code: def _test_merge(
      self,
      left=None,
      right=None,
      by=["id"],
      fn=lambda lft, rgt: pd.merge(lft, rgt, on=["id", "k"]),
      output_schema="id long, k int, v int, v2 int",
      expected=None,
  ):
      def fn_with_key(_, lft, rgt):
          return fn(lft, rgt)
  
      # Test fn with and without key argument
      with self.subTest("without key"):
          self.__test_merge(
              left, right, by, fn, output_schema, expected
          )
      with self.subTest("with key"):
          self.__test_merge(
              left, right, by, fn_with_key, output_schema, expected
          )
  
imports: import unittest
  from typing import cast
  from pyspark.sql.functions import array, explode, col, lit, udf, pandas_udf, sum
  from pyspark.sql.types import ArrayType, DoubleType, LongType, StructType, StructField, YearMonthIntervalType, Row
  from pyspark.sql.window import Window
  from pyspark.errors import IllegalArgumentException, PythonException
  from pyspark.testing.sqlutils import ReusedSQLTestCase, have_pandas, have_pyarrow, pandas_requirement_message, pyarrow_requirement_message
  from pyspark.testing.utils import QuietTest
label_desc: Use image node_id 40 for calling the CogroupedApplyInPandasTestsMixin obj's underlying member method code with example usage: obj._test_merge(left, right, by, fn, output_schema, expected) and returns: fn