code: def flat_map(self, flatmap_fn):
          
          op = Operator(
              _generate_uuid(),
              OpType.FlatMap,
              "FlatMap",
              flatmap_fn,
              num_instances=self.env.config.parallelism)
          return self.__register(op)
docs: Applies a flatmap operator to the stream.
  
          Attributes:
               flatmap_fn (function): The user-defined logic of the flatmap
               (e.g. split()).