code: def test_shift_preserve_freqstr(self, periods, frame_or_series):
      # GH#21275
      obj = frame_or_series(
          range(periods),
          index=date_range(
              "2016-1-1 00:00:00", periods=periods, freq="h"
          ),
      )
  
      result = obj.shift(1, "2h")
  
      expected = frame_or_series(
          range(periods),
          index=date_range(
              "2016-1-1 02:00:00", periods=periods, freq="h"
          ),
      )
      tm.assert_equal(result, expected)
  
imports: import numpy
  import pytest
  import pandas
  from pandas import CategoricalIndex, DataFrame, Index, NaT, Series, date_range, offsets
  import pandas._testing
label_desc: Use image node_id 9 for calling the TestDataFrameShift obj's underlying member method code with example usage: obj.test_shift_preserve_freqstr(periods, frame_or_series) without return types