code: def test_at_getitem_mixed_index_no_fallback(self):
      # GH#19860
      ser = Series([1, 2, 3, 4, 5], index=["a", "b", "c", 1, 2])
      with pytest.raises(KeyError, match="^0$"):
          ser.at[0]
      with pytest.raises(KeyError, match="^4$"):
          ser.at[4]
  
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 6 for calling the TestAtErrors obj's underlying member method code with example usage: obj.test_at_getitem_mixed_index_no_fallback() without return types