[reposting the example, with source]

example:
  class MyException(Exception):
      pass

  @contextmanager
  def my_cm():
      try:
          yield
      except BaseException:
          exc = MyException()
          try:
              raise exc
          finally:
              exc.__context__ = None

  print('\n=== `with` statement ===')
  try:
      with my_cm():
          assert False
  except BaseException as e:
      traceback.print_exc()

  print('\n=== enter_context() ===')
  try:
      with ExitStack() as stack:
          stack.enter_context(my_cm())
          assert False
  except BaseException as e:
      traceback.print_exc()


output:
  === `with` statement ===
  Traceback (most recent call last):
    File "exit_stack_test.py", line 251, in <module>
      assert False
    File "/.../python3.7/contextlib.py", line 130, in __exit__
      self.gen.throw(type, value, traceback)
    File "exit_stack_test.py", line 244, in my_cm
      raise exc
  MyException

  === enter_context() ===
  Traceback (most recent call last):
    File "exit_stack_test.py", line 240, in my_cm
      yield
    File "exit_stack_test.py", line 259, in <module>
      assert False
  AssertionError

  During handling of the above exception, another exception occurred:

  Traceback (most recent call last):
    File "exit_stack_test.py", line 259, in <module>
      assert False
    File "/.../python3.7/contextlib.py", line 524, in __exit__
      raise exc_details[1]
    File "/.../python3.7/contextlib.py", line 509, in __exit__
      if cb(*exc_details):
    File "/.../python3.7/contextlib.py", line 377, in _exit_wrapper
      return cm_exit(cm, exc_type, exc, tb)
    File "/.../python3.7/contextlib.py", line 130, in __exit__
      self.gen.throw(type, value, traceback)
    File "exit_stack_test.py", line 244, in my_cm
      raise exc
  MyException