code: def __len__(self):
      return len(self.d)
  
imports: from __future__ import annotations
  import heapq
  import operator
  import pickle
  import random
  from collections.abc import Mapping
  import pytest
  from distributed.collections import LRU, HeapSet, sum_mappings
label_desc: Use image node_id 4 for calling the ReadOnlyMapping obj's underlying member method code with example usage: obj.__len__() and returns: len