我们可以在Django的验证函数中调整ImageField的大小吗？

Question

Is it legal to modify data from inside a validation function in Django? I need to autoresize images from ImageFields .

In the docs it says that these kind of functions should raise ValidationError , but instead I need to actually modify the data ( image in this case). I'd like to do it like this because I have to apply this to several models. If I could do this, I wouldn't need to change the forms in the admin.

def image_field_autoresize(image):
    if image.width > MAX_IMAGE_WIDTH:
        raise ValidationError(u'image width error')
    if image.height > MAX_IMAGE_HEIGHT:
        raise ValidationError(u'image height error')

Answer 1

After a discussion with @karthikr, I can conclude with the following:

If you need to change data in a validation process, you can override clean_<fieldname>() from the ModelForm class. In your clean() method in ModelForm you shouldn't do that. And if you need to do some image resizing I found that the best place is Model's save() method.

I've solved the problem like this:

I've created a new module named image.py that contains a function that takes as parameter the image's absolute path, implements the resizing logic and modifies the image size. In my case, the "resizing logic" reduces the image proportionally if the width is bigger that a given size.

from PIL import Image
from django.conf import settings


def autoresize_image(image_path):
    image = Image.open(image_path)
    width = image.size[0]
    if width > settings.IMAGE_MAX_WIDTH:
        height = image.size[1]
        reduce_factor = settings.IMAGE_MAX_WIDTH / float(width)
        reduced_width = int(width * reduce_factor)
        reduced_height = int(height * reduce_factor)
        image = image.resize((reduced_width, reduced_height), Image.ANTIALIAS)
        image.save(image_path)

I call this function from save() method in the Model that contains an ImageField to be resized. Points to keep in mind here: call super() before trying to resize the image or it won't be found as it won't be already saved in your hdd and if the ImageField is not required, check if value is given. It looks like:

def save(self, *args, **kwargs):
    super(Startup, self).save(*args, **kwargs)
    if self.logo:
        autoresize_image(self.logo.path)

Thanks to @Jingo as well for his comment. It's been very useful :)