Question

我有一个基类Image，我在其中设置了上传目录。这完全适用。因为，据我所知，到目前为止，不可能覆盖Django中父类的字段，我想我会这样尝试。

class Image(models.Model):
    """
    Images base class
    """
    upload_directory = "uploads/images"
    image = models.ImageField(upload_to=upload_directory)

但是当覆盖子类中的upload_directory时，这根本就没有结果。现在，图像仍然上载到父类中设置的目录中。

class ActivityThumbnail(Image):
    """
    Thumbnail images for activities
    """
    upload_directory = "uploads/images/thumbnails/activities"

为每个子类设置上传目录路径的正确方法是什么？

Answer 1

在您的情况下，您应该使用函数来获取上传路径，包括文件名。

def upload_to_path(instance, filename):
    return '{upload_dir}/{filename}'.format(
        upload_dir=instance.upload_dir,
        filename=filename
    )

class Image(models.Model):
    """
    Images base class
    """
    upload_dir = "uploads/images"
    image = models.ImageField(upload_to=upload_to_path)


class ActivityThumbnail(Image):
    """
    Thumbnail images for activities
    """
    upload_dir = "uploads/images/thumbnails/activities"

看看这个https://docs.djangoproject.com/en/1.8/ref/models/fields/#django.db.models.FileField.upload_to

Answer 2

您正在混合类变量和实例变量。

当您处理类变量（相当于Java和C ++中的静态变量）时，您应始终使用<class-name>.<variable-name> 惯例。虽然使用实例名称访问它是合法的。

这样做会让您意识到您不需要为每个实例设置新的upload_dir，因此您可以在该流/循环之外进行更多全局初始化。

class base: var1 = 'from base' # can do without this @classmethod def set_var1(cls, v): cls.var1 = v print 'before derived definitions: base.var1', base.var1 # wrong.. class derived1(base): # This is a different variable (FQDN: derived1.var1), it DOES NOT override the base.var1 var1 = 'from derived1' print 'after derived1 definition: base.var1', base.var1 # correct.. class derived2(base): # don't create a new variable derived2.var1 pass b = base() d1 = derived1() d2 = derived2() base.var1 = 'overridden from main' print '\nbase.var1', base.var1, 'b.var1', b.var1 print 'derived1.var1', derived1.var1, 'd1.var1', d1.var1 print 'derived2.var1', derived2.var1, 'd2.var1', d2.var1 # if you want to complicate things and make it look like you know $#!T # create a classmethod, annotated appropriately to set the new value base.set_var1('overridden from main 2') print '\nbase.var1', base.var1, 'b.var1', b.var1 print 'derived1.var1', derived1.var1, 'd1.var1', d1.var1 print 'derived2.var1', derived2.var1, 'd2.var1', d2.var1 # if you REALLY wanna complicate things, e.g. because you have no control over # code in base.py then you should be able to use the python RTTI functions and # hack your way to set teh value on appropriate cls object using __set_attribute__

打印：

before derived definitions: base.var1 from base after derived1 definition: base.var1 from base base.var1 overridden from main b.var1 overridden from main derived1.var1 from derived1 d1.var1 from derived1 derived2.var1 overridden from main d2.var1 overridden from main base.var1 overridden from main 2 b.var1 overridden from main 2 derived1.var1 from derived1 d1.var1 from derived1 derived2.var1 overridden from main 2 d2.var1 overridden from main 2

覆盖硬编码值Python Django类

2 个答案: