How to mock exception in method

test.py

import gitlab
from gitlab.exceptions import GitlabError

class MainLogic():
  def open_gitlab(self):
    try:
      gl = gitlab.Gitlab('https://example.com', 'MyToken')
      gl.auth()
      log.error('Connected to GitLab.')
      return gl
    except GitlabError as e:
      log.error('Wrong configuration and/or unable to connect GitLab.')
      raise GitlabError from e


import logging

import unittest
from unittest import mock

log = logging.getLogger(__name__)


class MyTestCase(unittest.TestCase):
  def test_open_gitlab_exception(self):
    ml = MainLogic()
    gitlab.Gitlab = mock.Mock()
    #gitlab.Gitlab.auth = mock.Mock(side_effect=GitlabError('I need exception in open_gitlab'))
    gitlab.Gitlab.auth.side_effect = GitlabError('I need exception in open_gitlab')
    try:
      gl = ml.open_gitlab()
    except GitlabError:
      print('OK')
      assert True
    else:
      print('NOK')
      assert False

mtc = MyTestCase()
mtc.test_open_gitlab_exception()