Observateur en Python

L’Observateur est un patron de conception comportemental qui permet à certains objets d’envoyer des notifications concernant leur état à d’autres objets.

Ce patron fournit la possibilité aux objets qui implémentent une interface de souscription, de s’inscrire et de se désinscrire de ces événements.

En savoir plus sur l’observateur

Utilisation du patron de conception en Python

Complexité :

Popularité :

Exemples d’utilisation : L’observateur est assez répandu en Python, surtout dans les composants GUI. Il fournit une manière de réagir aux événements qui se produisent chez d’autres objets sans se coupler à leurs classes.

Identification : Ce patron peut être reconnu dans les méthodes de souscription qui stockent des objets dans une liste et par les appels des objets de cette liste à la méthode update.

Exemple conceptuel

Dans cet exemple, nous allons voir la structure de l’Observateur. Nous allons répondre aux questions suivantes :

Que contiennent les classes ?
Quels rôles jouent-elles ?
Comment les éléments du patron sont-ils reliés ?

main.py: Exemple conceptuel

from __future__ import annotations
from abc import ABC, abstractmethod
from random import randrange
from typing import List


class Subject(ABC):
    """
    The Subject interface declares a set of methods for managing subscribers.
    """

    @abstractmethod
    def attach(self, observer: Observer) -> None:
        """
        Attach an observer to the subject.
        """
        pass

    @abstractmethod
    def detach(self, observer: Observer) -> None:
        """
        Detach an observer from the subject.
        """
        pass

    @abstractmethod
    def notify(self) -> None:
        """
        Notify all observers about an event.
        """
        pass


class ConcreteSubject(Subject):
    """
    The Subject owns some important state and notifies observers when the state
    changes.
    """

    _state: int = None
    """
    For the sake of simplicity, the Subject's state, essential to all
    subscribers, is stored in this variable.
    """

    _observers: List[Observer] = []
    """
    List of subscribers. In real life, the list of subscribers can be stored
    more comprehensively (categorized by event type, etc.).
    """

    def attach(self, observer: Observer) -> None:
        print("Subject: Attached an observer.")
        self._observers.append(observer)

    def detach(self, observer: Observer) -> None:
        self._observers.remove(observer)

    """
    The subscription management methods.
    """

    def notify(self) -> None:
        """
        Trigger an update in each subscriber.
        """

        print("Subject: Notifying observers...")
        for observer in self._observers:
            observer.update(self)

    def some_business_logic(self) -> None:
        """
        Usually, the subscription logic is only a fraction of what a Subject can
        really do. Subjects commonly hold some important business logic, that
        triggers a notification method whenever something important is about to
        happen (or after it).
        """

        print("\nSubject: I'm doing something important.")
        self._state = randrange(0, 10)

        print(f"Subject: My state has just changed to: {self._state}")
        self.notify()


class Observer(ABC):
    """
    The Observer interface declares the update method, used by subjects.
    """

    @abstractmethod
    def update(self, subject: Subject) -> None:
        """
        Receive update from subject.
        """
        pass


"""
Concrete Observers react to the updates issued by the Subject they had been
attached to.
"""


class ConcreteObserverA(Observer):
    def update(self, subject: Subject) -> None:
        if subject._state < 3:
            print("ConcreteObserverA: Reacted to the event")


class ConcreteObserverB(Observer):
    def update(self, subject: Subject) -> None:
        if subject._state == 0 or subject._state >= 2:
            print("ConcreteObserverB: Reacted to the event")


if __name__ == "__main__":
    # The client code.

    subject = ConcreteSubject()

    observer_a = ConcreteObserverA()
    subject.attach(observer_a)

    observer_b = ConcreteObserverB()
    subject.attach(observer_b)

    subject.some_business_logic()
    subject.some_business_logic()

    subject.detach(observer_a)

    subject.some_business_logic()

Output.txt: Résultat de l’exécution

Subject: Attached an observer.
Subject: Attached an observer.

Subject: I'm doing something important.
Subject: My state has just changed to: 0
Subject: Notifying observers...
ConcreteObserverA: Reacted to the event
ConcreteObserverB: Reacted to the event

Subject: I'm doing something important.
Subject: My state has just changed to: 5
Subject: Notifying observers...
ConcreteObserverB: Reacted to the event

Subject: I'm doing something important.
Subject: My state has just changed to: 0
Subject: Notifying observers...
ConcreteObserverB: Reacted to the event