Python: Callbacks, Delegates, ... ? What is common?

Question

Just want to know what's the common way to react on events in python. There are several ways in other languages like callback functions, delegates, listener-structures and so on. Is there a common way? Which default language concepts or additional modules are there and which can you recommend?

Answer 1

Personally I don't see a difference between callbacks, listeners, and delegates.

The observer pattern (a.k.a listeners, a.k.a "multiple callbacks") is easy to implement - just hold a list of observers, and add or remove callables from it. These callables can be functions, bound methods, or classes with the __call__ magic method. All you have to do is define the interface you expect from these - e.g. do they receive any parameters.

class Foo(object):   def __init__(self):     self._bar_observers = []    def add_bar_observer(self, observer):     self._bar_observers.append(observer)    def notify_bar(self, param):     for observer in self._bar_observers:       observer(param)  def observer(param):   print "observer(%s)" % param  class Baz(object):   def observer(self, param):     print "Baz.observer(%s)" % param  class CallableClass(object):   def __call__(self, param):     print "CallableClass.__call__(%s)" % param  baz = Baz()  foo = Foo()  foo.add_bar_observer(observer) # function foo.add_bar_observer(baz.observer) # bound method foo.add_bar_observer(CallableClass()) # callable instance  foo.notify_bar(3) 

Answer 2

I can't speak for common approaches, but this page (actual copy is unavailable) has an implementation of the observer pattern that I like.

Here's the Internet Archive link: http://web.archive.org/web/20060612061259/http://www.suttoncourtenay.org.uk/duncan/accu/pythonpatterns.html