code: def test_combine_first_dt64(self, unit):
      s0 = to_datetime(Series(["2010", np.nan])).dt.as_unit(unit)
      s1 = to_datetime(Series([np.nan, "2011"])).dt.as_unit(unit)
      rs = s0.combine_first(s1)
      xp = to_datetime(Series(["2010", "2011"])).dt.as_unit(unit)
      tm.assert_series_equal(rs, xp)
  
      s0 = to_datetime(Series(["2010", np.nan])).dt.as_unit(unit)
      s1 = Series([np.nan, "2011"])
      rs = s0.combine_first(s1)
  
      xp = Series(
          [datetime(2010, 1, 1), "2011"], dtype="datetime64[ns]"
      )
  
      tm.assert_series_equal(rs, xp)
  
file_path: pandas/pandas/tests/series/methods/test_combine_first.py
imports: from datetime import datetime
  import numpy
  import pandas
  from pandas import Period, Series, date_range, period_range, to_datetime
  import pandas._testing
label_desc: Use image node_id 4 for calling the TestCombineFirst obj's underlying member method code with example usage: obj.test_combine_first_dt64(unit) without return types