code: def test_string_add_str_str(self):
      pdf = pd.DataFrame(
          dict(col1=["a", "b", "c"], col2=["1", "2", "3"])
      )
      psdf = ps.from_pandas(pdf)
  
      # TODO: Fix the Series names
      self.assert_eq(
          psdf["col1"] + psdf["col2"], pdf["col1"] + pdf["col2"]
      )
      self.assert_eq(
          psdf["col2"] + psdf["col1"], pdf["col2"] + pdf["col1"]
      )
  
imports: import pandas
  import numpy
  import re
  from pyspark import pandas
  from pyspark.testing.pandasutils import PandasOnSparkTestCase
  from pyspark.testing.sqlutils import SQLTestUtils
label_desc: Use image node_id 6 for calling the SeriesStringOpsMixin obj's underlying member method code with example usage: obj.test_string_add_str_str() without return types