code: def test_merge_outer(self):
      result = merge_dataframes(
          [self.df1, self.df2, self.df3], on="x", how="outer"
      )
      result = sort_df_by_columns(result)
      assert_frame_equal(result, self.df_merged_outer)
  
imports: import unittest
  import numpy
  import pandas
  from pandas.testing import assert_frame_equal
  from neptune.common.utils import align_channels_on_x, as_list, get_channel_name_stems, map_keys, map_values, merge_dataframes
  from tests.unit.neptune.legacy.random_utils import sort_df_by_columns
label_desc: Use image node_id 2 for calling the TestMergeDataFrames obj's underlying member method code with example usage: obj.test_merge_outer() without return types