code: def test_mapnode_iterfield_type(x_inp, f_exp):
      from nipype import MapNode, Function
  
      def double_func(x):
          return 2 * x
  
      double = Function(["x"], ["f_x"], double_func)
  
      double_node = MapNode(double, name="double", iterfield=["x"])
      double_node.inputs.x = x_inp
  
      res = double_node.run()
      assert res.outputs.f_x == f_exp
  
imports: import os
  from copy import deepcopy
  import pytest
  from ....None import config
  from ....interfaces import utility
  from ....interfaces import base
  from ...None import engine
  from ..utils import merge_dict
  from .test_base import EngineTestInterface
  from .test_utils import UtilsTestInterface
label_desc: Use image node_id 4 for calling a global function with example usage: test_mapnode_iterfield_type(x_inp, f_exp) and returns: unknown