Frequently, when dealing with remote APIs, there's a progression of states with a definitive order and an endpoint.  Often, when comparing the existing state to the proposed state, I want to make sure state is progressing in the correct direction.  The stupid simple version of this is something like:

    ('PROCESSING', _("Processing")),
    ('APPROVED',   _("Approved")),
    ('PROCESSED',  _("Processed")),
    ('SHIPPED',    _("Shipped")),
    ('CANCELLED',  _("Cancelled")),
    ('DECLINED',   _("Card Declined")),
class StateIndexManager(object):
    def __init__(self, states):
        self.states = states
    def __call__(self, value):
        cp = [i[0] for i in self.states]
        return cp.index(value)
state_index = StateIndexManager(ORDER_STATES)

This is obviously for a shopping cart, my arch nemesis. (Actually, every programmer's arch nemesis. Shopping carts are teh evulzh).  With something like this, you can create code in a Django model that reads:

    def _get_status(self):
        return self.order_status
    def _set_status(self, value):
        if state_index(value) > state_index(self.order_status):
            self.order_status = value
    status = property(_get_status, _set_status)

Feel free to add a raise() in there if the state is going in the wrong direction.

I just find myself doing this often enough that having a "personal snippet" location seemed like a good idea.