          """
          Handle any exception that occurs, by returning an appropriate response,
          or re-raising the error.
          """
          if isinstance(exc, exceptions.Throttled):
              # Throttle wait header
              self.headers['X-Throttle-Wait-Seconds'] = '%d' % exc.wait
  
          if isinstance(exc, (exceptions.NotAuthenticated,
                              exceptions.AuthenticationFailed)):
 
</s> remove from rest_framework.throttling import UserRateThrottle, ScopedRateThrottle
 </s> add from rest_framework.throttling import BaseThrottle, UserRateThrottle, ScopedRateThrottle
        if isinstance(exc, exceptions.Throttled) and exc.wait is not None:
          """
          Handle any exception that occurs, by returning an appropriate response,
          or re-raising the error.
          """
        if isinstance(exc, exceptions.Throttled) and exc.wait is not None:
              # Throttle wait header
              self.headers['X-Throttle-Wait-Seconds'] = '%d' % exc.wait
  
          if isinstance(exc, (exceptions.NotAuthenticated,
                              exceptions.AuthenticationFailed)):
keep keep keep keep replace keep keep keep keep keep
 don't set X-Throttle-Wait-Second header if throttle wait is None 
rest_framework/views.py