code: def test_astype_to_timedelta_unit_ns(self):
      # preserver the timedelta conversion
      # GH#19223
      dtype = "m8[ns]"
      arr = np.array([[1, 2, 3]], dtype=dtype)
      df = DataFrame(arr)
      result = df.astype(dtype)
      expected = DataFrame(arr.astype(dtype))
  
      tm.assert_frame_equal(result, expected)
  
imports: import re
  import numpy
  import pytest
  import pandas.util._test_decorators
  import pandas
  from pandas import Categorical, CategoricalDtype, DataFrame, DatetimeTZDtype, Index, Interval, IntervalDtype, NaT, Series, Timedelta, Timestamp, concat, date_range, option_context
  import pandas._testing
label_desc: Use image node_id 24 for calling the TestAstype obj's underlying member method code with example usage: obj.test_astype_to_timedelta_unit_ns() without return types