class_docstr: The implementation of InternalWindowProcessFunction for MergingWindowAssigner.
class_docstr_tok: ["The","implementation","of","InternalWindowProcessFunction","for","MergingWindowAssigner","."]
code: def open(self, ctx: Context[K, W]):
      super(MergingWindowProcessFunction, self).open(ctx)
      self._window_mapping = self._state_backend.get_map_state(
          "session-window-mapping",
          self._state_backend.namespace_coder,
          self._state_backend.namespace_coder,
      )
  
imports: from abc import abstractmethod, ABC
  from typing import Generic, List, Iterable, Dict, Set
  from pyflink.common import Row
  from pyflink.common.constants import MAX_LONG_VALUE
  from pyflink.datastream.state import MapState
  from pyflink.fn_execution.table.window_assigner import WindowAssigner, PanedWindowAssigner, MergingWindowAssigner
  from pyflink.fn_execution.table.window_context import Context, K, W
label_desc: Use image node_id 2 for calling the MergingWindowProcessFunction obj's underlying member method code with example usage: obj.open(ctx) without return types