Python 2.7 未绑定方法与 Django 1.7 迁移

Question

我正在升级到 Django 1.7.4 并使用新的内置迁移，但是在尝试运行 makemigrations:

时出现以下错误

ValueError: Could not find function upload_callback in app_utils.
Please note that due to Python 2 limitations, you cannot serialize unbound 
method functions (e.g. a method declared and used in the same class body). 
Please move the function into the main module body to use migrations.For more information, 
see https://docs.djangoproject.com/en/1.7/topics/migrations/#serializing-values

我的模型定义：

class Discount(models.Model):
    banner = models.ImageField(
        help_text=_("Banner image for this discount"),
        upload_to=upload_to('discounts/', 'title'),
        blank=True,
        null=True
    )

以及我在 app_utils.py 中的上传回调：

def upload_to(path, attribute=None):
    def upload_callback(instance, filename):
        compact = filename[:50]
        try:
            attr= unicode( slugify( getattr(instance, attribute) ) )
            mypath = '%s/%s/%s' % (path, attr, compact)
        except:
            mypath = '%s%s' % (path, compact)
        return mypath
    return upload_callback

根据错误信息，表明upload_callback未绑定。所以我尝试将 upload_callback 拉出包装函数，但找不到将额外的 path 和 attribute 参数传入 upload_to 的方法。 Django 的文档并不清楚如何做到这一点，它只指定 instance 和 filename 是必需的。

理想情况下，我想要的是：

def upload_to(instance, filename, path, attribute=None):
    ...

我有什么想法可以实现这个目标吗？

Answer 1

在网络上认真深入挖掘之后，我发现了这个 Django 错误报告。该解决方案似乎创建了一个可调用工厂 class:

https://code.djangoproject.com/ticket/22999

更多讨论在这里：Django - Cannot create migrations for ImageField with dynamic upload_to value

Python 2.7 未绑定方法与 Django 1.7 迁移

Python 2.7 unbound method with Django 1.7 migrations

python

django

python-2.7

django-models

django-south