code: def test_at_datetime_index(self, row):
      # Set float64 dtype to avoid upcast when setting .5
      df = DataFrame(
          data=[[1] * 2],
          index=DatetimeIndex(data=["2019-01-01", "2019-01-02"]),
      ).astype({0: "float64"})
      expected = DataFrame(
          data=[[0.5, 1], [1.0, 1]],
          index=DatetimeIndex(data=["2019-01-01", "2019-01-02"]),
      )
  
      df.at[row, 0] = 0.5
      tm.assert_frame_equal(df, expected)
  
imports: from datetime import datetime, timezone
  import numpy
  import pytest
  from pandas.errors import InvalidIndexError
  from pandas import CategoricalDtype, CategoricalIndex, DataFrame, DatetimeIndex, Index, MultiIndex, Series, Timestamp
  import pandas._testing
label_desc: Use image node_id 5 for calling the TestAtSetItem obj's underlying member method code with example usage: obj.test_at_datetime_index(row) without return types