sorl-snippet.py

#inside template tag called app_tags.py
@register.filter
def image_dimensions(dimensions, ratio):
    print(dimensions)
    width, height = dimensions
    if 'x' in ratio:
        return ratio
    x,y = ratio.split(":")
    x,y = int(x),int(y)
    ratio = x/y 
    req_width = int(height*ratio)
    if(req_width<width):
        print('{0}x{1}'.format(req_width,height))
        return '{0}x{1}'.format(req_width,height)
    else:
        print('{0}x{1}'.format(width,int(width/ratio)))
        return '{0}x{1}'.format(width,int(width/ratio))
      
#models for image
class ModelName(models.Model):
    ...

    def get_dimensions(self):
        return [self.image.width, self.image.height]