(), pd.offsets.Day(n=2)])
-        res = dti + other
+
+        with tm.assert_produces_warning(PerformanceWarning):
+            res = dti + other
         expected = DatetimeIndex([dti[n] + other[n] for n in range(len(dti))],
                                  name=dti.name, freq='infer')
         tm.assert_index_equal(res, expected)
 
-        res2 = other + dti
+        with tm.assert_produces_warning(PerformanceWarning):
+            res2 = other + dti
         tm.assert_index_equal(res2, expected)
 
     @pytest.mark.parametrize('box', [np.array, pd.Index])
     def test_dti_sub_offset_array(self, tz, box):
         # GH#18824
         dti = pd.date_range('2017-01-01', periods=2, tz=tz)
         other = box([pd.offsets.MonthEnd(), pd.offsets.Day(n=2)])
-        res = dti - other
+
+        with tm.assert_produces_warning(PerformanceWarning):
+            res = dti - other
         expected = DatetimeIndex([dti[n] - other[n] for n in range(len(dti))],
                                  name=dti.name, freq='infer')
         tm.assert_index_equal(res, expected)
@@ -392,20 +397,25 @@ def test_dti_sub_offset_array(self, tz, box):
     def test_dti_with_offset_series(self, tz, names):
         # GH#18849
         dti = pd.date_range('2017-01-01', periods=2, tz=tz, name=names[0])
-        other = pd.Series([pd.offsets.MonthEnd(), pd.offsets.Day(n=2)],
-                          name=names[1])
+        other = Series([pd.offsets.MonthEnd(), pd.offsets.Day(n=2)],
+                       name=names[1])
 
-        expected_add = pd.Series([dti[n] + other[n] for n in range(len(dti))],
-                                 name=names[2])
-        res = dti + other
+        expected_add = Series([dti[n] + other[n] for n in range(len(dti))],
+                              name=names[2])
+
+        with tm.assert_produces_warning(PerformanceWarning):
+            res = dti + other
         tm.assert_series_equal(res, expected_add)
-        res2 = other + dti
+
+        with tm.assert_produces_warning(PerformanceWarning):
+            res2 = other + dti
         tm.assert_series_equal(res2, expected_add)
 
-        expected_sub = pd.Series([dti[n] - other[n] for n in range(len(dti))],
-                                 name=names[2])
+        expected_sub = Series([dti[n] - other[n] for n in range(len(dti))],
+                              name=names[2])
 
-        res3 = dti - other
+        with tm.assert_produces_warning(PerformanceWarning):
+            res3 = dti - other
         tm.assert_series_equal(res3, expected_sub)