code: def test_tuple_string_column_names():
      # GH#50372
      mi = MultiIndex.from_tuples(
          [("a", "aa"), ("a", "ab"), ("b", "ba"), ("b", "bb")]
      )
      df = DataFrame([range(4), range(1, 5), range(2, 6)], columns=mi)
      df["single_index"] = 0
  
      df_flat = df.copy()
      df_flat.columns = df_flat.columns.to_flat_index()
      df_flat["new_single_index"] = 0
  
      result = df_flat[[("a", "aa"), "new_single_index"]]
      expected = DataFrame(
          [[0, 0], [1, 0], [2, 0]],
          columns=Index([("a", "aa"), "new_single_index"]),
      )
      tm.assert_frame_equal(result, expected)
  
imports: import numpy
  import pytest
  from pandas import DataFrame, Index, MultiIndex, Series
  import pandas._testing
  from pandas.core.indexing import IndexingError
label_desc: Use image node_id 9 for calling a global function with example usage: test_tuple_string_column_names() without return types