Python subclass method decoration
Modifying the class after defining it smells. That is, its not necessarily a bad idea but one should explore options before resorting to it.

When "decorating" a function you should use the functools.wraps decorator. It will make the decorated function look more like the original.

The code you are replacing looks like:

def importance_changed(self):
    return_value = super(Model, self).importance_changed()
    self.modelChanged.emit()
    return return_value


Your code is harder to read and longer then just writing those two functions. Its not really worth implementing what you've done just for two functions. However, if you have a lot of functions then the situation changes.

Your emit_decorator function isn't a decorator. Dectorators have to take exactly one argument, (the function they are wrapping) to be used with the @syntax.