Pytest how to mock external libraries calls in class

I'm having a class with watchdog and i want to create a unit test that check that at a call to a method in my class, methods of watchdog.Observer calls occur

this is the class i have

from watchdog.observers.polling import PollingObserver as Observer


class FolderObserver:
    def __init__(self):
        self.__event_handler = MyObserver()
        self.__event_observer = Observer()

    def watch(self, path):
        self.start(path)
        try:
            while True:
                time.sleep(1)
        except:
            self.stop()

    def start(self, path):
        self.__schedule(path)
        self.__event_observer.start()

    def stop(self):
        self.__event_observer.stop()
        self.__event_observer.join()

    def __schedule(self, path):
        self.__event_observer.schedule(self.__event_handler, path, recursive=True)

and this is a unit test I tried

class TestWatch(TestCase):
    """Unit test for loadConfig function"""

    @patch.object(watchdog.observers.polling.PollingObserver,'schedule')    
    @patch.object(watchdog.observers.polling.PollingObserver,'start')
    def test_start_calls_schedule(self, start, schedule):
        src_path = "random_path"
        observer = FolderObserver()
        observer.start(src_path)
        start.assert_called_with(mockObserver, src_path, recursive=True)
        schedule.assert_called_with(mockObserver, src_path, recursive=True)

and I cannot get it to work,

what am I doing wrong?