Using decorators to implement Observer Pattern in Python3

Question

This question is not in general about the observer pattern. It is focused on the use of decorators in that pattern. The question is based on the answer of a similar question.

#!/usr/bin/env python3

class Observable:
    """
        The object that need to be observed. Alternative names are 'Subject'.
        In the most cases it is a data object.
    """
    def __init__(self):
        self._observers = []

    def register_observer(self, callback):
        self._observers.append(callback)
        return callback

    def _broadcast_observers(self, *args, **kwargs):
        for callback in self._observers:
            callback(*args, **kwargs)


class TheData(Observable):
    """
        Example of a data class just for demonstration.
    """
    def __init__(self, data):
        Observable.__init__(self)
        self._data = data

    @property
    def data(self):
        return self._data

    @data.setter
    def data(self, data):
        self._data = data
        self._broadcast_observers()


class TheGUIElement:
    """
        Example of a gui class (Widget) just for demonstration.
        e. g. it could be a text field in GUI.
    """
    def __init__(self, data):
        self._data = data
        #data.register_observer(self._data_updated)
        self._redraw()

    def _redraw(self):
        print('in _redraw(): ' + data.data)

    @Observable.register_observer
    def _data_updated(self, **kwargs):
        """
            This is the callback that is called by the Observable if the
            data changed.
        """
        print('in _data_updated() - kwargs: {}'.format(kwargs))
        self._redraw()


if __name__ == '__main__':
    data = TheData('DATA')
    gui = TheGUIElement(data)

    data.data = 'SECOND DATA'

This code doesn't work because of this error.

Traceback (most recent call last):
  File "./o.py", line 42, in <module>
    class TheGUIElement:
  File "./o.py", line 55, in TheGUIElement
    @Observable.register_observer
TypeError: register_observer() missing 1 required positional argument: 'callback'

It is unclear to me how to use a decorator for to register the observers (e.g. TheGUIElement).

Answer 1

To register the callback, you need to have an actual object. In your code, how is @Observable.register_observer supposed to find which instance is should register on?

Please drop that Observable thing that's a javaism, cumbersome in python.

Look at this.

#!/usr/bin/env python

class SomeData(object):
    def __init__(self, value):
        self.callbacks = []
        self.foo = value

    def register(self, callback):
        self.callbacks.append(callback)
        return callback

    def notify(self, *args, **kwargs):
        for callback in self.callbacks:
            callback(self, *args, **kwargs)

class SomeGUI(object):
    def redraw(self, obj, key, newvalue):
        print('redrawing %s with value %s' % (self, newvalue))


if __name__ == '__main__':
    my_data = SomeData(42)

    # Register some function using decorator syntax
    @my_data.register
    def print_it(obj, key, value):
        print('Key %s changed to %s' % (key, value))

    # Register the SomeGUI element
    my_gui = SomeGUI()
    my_data.register(my_gui.redraw)

    # Try changing it. Note my_data is dumb for now, notify manually.
    my_data.foo = 10
    my_data.notify("foo", 10)

I intentionally removed automatic notifications to illustrate registration by itself.

Let's add it back. But there is no point using that Observable class. Let's make it lighter, simply defining an event class.

#!/usr/bin/env python3


class Event(object):
    def __init__(self):
        self.callbacks = []

    def notify(self, *args, **kwargs):
        for callback in self.callbacks:
            callback(*args, **kwargs)

    def register(self, callback):
        self.callbacks.append(callback)
        return callback

class SomeData(object):
    def __init__(self, foo):
        self.changed = Event()
        self._foo = foo

    @property
    def foo(self):
        return self._foo

    @foo.setter
    def foo(self, value):
        self._foo = value
        self.changed.notify(self, 'foo', value)

class SomeGUI(object):
    def redraw(self, obj, key, newvalue):
        print('redrawing %s with value %s' % (self, newvalue))


if __name__ == '__main__':
    my_data = SomeData(42)

    # Register some function using decorator syntax
    @my_data.changed.register
    def print_it(obj, key, value):
        print('Key %s changed to %s' % (key, value))

    # Register the SomeGUI element
    my_gui = SomeGUI()
    my_data.changed.register(my_gui.redraw)

    # Try changing it.
    my_data.foo = 10

As you probably noted now, the decorator syntax is useful in those circumstances:

• You have a single registry. Either a singleton or the class itself class are first-order objects, and most are singletons.
• You dynamically define the function and register it as you go.

Now, those manual getters/setters you have are cumbersome as well, if you have many why not factor them out?

#!/usr/bin/env python3


class Event(object):
    def __init__(self):
        self.callbacks = []

    def notify(self, *args, **kwargs):
        for callback in self.callbacks:
            callback(*args, **kwargs)

    def register(self, callback):
        self.callbacks.append(callback)
        return callback

    @classmethod
    def watched_property(cls, event_name, key):
        actual_key = '_%s' % key

        def getter(obj):
            return getattr(obj, actual_key)

        def setter(obj, value):
            event = getattr(obj, event_name)
            setattr(obj, actual_key, value)
            event.notify(obj, key, value)

        return property(fget=getter, fset=setter)


class SomeData(object):
    foo = Event.watched_property('changed', 'foo')

    def __init__(self, foo):
        self.changed = Event()
        self.foo = foo



class SomeGUI(object):
    def redraw(self, obj, key, newvalue):
        print('redrawing %s with value %s' % (self, newvalue))


if __name__ == '__main__':
    my_data = SomeData(42)

    # Register some function using decorator syntax
    @my_data.changed.register
    def print_it(obj, key, value):
        print('Key %s changed to %s' % (key, value))

    # Register the SomeGUI element
    my_gui = SomeGUI()
    my_data.changed.register(my_gui.redraw)

    # Try changing it.
    my_data.foo = 10

For reference, all three programs output the exact same thing:

$ python3 test.py
Key foo changed to 10
redrawing <__main__.SomeGUI object at 0x7f9a90d55fd0> with value 10

Answer 2

Even though the thread is kinda old (probably the problem is already solved), I would like to share a solution of mine to the "Decorated Observer Pattern" problem:

https://pypi.org/project/notifyr/

I created a package that implements decorators which add the observer-observed methods/attributes to python classes. I managed to use the package in a Django project too, but with a few adaptations (the .observers attribute is not persisted in the database, so I had to load the list of observers into it every time I expected to notify them).

Here is an implementation example:

Original Code:

class Dog(object):
  def __init__(self, name):
      self.name = name

  def bark(self):
      print('Woof')

  def sleep(self):
      print(self.name, 'is now asleep: ZZzzzzZzzZ...')

class Person(object):
  def __init__(self, name):
      self.name = name

  def educate_dog(self, dog):
      print(self.name + ':','Sleep,', dog.name)
      dog.sleep()

Suppose we want a person to educate a dog every time the animal barks:

from notifyr.agents import observed, observer
from notifyr.functions import target

@observed
class Dog(object):
  def __init__(self, name):
      self.name = name

  @target
  def bark(self):
      print('Woof')

  def sleep(self):
      print(self.name, 'is now asleep: ZZzzzzZzzZ...')

@observer('educate_dog')
class Person(object):
  def __init__(self, name):
      self.name = name

  def educate_dog(self, dog):
      print(self.name + ':','Sleep,', dog.name)
      dog.sleep()

Given the decorated classes, it is possible to achieve the following result:

d = Dog('Tobby')
p = Person('Victor')

d.attach(p) # Victor is now observing Tobby

d.bark()
# Woof
# Victor: Sleep, Tobby
# Tobby is now asleep: ZZzzzzZzzZ...

The package is still very primitive, but it presents a working solution to this type of situation.