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: Return all keys in self._buckets.
  
  Returns:
      All keys in reservoir buckets.
  :return:
docstr_tok: ["Return","all","keys","in","self._buckets",".","Returns",":","All","keys","in","reservoir","buckets",".",":","return",":"]
code: def keys(self):
      """Return all keys in self._buckets.
  
      Returns:
          All keys in reservoir buckets.
      :return:
      """
      with self._mutex:
          return list(self._buckets.keys())
  
imports: import collections
  import random
  import threading
label_desc: Use image node_id 2 for calling the Reservoir obj's underlying member method code with example usage: obj.keys() and returns: list