class_docstr: A map-to-arrays dict, with deterministic Reservoir Sampling.
  
  Store each reservoir bucket by key, and each bucket is a list sampling
  with reservoir algorithm.
docstr: Determine if key exists.
  
  Args:
      key: Key to determine if exists.
  
  Returns:
      True if key exists in buckets.keys, otherwise False.
docstr_tok: ["Determine","if","key","exists",".","Args",":","key",":","Key","to","determine","if","exists",".","Returns",":","True","if","key","exists","in","buckets.keys",",","otherwise","False","."]
code: def _exist_in_keys(self, key):
      """Determine if key exists.
  
      Args:
          key: Key to determine if exists.
  
      Returns:
          True if key exists in buckets.keys, otherwise False.
      """
      return True if key in self._buckets.keys() else False
  
imports: import collections
  import random
  import threading
label_desc: Use image node_id 3 for calling the Reservoir obj's underlying member method code with example usage: obj._exist_in_keys(key) and returns: unknown