code: def check_apply_in_pandas_not_returning_pandas_dataframe(self):
      self._test_merge_error(
          fn=lambda lft, rgt: lft.size + rgt.size,
          error_class=PythonException,
          error_message_regex="Return type of the user-defined function "
          "should be pandas.DataFrame, but is int.",
      )
  
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 13 for calling the CogroupedApplyInPandasTestsMixin obj's underlying member method code with example usage: obj.check_apply_in_pandas_not_returning_pandas_dataframe() without return types