Observer Pattern with Mock Metrics in Python

import unittest
from unittest.mock import Mock


class Subject:
    def __init__(self):
        self._state = 0
        self._observers = []

    def attach(self, observer):
        self._observers.append(observer)

    def set_state(self, value):
        if value != self._state:
            self._state = value
            self._notify()

    def _notify(self):
        for observer in self._observers:
            observer.update(self._state)


class ObserverMetrics:
    def __init__(self):
        self.update_calls = []

    def update(self, state):
        self.update_calls.append(state)


class TestSubjectObserverMetrics(unittest.TestCase):
    def test_observer_notified_on_change(self):
        subject = Subject()
        metrics = ObserverMetrics()
        subject.attach(metrics)

        subject.set_state(1)
        subject.set_state(2)
        subject.set_state(2)  # no change — no notification

        self.assertEqual(metrics.update_calls, [1, 2])


if __name__ == "__main__":
    subject = Subject()
    mock_observer = Mock()
    subject.attach(mock_observer)

    subject.set_state(10)
    subject.set_state(20)

    called_states = [call.args[0] for call in mock_observer.update.call_args_list]
    print(f"Observer notified with states: {called_states}")