code: def test_multiindex(df_multi):
      """Test output if df.columns is a multiindex"""
      actual = df_multi.explode_index(names_sep="_")
      assert_frame_equal(df_multi, actual)
  
file_path: pyjanitor/tests/functions/test_explode_index.py
imports: import pandas
  import pytest
  from pandas.testing import assert_frame_equal
label_desc: Use image node_id 10 for calling a global function with example usage: test_multiindex(df_multi) without return types