code: def test_double(self):
      self._dll.tf_d.restype = c_double
      self._dll.tf_d.argtypes = (c_double,)
      self.assertEqual(self._dll.tf_d(42.0), 14.0)
      self.assertEqual(self.S(), 42)
  
imports: import unittest
  from ctypes import *
  from ctypes.test import need_symbol
  import _ctypes_test
label_desc: Use image node_id 25 for calling the CFunctions obj's underlying member method code with example usage: obj.test_double() without return types