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.
code: def _get_num_items_index(self, key):
      keys = self.keys
      if key not in keys:
          raise KeyError("Key %s not in buckets.keys()" % key)
      return self._buckets[key].num_items_index
  
imports: import collections
  import random
  import threading
label_desc: Use image node_id 5 for calling the Reservoir obj's underlying member method code with example usage: obj._get_num_items_index(key) and returns: self