code: def test_window_reduce_process(self):
      data_stream = self.env.from_collection(
          [
              ("a", 1),
              ("a", 2),
              ("b", 3),
              ("a", 6),
              ("b", 8),
              ("b", 9),
              ("a", 15),
          ],
          type_info=Types.TUPLE([Types.STRING(), Types.INT()]),
      )  # type: DataStream
      watermark_strategy = WatermarkStrategy.for_monotonous_timestamps().with_timestamp_assigner(
          SecondColumnTimestampAssigner()
      )
  
      class MyProcessFunction(ProcessWindowFunction):
          def process(
              self,
              key,
              context: ProcessWindowFunction.Context,
              elements: Iterable[Tuple[str, int]],
          ) -> Iterable[str]:
              yield "current window start at {}, reduce result {}".format(
                  context.window().start,
                  next(iter(elements)),
              )
  
      data_stream.assign_timestamps_and_watermarks(
          watermark_strategy
      ).key_by(lambda x: x[0], key_type=Types.STRING()).window(
          EventTimeSessionWindows.with_gap(Time.milliseconds(2))
      ).reduce(
          lambda a, b: (b[0], a[1] + b[1]),
          window_function=MyProcessFunction(),
          output_type=Types.STRING(),
      ).add_sink(
          self.test_sink
      )
  
      self.env.execute("test_window_reduce_process")
      results = self.test_sink.get_results()
      expected = [
          "current window start at 1, reduce result ('a', 3)",
          "current window start at 15, reduce result ('a', 15)",
          "current window start at 3, reduce result ('b', 3)",
          "current window start at 6, reduce result ('a', 6)",
          "current window start at 8, reduce result ('b', 17)",
      ]
      self.assert_equals_sorted(expected, results)
  
imports: from typing import Iterable, Tuple, Dict
  from pyflink.common import Configuration
  from pyflink.common.time import Time
  from pyflink.common.typeinfo import Types
  from pyflink.common.watermark_strategy import WatermarkStrategy, TimestampAssigner
  from pyflink.datastream.data_stream import DataStream
  from pyflink.datastream.functions import ProcessWindowFunction, WindowFunction, AggregateFunction, ProcessAllWindowFunction
  from pyflink.datastream.output_tag import OutputTag
  from pyflink.datastream.window import TumblingEventTimeWindows, SlidingEventTimeWindows, EventTimeSessionWindows, CountSlidingWindowAssigner, SessionWindowTimeGapExtractor, CountWindow, PurgingTrigger, EventTimeTrigger, TimeWindow, GlobalWindows, CountTrigger
  from pyflink.datastream.tests.test_util import DataStreamTestSinkFunction
  from pyflink.java_gateway import get_gateway
  from pyflink.testing.test_case_utils import PyFlinkStreamingTestCase
  from pyflink.util.java_utils import get_j_env_configuration
label_desc: Use image node_id 11 for calling the WindowTests obj's underlying member method code with example usage: obj.test_window_reduce_process() without return types