How to give FileField's upload_to parameter a dynamic function?

[Xx Johnsua]

I wrote 3 models:

...
def generate_mugshot(instance, filename):
    filename, ext = os.path.splitext(filename)
    return "mugshot/%s/%s%s" % (instance.id, uuid4(), ext)

def generate_team_photo(instance, filename):
    filename, ext = os.path.splitext(filename)
    return "team/%s/%s%s" % (instance.id, uuid4(), ext)

def generate_gift_photo(instance, filename):
    filename, ext = os.path.splitext(filename)
    return "gift/%s/%s%s" % (instance.id, uuid4(), ext)


class User(AbstractUser):
    ...
    mugshot = models.ImageField('头像', upload_to= generate_mugshot, validators=[check_image_extension], null=True, blank=True)

class Team(models.Model):
    ...
    photo = models.ImageField('队标', upload_to= generate_team_photo, validators=[check_image_extension], blank=True, null=True)

class Gift(models.Model):
    ...
    photo = models.ImageField('图片', upload_to= generate_gift_photo, validators=[check_image_extension], null=True, blank=True)

If I want to add a new model, I have to write a new duplicate function "generate_something(instance, filename)", it is not pythonic. So I modify it to following code:

def generate_attachment_by_id(prefix):
    def _return_function(instance, filename):
        filename, ext = os.path.splitext(filename)
        return "%s/%s/%s%s" % (prefix, instance.id, uuid4(), ext)
    return _return_function

...

class User(AbstractUser):
    mugshot = models.ImageField('头像', upload_to=generate_attachment_by_id('mugshot'), validators=[check_image_extension], null=True, blank=True)

class Team(models.Model):
    photo = models.ImageField('队标', upload_to=generate_attachment_by_id('team'), validators=[check_image_extension], blank=True, null=True)

class Gift(models.Model):
    photo = models.ImageField('图片', upload_to=generate_attachment_by_id('gift'), validators=[check_image_extension], null=True, blank=True)

Then I execute makemigrations, an error raised:

ValueError: Could not find function _return_function in app.account.models.

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/1.10/topics/migrations/#serializing-values

So, how to solve the problem?

Thank you for helping me...

[ludovic coues]

Instead of passing prefix as an argument, you could set it as an
attribute of your models. Something like this:

class User(AbstractUser):
prefix = "mugshot"
mugshot = models.ImageField('AB', upload_to= generate_attachment, ...)

then generate_attachment would be something like that:

def generate_mugshot(instance, filename):
filename, ext = os.path.splitext(filename)

return "%s/%s/%s%s" % (instance.prefix, instance.id, uuid4(), ext)


Should work

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Cordialement, Coues Ludovic
+336 148 743 42