code: def open(self, function_context: FunctionContext):
      self._internal_timer_service = LegacyInternalTimerServiceImpl(
          self._state_backend
      )
      self._window_aggregator.open(
          PerWindowStateDataViewStore(
              function_context, self._state_backend
          )
      )
  
      if isinstance(self._window_assigner, PanedWindowAssigner):
          self._window_function = PanedWindowProcessFunction(
              self._allowed_lateness,
              self._window_assigner,
              self._window_aggregator,
          )
      elif isinstance(self._window_assigner, MergingWindowAssigner):
          self._window_function = MergingWindowProcessFunction(
              self._allowed_lateness,
              self._window_assigner,
              self._window_aggregator,
              self._state_backend,
          )
      else:
          self._window_function = GeneralWindowProcessFunction(
              self._allowed_lateness,
              self._window_assigner,
              self._window_aggregator,
          )
      self._trigger_context = TriggerContext(
          self._trigger,
          self._internal_timer_service,
          self._state_backend,
      )
      self._trigger_context.open()
      self._window_context = WindowContext(
          self,
          self._trigger_context,
          self._state_backend,
          self._state_value_coder,
          self._internal_timer_service,
          self._window_assigner.is_event_time(),
      )
      self._window_function.open(self._window_context)
  
imports: import datetime
  from abc import ABC, abstractmethod
  from typing import TypeVar, Generic, List, Dict
  import pytz
  from pyflink.common import Row, RowKind
  from pyflink.common.constants import MAX_LONG_VALUE
  from pyflink.fn_execution.datastream.timerservice import InternalTimer
  from pyflink.fn_execution.datastream.process.timerservice_impl import LegacyInternalTimerServiceImpl
  from pyflink.fn_execution.coders import PickleCoder
  from pyflink.fn_execution.table.aggregate_slow import DistinctViewDescriptor, RowKeySelector
  from pyflink.fn_execution.table.state_data_view import DataViewSpec, ListViewSpec, MapViewSpec, PerWindowStateDataViewStore
  from pyflink.fn_execution.table.window_assigner import WindowAssigner, PanedWindowAssigner, MergingWindowAssigner
  from pyflink.fn_execution.table.window_context import WindowContext, TriggerContext, K, W
  from pyflink.fn_execution.table.window_process_function import GeneralWindowProcessFunction, InternalWindowProcessFunction, PanedWindowProcessFunction, MergingWindowProcessFunction
  from pyflink.fn_execution.table.window_trigger import Trigger
  from pyflink.table.udf import ImperativeAggregateFunction, FunctionContext
label_desc: Use image node_id 2 for calling the GroupWindowAggFunctionBase obj's underlying member method code with example usage: obj.open(function_context) without return types