举一个例子：当学生名字发生改变之后发布一条公告。
from django.db.models import signals
from django.dispatch import receiver
from students.models import Student
from .models import Announcement
@receiver(signals.post_init, sender=Student)
def welcome_student(instance, **kwargs):
instance.__original_name = instance.name
@receiver(signals.post_save, sender=Student)
def welcome_student(instance, created, **kwargs):
if not created and instance.__original_name != instance.name:
Announcement.objects.create(content=
'Student %s has renamed to %s' % (instance.__original_name, instance.name))
简单的说就是在该模型广播 post_init 信号的时候，在模型对象中缓存当前的字段值；在模型广播 post_save （或 pre_save ）的时候，比较该模型对象的当前的字段值与缓存的字段值，如果不相同则认为该字段值发生了变化。