add如何使 django post_save 信号仅在创建期间运行

我在项目中使用 django-notifications，并且我想在使用信号创建模型时通知特定用户，但是在更新模型时 post_save 也会运行，如何防止这种情况并仅运行 post_save 方法当创建模型时。

模型.py

class Card(models.Model):
    user = models.ForeignKey(User,on_delete=models.CASCADE)
    title = models.CharField(max_length=100)
    description = models.TextField(blank=True)
    list = models.ForeignKey(List, related_name='cards')
    story_points = models.IntegerField(null=True, blank=True)
    business_value = models.IntegerField(null=True, blank=True)

    def __str__(self):
        return "Card: {}".format(self.title)

def my_handler(sender, instance, **kwargs):
    if instance.pk is None:
        notify.send(instance.user, recipient=User.objects.get(pk=1),   target=instance, verb='created')  

post_save.connect(my_handler, sender=Card)    

我尝试使用 if instance.pk is None，但是当我添加此条件时，它根本不运行。

编辑：检查代码是否已创建

def my_handler(sender, instance, created, **kwargs):
    if created:
        notify.send(instance.user, recipient=User.objects.get(pk=1), target=instance, verb='created')  

有一个创建的命名参数，如果它是一个新对象，它将被设置为 True。

看看这里——https://docs.djangoproject.com/en/1.10/ref/signals/#post-save https://docs.djangoproject.com/en/1.10/ref/signals/#post-save

def my_func(sender, instance, created, **kwargs):
    print("Created: ", created)

class MyModel(models.Model):
    x = models.CharField(max_length=255)

post_save.connect(my_func, sender=MyModel)

>>> MyModel.objects.create(f='asdf')
Created:  True
>>> m = MyModel.objects.all().first()
>>> m.x
'asdf'
>>> m.x = 'a'
>>> m.save()
Created:  False