code: def instance(cls, public_keys_dir):
          '''Please avoid create multi instance'''
          if public_keys_dir in cls._authenticators:
              return cls._authenticators[public_keys_dir]
          new_instance = cls(public_keys_dir)
          cls._authenticators[public_keys_dir] = new_instance
          return new_instancf instance(cls, public_keys_dir):
          '''Please avoid create multi instance'''
          if public_keys_dir in cls._authenticators:
              return cls._authenticators[public_keys_dir]
          new_instance = cls(public_keys_dir)
          cls._authenticators[public_keys_dir] = new_instance
          return new_instance
docs: Please avoid create multi instance