class_docstr: A variant of weakref.WeakKeyDictionary for unhashable objects.
  
  This datastructure is used to store futures for broadcasted data objects
  such as large numpy arrays or pandas dataframes that are not hashable and
  therefore cannot be used as keys of traditional python dicts.
  
  Furthermore using a dict with id(array) as key is not safe because the
  Python is likely to reuse id of recently collected arrays.
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code: def __len__(self):
      return len(self._data)
  
imports: from __future__ import print_function, division, absolute_import
  import asyncio
  import concurrent.futures
  import contextlib
  import time
  from uuid import uuid4
  import weakref
  from .parallel import parallel_config
  from .parallel import AutoBatchingMixin, ParallelBackendBase
label_desc: Use image node_id 4 for calling the _WeakKeyDictionary obj's underlying member method code with example usage: obj.__len__() and returns: len