Question

在Django中使用signal时遇到一些问题。

由于函数内部post_save，

instance.save()发生递归。

但奇怪的是只有一种情况发生递归。

不发生递归的情况。

models.py

class Product(TimeStampedModel):
    name = models.CharField(max_length=120)
    slug = models.SlugField(null=True, blank=True)
    description = models.CharField(max_length=400, blank=True)
    is_active = models.BooleanField(default=True)

    objects = ProductManager()

    class Meta:
        ordering = ('-created',)

    def __str__(self):
        return self.name

    def get_absolute_url(self):
        return reverse(
            "products:product_detail",
            kwargs={
                "slug": self.slug,
            }
        )

signals.py

@receiver(post_save, sender=Product)
def post_save_product(sender, instance, created, **kwargs):
    if not instance.slug:
        instance.slug = slugify(instance.name, allow_unicode=True)
        instance.save()

当我使用Product创建Product.objects.create()时，它不会发生递归。

发生递归的案例

models.py

class Variation(TimeStampedModel):
    COLOR_CHOICES = (
        ('black', '흑백'),
        ('single', '단색'),
        ('multi', '컬러'),
    )
    price = models.DecimalField(
        decimal_places=2,
        max_digits=15,
        blank=True,
        null=True,
    )
    product = models.ForeignKey(Product)
    color = models.CharField(
        max_length=10,
        choices=COLOR_CHOICES,
        default='흑백'
    )
    is_active = models.BooleanField(default=True)

    class Meta:
        ordering = ('product',)

    def __str__(self):
        return "{product} - {color}".format(
            product=self.product,
            color=self.color
        )

signals.py

@receiver(post_save, sender=Variation)
def post_save_variation(sender, instance, created, **kwargs):
    if not instance.price:
        if instance.color == '흑백':
            instance.price = 40000
        elif instance.color == '단색':
            instance.price = 50000
        elif instance.color == '컬러':
            instance.price = 60000
        instance.save()

这种情况发生递归错误：

File "/Users/Chois/.pyenv/versions/spacegraphy/lib/python3.5/site-packages/django/db/models/base.py", line 708, in save
    force_update=force_update, update_fields=update_fields)
  File "/Users/Chois/.pyenv/versions/spacegraphy/lib/python3.5/site-packages/django/db/models/base.py", line 745, in save_base
    update_fields=update_fields, raw=raw, using=using)
  File "/Users/Chois/.pyenv/versions/spacegraphy/lib/python3.5/site-packages/django/dispatch/dispatcher.py", line 192, in send
    response = receiver(signal=self, sender=sender, **named)
  File "/Users/Chois/Dropbox/Workspace/django/spacegraphy-project/spacegraphy/products/signals/post_save.py", line 24, in post_save_variation
    instance.save()
  File "/Users/Chois/.pyenv/versions/spacegraphy/lib/python3.5/site-packages/django/db/models/base.py", line 708, in save
    force_update=force_update, update_fields=update_fields)
  File "/Users/Chois/.pyenv/versions/spacegraphy/lib/python3.5/site-packages/django/db/models/base.py", line 745, in save_base
    update_fields=update_fields, raw=raw, using=using)
  File "/Users/Chois/.pyenv/versions/spacegraphy/lib/python3.5/site-packages/django/dispatch/dispatcher.py", line 192, in send
    response = receiver(signal=self, sender=sender, **named)
  File "/Users/Chois/Dropbox/Workspace/django/spacegraphy-project/spacegraphy/products/signals/post_save.py", line 24, in post_save_variation
    instance.save()
  File "/Users/Chois/.pyenv/versions/spacegraphy/lib/python3.5/site-packages/django/db/models/base.py", line 708, in save
    force_update=force_update, update_fields=update_fields)
  File "/Users/Chois/.pyenv/versions/spacegraphy/lib/python3.5/site-packages/django/db/models/base.py", line 736, in save_base
    updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)
  File "/Users/Chois/.pyenv/versions/spacegraphy/lib/python3.5/site-packages/django/db/models/base.py", line 796, in _save_table
    base_qs = cls._base_manager.using(using)
  File "/Users/Chois/.pyenv/versions/spacegraphy/lib/python3.5/site-packages/django/db/models/manager.py", line 122, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/Users/Chois/.pyenv/versions/spacegraphy/lib/python3.5/site-packages/django/db/models/manager.py", line 214, in get_queryset
    return self._queryset_class(model=self.model, using=self._db, hints=self._hints)
  File "/Users/Chois/.pyenv/versions/spacegraphy/lib/python3.5/site-packages/django/db/models/query.py", line 171, in __init__
    self.query = query or sql.Query(self.model)
  File "/Users/Chois/.pyenv/versions/spacegraphy/lib/python3.5/site-packages/django/db/models/sql/query.py", line 155, in __init__
    self.where = where()
RecursionError: maximum recursion depth exceeded while calling a Python object

我认为这两种情况具有相同的结构，但只有一种情况发生递归。

不知道为什么。需要帮助，谢谢。

Answer 1

保存前断开信号，然后重新连接。 https://docs.djangoproject.com/en/1.10/topics/signals/#disconnecting-signals

def post_save_product(sender, instance, **kwargs):
    post_save.disconnect(post_save_product, sender=sender)
    instance.do_stuff()
    instance.save()
    post_save.connect(post_save_product, sender=sender)
post_save.connect(post_save_product, sender= Product)

Answer 2

在第二种情况下，您将dojo/store/JsonRest的数据库值与显示值进行比较。这些永远不会匹配。您应该检查数据库值：

instance.color

同样，您应将默认值设置为数据库值，即@receiver(post_save, sender=Variation) def post_save_variation(sender, instance, created, **kwargs): if not instance.price: if instance.color == 'black': instance.price = 40000 elif instance.color == 'single': instance.price = 50000 elif instance.color == 'multi': instance.price = 60000 instance.save()。

在原始代码中，所有检查都将失败，default = 'black'永远不会更新为非空值。对instance.price的调用将再次触发instance.save()信号，post_save仍为真，并且在不设置价格的情况下再次保存实例。这是你所看到的无限递归。

在第一个示例中，not instance.price始终设置为非空值，因此当第二次触发slug信号时，post_save检查将失败，并且该实例不会第三次保存。

在这两种情况下，如果没有设置段塞/价格，您至少要保存实例两次。为防止这种情况，您可以使用if not instance.slug信号。您不必再在信号处理程序中保存实例：

pre_save

Answer 3

如果您想避免post_save信号中的递归，只需使用Model.objects.filter(id=id).update(object=object)

Answer 4

只需使用pre_save，就无需在其内部再次使用.save()方法。

如何在Django中阻止post_save递归？

4 个答案: