      """
      The default authorization token model.
      """
      key = models.CharField(max_length=40, primary_key=True)
      user = models.OneToOneField(AUTH_USER_MODEL, related_name='auth_token')
      created = models.DateTimeField(auto_now_add=True)
  
      def save(self, *args, **kwargs):
          if not self.key:
              self.key = self.generate_key()
 
</s> remove                                help_text='Target', verbose_name='Target')
 </s> add                                help_text='Target', verbose_name='Target',
                               on_delete=models.CASCADE) </s> remove                                   related_name='nullable_source')
 </s> add                                   related_name='nullable_source', on_delete=models.CASCADE) </s> remove                                verbose_name='Optional target object')
 </s> add                                verbose_name='Optional target object',
                               on_delete=models.CASCADE)
    user = models.OneToOneField(AUTH_USER_MODEL, related_name='auth_token',
                                on_delete=models.CASCADE)
      """
      The default authorization token model.
      """
      key = models.CharField(max_length=40, primary_key=True)
    user = models.OneToOneField(AUTH_USER_MODEL, related_name='auth_token',
                                on_delete=models.CASCADE)
      created = models.DateTimeField(auto_now_add=True)
  
      def save(self, *args, **kwargs):
          if not self.key:
              self.key = self.generate_key()
keep keep keep keep replace keep keep keep keep keep
 Regarding #3729

python -Werror generates warnings informing that on_delete is a required keyword in Django 2.0 
rest_framework/authtoken/models.py