python - 使用 lambda 作为属性的默认值时，Django 1.7.1 Makemigrations 失败

Question

我使用的是 Django 1.7.1。我的模型如下所示:

from datetime import datetime
from django.db import models

class myModel(models.Model):
    x = models.CharField(max_length=254,null=True, blank=True,)

一切都运行得很好。

但是，当我将以下属性添加到 myModel 时，它会中断:

    y = models.DateTimeField(default=lambda: datetime.utcnow() + timedelta(days=1), editable=False)

manage.py makemigrations 给出以下错误:

ValueError: Cannot serialize function: lambda

这似乎是一个已知的错误:http://comments.gmane.org/gmane.comp.python.django.scm/125724

那么我该如何解决这个问题呢？我需要将 y 的值默认自动设置为从模型创建之日起的 24 小时。

Answer 1

migrations documentation解决这个问题:

Migrations are just Python files containing the old definitions of your models - thus, to write them, Django must take the current state of your models and serialize them out into a file. While Django can serialize most things, there are some things that we just can’t serialize out into a valid Python representation....

Django can serialize the following: Any function or method reference... in module’s top-level scope

Django cannot serialize: Lambdas

因此解决方案很简单:定义一个常规函数并按名称引用它，而不是使用 lambda。

def one_day_hence():
    return datetime.utcnow() + timezone.timedelta(days=1)

class MyModel(models.Model):
    y = models.DateTimeField(default=one_day_hence)