code: def test_builtin_func(self):
      def f(x):
          return len(x)
  
      func_map = free_vars_detect._detect_function_free_vars(f)
      self.assertEmpty(func_map)
  
imports: import functools
  import unittest
  from absl.testing import parameterized
  import numpy
  from tensorflow.core.function.capture import free_vars_detect
  from tensorflow.python.util import tf_decorator
label_desc: Use image node_id 4 for calling the FreeVarDetectionTest obj's underlying member method code with example usage: obj.test_builtin_func() and returns: len