code: def test_sub_datetime_preserves_freq_across_dst(self):
      # GH#48818
      ts = Timestamp("2016-03-11", tz="US/Pacific")
      dti = date_range(ts, periods=4)
  
      res = dti - dti[0]
      expected = TimedeltaIndex(
          [
              Timedelta(days=0),
              Timedelta(days=1),
              Timedelta(days=2),
              Timedelta(days=2, hours=23),
          ]
      )
      tm.assert_index_equal(res, expected)
      assert res.freq == expected.freq
  
imports: import pytest
  from pandas import Timedelta, TimedeltaIndex, Timestamp, date_range, timedelta_range
  import pandas._testing
label_desc: Use image node_id 3 for calling the TestDatetimeIndexArithmetic obj's underlying member method code with example usage: obj.test_sub_datetime_preserves_freq_across_dst() without return types