Unique token for model

gistfile1.py

class MyModel(models.Models):
    token = models.CharField(max_length=22, unique=True, editable=False)

    def save(self, *args, **kwargs):
        if not self.token:
            import uuid
            import base64
            self.token = base64.urlsafe_b64encode(uuid.uuid4().bytes).strip('=')
        return super(MyModel, self).save(*args, **kwargs)