code: def test_len(self):
      d = self._empty_mapping()
      self.assertEqual(len(d), 0)
  
imports: import unittest
  import collections
  from test.support import Py_C_RECURSION_LIMIT
label_desc: Use image node_id 12 for calling the BasicTestMappingProtocol obj's underlying member method code with example usage: obj.test_len() without return types