code: def test_value_vars(self, df):
      result3 = df.melt(id_vars=["id1", "id2"], value_vars="A")
      assert len(result3) == 10
  
      result4 = df.melt(id_vars=["id1", "id2"], value_vars=["A", "B"])
      expected4 = DataFrame(
          {
              "id1": df["id1"].tolist() * 2,
              "id2": df["id2"].tolist() * 2,
              "variable": ["A"] * 10 + ["B"] * 10,
              "value": (df["A"].tolist() + df["B"].tolist()),
          },
          columns=["id1", "id2", "variable", "value"],
      )
      tm.assert_frame_equal(result4, expected4)
  
imports: import re
  import numpy
  import pytest
  import pandas
  from pandas import DataFrame, Index, date_range, lreshape, melt, wide_to_long
  import pandas._testing
label_desc: Use image node_id 4 for calling the TestMelt obj's underlying member method code with example usage: obj.test_value_vars(df) without return types