Django 使用slug field时遇到的问题

Question

导入models时，它就会报错，不管我改成self.slug或是Post.slug都是这样。提示说self或者Post not defined

class Post(models.Model):
    title=models.CharField(max_length=70)
    body=models.TextField()
    created_time=models.DateTimeField()
    modified_time=models.DateTimeField()
    excerpt=models.CharField(max_length=200,blank=True)
    category=models.ForeignKey(Category)
    tags=models.ManyToManyField(Tag,blank=True)
    author=models.ForeignKey(User)
    views=models.PositiveIntegerField(default=0)
    excerpt=models.CharField(max_length=100,blank=True)
    sluglookup = models.SlugField(
        unique=True,
        default=Post.slug,   ######这里这里这里报错！！！
        max_length=15,
        )
    def __str__(self):
        return self.title

    def get_absolute_url(self):
        return reverse('blog:detail',kwargs={'pk':self.pk})
        
    def increase_views(self):
        self.views+=1
        self.save(update_fields=['views'])
    def save(self,*args,**kwargs):
        if not self.excerpt:
            md=markdown.Markdown(extensions=[
                'markdown.extensions.extra',
                'markdown.extensions.codehilite',
                ])
            self.excerpt=strip_tags(md.convert(self.body))[:54]
        super(Post,self).save(*args,**kwargs)
    def slug(self):
        return slugify(self.title)