class POSLocation(models.Model): '''A POSLocation is a point-of-sale location''' name = models.CharField(_('''the name of this point-of-sale location''')) tax_policy = '''What should I put here?'''class ITaxPolicy(object): def calculateTax(self, Order): # Order is another Django model class raise NotImplementedError, 'Subclass me!'class MASalesTax(ITaxPolicy): def calculateTax(self, Order): return taxEverythingExceptClothes(0.05, Order)question = '''How should I make POSLocation.tax_policy reference ITaxPolicy classes so that a POSLocation object can get the appropriate ITaxPolicy object?'''