也许只需要在 初始化 在那里决定你是否有一个有效的号码?

class FormattedPhoneNumber(object):
  def __init__(self, number):
    try:
      as_number = int(number)
    except (TypeError, ValueError):
      self._formatted = None
    else:
      # you have a number, do whatever formatting you need to do
      area_code = number[ 0:3 ]
      exchange = number[ 3:6 ]
      line = number[ 6:13 ]
      self._formatted = "({0}) {1}-{2}".format(area_code,exchange,line)

  def __str__(self):
    return self._formatted

变化

context['colors']=Student.objects.all(
       ).order_by('color').distinct('color')

到

context['colors']=self.student_set.all(
       ).order_by('color').distinct('color')