"""

http://code.activestate.com/recipes/131499-observer-pattern/

*TL;DR

Maintains a list of dependents and notifies them of any state changes.

*Examples in Python ecosystem:

Django Signals: https://docs.djangoproject.com/en/3.1/topics/signals/

Flask Signals: https://flask.palletsprojects.com/en/1.1.x/signals/

"""

# observer.py

from __future__ import annotations

class Observer:

def update(self, subject: Subject) -> None:

"""

Receive update from the subject.

Args:

subject (Subject): The subject instance sending the update.

"""

pass

class Subject:

_observers: list[Observer]

def __init__(self) -> None:

"""

Initialize the subject with an empty observer list.

"""

self._observers = []

def attach(self, observer: Observer) -> None:

"""

Attach an observer to the subject.

Args:

observer (Observer): The observer instance to attach.

"""

if observer not in self._observers:

self._observers.append(observer)

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

"""

Detach an observer from the subject.

Args:

observer (Observer): The observer instance to detach.

"""

try:

self._observers.remove(observer)

except ValueError:

pass

def notify(self) -> None:

"""

Notify all attached observers by calling their update method.

"""

for observer in self._observers:

observer.update(self)

class Data(Subject):

def __init__(self, name: str = "") -> None:

super().__init__()

self.name = name

self._data = 0

@property

def data(self) -> int:

return self._data

@data.setter

def data(self, value: int) -> None:

self._data = value

self.notify()

class HexViewer:

def update(self, subject: Data) -> None:

print(f"HexViewer: Subject {subject.name} has data 0x{subject.data:x}")

class DecimalViewer:

def update(self, subject: Data) -> None:

print(f"DecimalViewer: Subject {subject.name} has data {subject.data}")

def main():

"""

>>> data1 = Data('Data 1')

>>> data2 = Data('Data 2')

>>> view1 = DecimalViewer()

>>> view2 = HexViewer()

>>> data1.attach(view1)

>>> data1.attach(view2)

>>> data2.attach(view2)

>>> data2.attach(view1)

>>> data1.data = 10

DecimalViewer: Subject Data 1 has data 10

HexViewer: Subject Data 1 has data 0xa

>>> data2.data = 15

HexViewer: Subject Data 2 has data 0xf

DecimalViewer: Subject Data 2 has data 15

>>> data1.data = 3

DecimalViewer: Subject Data 1 has data 3

HexViewer: Subject Data 1 has data 0x3

>>> data2.data = 5

HexViewer: Subject Data 2 has data 0x5

DecimalViewer: Subject Data 2 has data 5

# Detach HexViewer from data1 and data2

>>> data1.detach(view2)

>>> data2.detach(view2)

>>> data1.data = 10

DecimalViewer: Subject Data 1 has data 10

>>> data2.data = 15

DecimalViewer: Subject Data 2 has data 15

"""

if __name__ == "__main__":

import doctest

doctest.testmod()