code: def test_string_add_assign(self):
      pdf = pd.DataFrame(
          dict(col1=["a", "b", "c"], col2=["1", "2", "3"])
      )
      psdf = ps.from_pandas(pdf)
      psdf["col1"] += psdf["col2"]
      pdf["col1"] += pdf["col2"]
      self.assert_eq(psdf["col1"], 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 5 for calling the SeriesStringOpsMixin obj's underlying member method code with example usage: obj.test_string_add_assign() without return types