code: def test_missing_start_boundary(self):
      source = textwrap.dedent(
          """\
              Content-Type: multipart/mixed; boundary="AAA"
              From: Mail Delivery Subsystem <xxx@example.com>
              To: yyy@example.com
  
              --AAA
  
              Stuff
  
              --AAA
              Content-Type: message/rfc822
  
              From: webmaster@python.org
              To: zzz@example.com
              Content-Type: multipart/mixed; boundary="BBB"
  
              --BBB--
  
              --AAA--
  
              """
      )
      # The message structure is:
      #
      # multipart/mixed
      #    text/plain
      #    message/rfc822
      #        multipart/mixed [*]
      #
      # [*] This message is missing its start boundary
      with self._raise_point(errors.StartBoundaryNotFoundDefect):
          outer = self._str_msg(source)
      if self.raise_expected:
          return
      bad = outer.get_payload(1).get_payload(0)
      self.assertEqual(len(self.get_defects(bad)), 1)
      self.assertIsInstance(
          self.get_defects(bad)[0], errors.StartBoundaryNotFoundDefect
      )
  
imports: import textwrap
  import unittest
  import contextlib
  from email import policy
  from email import errors
  from test.test_email import TestEmailBase
label_desc: Use image node_id 8 for calling the TestDefectsBase obj's underlying member method code with example usage: obj.test_missing_start_boundary() without return types