code: def test_multiindex_at_set(self):
      # GH 26989
      # DataFrame.at and DataFrame.loc setter works with MultiIndex
      df = DataFrame({"a": [1, 2]}, index=[[1, 2], [3, 4]])
      assert df.index.nlevels == 2
      df.at[(1, 3), "a"] = 3
      assert df.at[(1, 3), "a"] == 3
      df.loc[(1, 3), "a"] = 4
      assert df.loc[(1, 3), "a"] == 4
  
      # Series.at and Series.loc setter works with MultiIndex
      series = df["a"]
      assert series.index.nlevels == 2
      series.at[1, 3] = 5
      assert series.at[1, 3] == 5
      series.loc[1, 3] = 6
      assert series.loc[1, 3] == 6
  
imports: from datetime import datetime, timedelta
  import itertools
  import numpy
  import pytest
  from pandas import DataFrame, Index, Series, Timedelta, Timestamp, date_range
  import pandas._testing
label_desc: Use image node_id 2 for calling the TestMultiIndexScalar obj's underlying member method code with example usage: obj.test_multiindex_at_set() without return types