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?'''