code: def test_pickle_unpickle(self, freq):
      rng = date_range("2009-01-01", "2010-01-01", freq=freq)
      unpickled = tm.round_trip_pickle(rng)
      assert unpickled.freq == freq
  
file_path: pandas/pandas/tests/indexes/datetimes/test_pickle.py
imports: import pytest
  from pandas import NaT, date_range, to_datetime
  import pandas._testing
label_desc: Use image node_id 5 for calling the TestPickle obj's underlying member method code with example usage: obj.test_pickle_unpickle(freq) without return types