BUG: freq not retained on apply_index (#33779)

* BUG: freq not retained on apply_index * whatsnew

Author: jbrockmendel

doc/source/whatsnew/v1.1.0.rst

@@ -462,6 +462,7 @@ Datetimelike
 - Bug in :meth:`DatetimeIndex.to_period` not infering the frequency when called with no arguments (:issue:`33358`)
 - Bug in :meth:`DatetimeIndex.tz_localize` incorrectly retaining ``freq`` in some cases where the original freq is no longer valid (:issue:`30511`)
 - Bug in :meth:`DatetimeIndex.intersection` losing ``freq`` and timezone in some cases (:issue:`33604`)
+- Bug in :class:`DatetimeIndex` addition and subtraction with some types of :class:`DateOffset` objects incorrectly retaining an invalid ``freq`` attribute (:issue:`33779`)
 
 Timedelta
 ^^^^^^^^^

pandas/tests/tseries/offsets/test_yqm_offsets.py

@@ -64,6 +64,7 @@ def test_apply_index(cls, n):
  ser = pd.Series(rng)
 
  res = rng + offset
+ assert res.freq is None # not retained
  res_v2 = offset.apply_index(rng)
  assert (res == res_v2).all()
  assert res[0] == rng[0] + offset

pandas/tseries/offsets.py

@@ -1147,9 +1147,7 @@ def apply(self, other):
  @apply_index_wraps
  def apply_index(self, i):
  shifted = liboffsets.shift_months(i.asi8, self.n, self._day_opt)
- # TODO: going through __new__ raises on call to _validate_frequency;
- # are we passing incorrect freq?
- return type(i)._simple_new(shifted, freq=i.freq, dtype=i.dtype)
+ return type(i)._simple_new(shifted, dtype=i.dtype)
 
 
 class MonthEnd(MonthOffset):
@@ -1868,11 +1866,7 @@ def apply_index(self, dtindex):
  shifted = liboffsets.shift_quarters(
  dtindex.asi8, self.n, self.startingMonth, self._day_opt
  )
- # TODO: going through __new__ raises on call to _validate_frequency;
- # are we passing incorrect freq?
- return type(dtindex)._simple_new(
- shifted, freq=dtindex.freq, dtype=dtindex.dtype
- )
+ return type(dtindex)._simple_new(shifted, dtype=dtindex.dtype)
 
 
 class BQuarterEnd(QuarterOffset):
@@ -1954,11 +1948,7 @@ def apply_index(self, dtindex):
  shifted = liboffsets.shift_quarters(
  dtindex.asi8, self.n, self.month, self._day_opt, modby=12
  )
- # TODO: going through __new__ raises on call to _validate_frequency;
- # are we passing incorrect freq?
- return type(dtindex)._simple_new(
- shifted, freq=dtindex.freq, dtype=dtindex.dtype
- )
+ return type(dtindex)._simple_new(shifted, dtype=dtindex.dtype)
 
  def is_on_offset(self, dt: datetime) -> bool:
  if self.normalize and not _is_normalized(dt):