code: def test_merge_statistics():
      df1 = pd.DataFrame(
          {
              "A": ["A0", "A1", "A2", "A3"],
              "B": ["B0", "B1", "B2", "B3"],
              "C": ["C0", "C1", "C2", "C3"],
              "D": ["D0", "D1", "D2", "D3"],
          },
          index=[0, 1, 2, 3],
      )
  
      df2 = pd.DataFrame(
          {
              "B": ["B2_", "B3_", "B6_", "B7_"],
              "D": ["D2_", "D3_", "D6_", "D7_"],
              "F": ["F2", "F3", "F6", "F7"],
          },
          index=[2, 3, 6, 7],
      )
  
      out = pd.DataFrame(
          {
              "A": ["A0", "A1", "A2", "A3", None, None],
              "B": ["B0", "B1", "B2", "B3", "B6_", "B7_"],
              "C": ["C0", "C1", "C2", "C3", None, None],
              "D": ["D0", "D1", "D2", "D3", "D6_", "D7_"],
              "F": [None, None, "F2", "F3", "F6", "F7"],
          },
          index=[0, 1, 2, 3, 6, 7],
      )
  
      datastore = {
          "first_df": {
              "feature_1": df1,
              "feature_2": df2,
              "feature_3": df1,
          },
          "second_df": {"feature_1": df2, "feature_3": None},
      }
      datastore = MergeStatistics(
          read_keys=["first_df", "second_df"], store_key="output_df"
      ).transform(datastore)
  
      pd.testing.assert_frame_equal(df1.combine_first(df2), out)
      pd.testing.assert_frame_equal(
          datastore["output_df"]["feature_1"], out
      )
      pd.testing.assert_frame_equal(
          datastore["output_df"]["feature_2"], df2
      )
      pd.testing.assert_frame_equal(
          datastore["output_df"]["feature_3"], df1
      )
  
label_desc: Use image node_id 1 for calling a global function with example usage: test_merge_statistics() without return types