Question

I’m working on my first Python project, and I’m already missing events in my classes. Perhaps it’s not even called events in Python, but I would like to create “groups” in my classes to which function references can be added. At some point in my class all function references in my group would execute.

Is this built into Python? (I’m using 2.7 at the moment)

Answer 1

Python doesn’t have any sort of event system built-in, but it’s could be implemented pretty simply. For example:

class ObjectWithEvents(object):
    callbacks = None

    def on(self, event_name, callback):
        if self.callbacks is None:
            self.callbacks = {}

        if event_name not in self.callbacks:
            self.callbacks[event_name] = [callback]
        else:
            self.callbacks[event_name].append(callback)

    def trigger(self, event_name):
        if self.callbacks is not None and event_name in self.callbacks:
            for callback in self.callbacks[event_name]:
                callback(self)

class MyClass(ObjectWithEvents):
    def __init__(self, contents):
        self.contents = contents

    def __str__(self):
        return "MyClass containing " + repr(self.contents)

def echo(value): # because "print" isn't a function...
    print value

o = MyClass("hello world")
o.on("example_event", echo)
o.on("example_event", echo)
o.trigger("example_event") # prints "MyClass containing \"Hello World\"" twice