ETag "ready" Django model using a modified datetime

etag_modified_model.py

from datetime import datetime
from django.db import models
from django.core.cache import cache

class ModifiedModel(models.Model):
    modified = models.DateTimeField()

    class Meta(object):
        abstract = True

    def save(self, *args, **kwargs):
        etag = self.invalidate_cache()
        last_modified = self.modified
        self.modified = datetime.now()
        try:
            super(ModifiedModel, self).save(*args, **kwargs)
        except Exception, e:
            # Revert if the save failed
            self.modified = last_modified
            cache.set(etag, 1)
            raise e

    def invalidate_cache(self):
        etag = self.generate_etag()
        cache.delete(etag)
        return etag

    def generate_etag(self):
        kwargs = {
            'app': self._meta.app_label,
            'model': self._meta.module_name,
            'id': self.id,
            'version': self.modified.isoformat(),
        }
        return '{app}.{model}-{id},{version}'.format(**kwargs)