code: def test_parser_propagates_policy_to_sub_messages(self):
      msg = self._make_msg(
          textwrap.dedent(
              """\
              Subject: mime test
              MIME-Version: 1.0
              Content-Type: multipart/mixed, boundary="XXX"
  
              --XXX
              Content-Type: text/plain
  
              test
              --XXX
              Content-Type: text/plain
  
              test2
              --XXX--
              """
          )
      )
      for part in msg.walk():
          self.assertIs(part.policy, self.policy)
  
imports: import io
  import types
  import textwrap
  import unittest
  import email.errors
  import email.policy
  import email.parser
  import email.generator
  import email.message
  from email import headerregistry
label_desc: Use image node_id 9 for calling the TestPolicyPropagation obj's underlying member method code with example usage: obj.test_parser_propagates_policy_to_sub_messages() without return types