How to mock an object attribute, I can't access?

I'm trying to mock the parser.html_source attribute of the following view in my unit test. I want to overwrite it with a former downloaded html of that page.

views.py:

    def parse_online_page(request):
        site_manager = SiteManager(Constants.LOGIN)
        site_manager.login()
        site_manager.change_to_frame(Constants.HEAD)

        parser = HeadParser(html_source=site_manager.page_source)
        parser.parse()

        return redirect('another_page')

Here's my unit test:

    TESTDATA_PATH = 'core/tests/assets'


    class TestSession(TestCase):
        def test_parser_view(self):
            with patch('core.parsers.HeadParser') as parser_mock:
                with open(os.path.join(TESTDATA_PATH, 'frame_head.html'), encoding='utf8') as f:
                    parser_mock.html_source = f.read()

                    response = self.client.get(reverse('core:parse_online_page'))
                    self.assertEqual(response.status_code, 302)

                    # assert that my html test asset was parsed correctly

Unfortunatly this does not overwrite the HeadParser's html_source attribute in all instances generated out of this class.

I don't even know if this is the right attribute to mock, I'm totally clueless.