code: def test_cancel_post_init(self):
      async def cancel_make_transport():
          coro = self.loop.subprocess_exec(
              asyncio.SubprocessProtocol, *PROGRAM_BLOCKED
          )
          task = self.loop.create_task(coro)
  
          self.loop.call_soon(task.cancel)
          try:
              await task
          except asyncio.CancelledError:
              pass
  
      # ignore the log:
      # "Exception during subprocess creation, kill the subprocess"
      with test_utils.disable_logger():
          self.loop.run_until_complete(cancel_make_transport())
          test_utils.run_briefly(self.loop)
  
imports: import os
  import signal
  import sys
  import textwrap
  import unittest
  import warnings
  from unittest import mock
  import asyncio
  from asyncio import base_subprocess
  from asyncio import subprocess
  from test.test_asyncio import utils
  from test import support
  from test.support import os_helper
label_desc: Use image node_id 21 for calling the SubprocessMixin obj's underlying member method code with example usage: obj.test_cancel_post_init() without return types