code: def test_default_content_manager_for_add_comes_from_policy(self):
      cm = self._TestSetRaisingContentManager()
      m = self.message(policy=self.policy.clone(content_manager=cm))
      for method in (
          "add_related",
          "add_alternative",
          "add_attachment",
      ):
          with self.assertRaises(
              self._TestSetRaisingContentManager.CustomError
          ) as ar:
              getattr(m, method)("")
          self.assertEqual(str(ar.exception), "test")
  
imports: import unittest
  import textwrap
  from email import policy, message_from_string
  from email.message import EmailMessage, MIMEPart
  from test.test_email import TestEmailBase, parameterize
label_desc: Use image node_id 15 for calling the TestEmailMessageBase obj's underlying member method code with example usage: obj.test_default_content_manager_for_add_comes_from_policy() without return types