code: def test_at_setitem_categorical_missing(self):
      df = DataFrame(
          index=range(3),
          columns=range(3),
          dtype=CategoricalDtype(["foo", "bar"]),
      )
      df.at[1, 1] = "foo"
  
      expected = DataFrame(
          [
              [np.nan, np.nan, np.nan],
              [np.nan, "foo", np.nan],
              [np.nan, np.nan, np.nan],
          ],
          dtype=CategoricalDtype(["foo", "bar"]),
      )
  
      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 3 for calling the TestAtSetItem obj's underlying member method code with example usage: obj.test_at_setitem_categorical_missing() without return types