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Observer Pattern with Mock Metrics in Python

Implement the Observer pattern with a mock metrics collector to track state changes and verify notifications.

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
      …

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