'Remove a callback from this object'

def remove_on_change(self, attr, *callbacks):

    if (len(callbacks) == 0):
        raise ValueError('remove_on_change    takes    an    attribute    name    and    one    or    more    callbacks,    got    only    one    parameter')
    _callbacks = self._callbacks.setdefault(attr, [])
    for callback in callbacks:
        _callbacks.remove(callback)
