A simple Python observer pattern implementation

The classic observer pattern uses two classes:

• Observable: This class is the provider of the observed state as implemented by the sub type
• Observer: This class is a registered subscriber listening to updates from the provider

Example usage

The below example application resides in observer.py and can be run by running the module.

from observer import Observable, Observer

class NewValueSubscriber(Observer):
    def __init__(self):
        super(NewValueSubscriber, self).__init__()

    def update(self, new_value):
        print('{} received new value: {}'.format(self.name, new_value[0]))

class NewValuePublisher(Observable):
    def __init__(self):
        super(NewValuePublisher, self).__init__()
        self._value = 0

    @property
    def value(self):
        return self._value

    @value.setter
    def value(self, value):
        self._value = value
        print('{} updating new value: {} to observers'.format(self.name, value))
        self.notify(value)

publisher = NewValuePublisher()
listener = NewValueSubscriber()
listener2 = NewValueSubscriber()
publisher.attach(listener)
publisher.attach(listener)  # this is ignored
publisher.attach(listener2)
publisher.value = 5
publisher.detach(listener)
publisher.value = 6  # only listener2 is updated
publisher.detach(listener2)  # last listener is detached
publisher.value = 7  # nobody is updated

This gives the following console output:

$ python observer.py 
New value: 5
NewValuePublisher updating new value: 5 to observers
NewValueSubscriber0 received new value: 5
NewValueSubscriber1 received new value: 5
New value: 6
NewValuePublisher updating new value: 6 to observers
NewValueSubscriber1 received new value: 6
New value: 7

Prerequisites

Works for both Python2.6+ or python3 Install python

Install this package directly

Get this package by issuing the following command:

$ pip install git+https://github.com/FrederikBjorne/python-observer

then import it like this:

from observer import ...