code: def retrieve(self, key):
          
          column_file = os.path.join(self._hash_dir, '%s.json' % key)
          cache_file = os.path.join(self._hash_dir, '%s.npy' % key)
  
          if os.path.exists(cache_file):
              data = np.load(cache_file)
              if os.path.exists(column_file):
                  with open(column_file, 'r') as json_file:
                      columns = json.load(json_file)
                  data = pd.DataFrame(data, columns=columns)
          else:
              return None
  
          return data
docs: Retrieves a cached array if possible.