code: def test_at_setitem_mixed_index_assignment(self):
      # GH#19860
      ser = Series([1, 2, 3, 4, 5], index=["a", "b", "c", 1, 2])
      ser.at["a"] = 11
      assert ser.iat[0] == 11
      ser.at[1] = 22
      assert ser.iat[3] == 22
  
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 2 for calling the TestAtSetItem obj's underlying member method code with example usage: obj.test_at_setitem_mixed_index_assignment() without return types