Decorating A Class To Monitor Attribute Changes

I want to have classes that automatically send notifications to subscribers whenever one of their attributes change. So if I would write this code: @ChangeMonitor class ChangingCla

Solution 1:

You'd have to add a __setattr__() method:

defChangeMonitor(cls):
    _sentinel = object()
    old_setattr = getattr(cls, '__setattr__', None)
    def__setattr__(self, name, value):
        old = getattr(self, name, _sentinel)
        if old notis _sentinel and old != value:
            print"Old {0} = {1!r}, new {0} = {2!r}".format(name, old, value)
        if old_setattr:
            old_setattr(self, name, value)
        else:
            # Old-style class
            self.__dict__[name] = value

    cls.__setattr__ = __setattr__

    return cls

This should handle existing __setattr__ hooks as well. The _sentinel is used to allow None as the old value too.

Demo:

>>>changer = ChangingClass(5)>>>changer.x = 6
Old x = 5, new x = 6
>>>changer.x = 6>>># nothing printed...>>>changer.x = None
Old x = 6, new x = None
>>>changer.x = 6
Old x = None, new x = 6