class Prefs(object):

def __init__(self):

self.prefs = {}

self.callbacks = {}

def set(self, key, value):

"""Set the value of `key` preference to `value`."""

if key in self.callbacks:

self.callbacks[key](value)

else:

self.prefs[key] = value

def add(self, key, value):

"""Add an entry to a list preference

Add `value` to the list of entries for the `key` preference.

"""

if not key in self.prefs:

self.prefs[key] = []

self.prefs[key].append(value)

def get(self, key, default=None):

"""Get the value of the key preference"""

return self.prefs.get(key, default)

def add_callback(self, key, callback):

"""Add `key` preference with `callback` function

Whenever `key` is set the callback is called with the

given `value` as parameter.

"""

self.callbacks[key] = callback

def __setitem__(self, key, value):

self.set(key, value)

def __getitem__(self, key):

return self.get(key)