              if self.should_redirect_with_slash(request):
                  return self.response_redirect_class(self.get_full_path_with_slash(request))
  
          if settings.USE_ETAGS and self.needs_etag(response):
              if not response.has_header('ETag'):
                  set_response_etag(response)
  
              if response.has_header('ETag'):
                  return get_conditional_response(
 
</s> remove     If-Modified-Since, the response is replaced by an HttpNotModified.
 </s> add     If-Modified-Since, the response is replaced by an HttpNotModified. An ETag
    header is added if needed. </s> remove Handles conditional GET operations. If the response has a ``ETag`` or
 </s> add Handles conditional GET operations. If the response doesn't have an ``ETag``
header, the middleware adds one if needed. If the response has a ``ETag`` or
            warnings.warn(
                "The USE_ETAGS setting is deprecated in favor of "
                "ConditionalGetMiddleware which sets the ETag regardless of "
                "the setting. CommonMiddleware won't do ETag processing in "
                "Django 2.1.",
                RemovedInDjango21Warning
            )
              if self.should_redirect_with_slash(request):
                  return self.response_redirect_class(self.get_full_path_with_slash(request))
  
          if settings.USE_ETAGS and self.needs_etag(response):
            warnings.warn(
                "The USE_ETAGS setting is deprecated in favor of "
                "ConditionalGetMiddleware which sets the ETag regardless of "
                "the setting. CommonMiddleware won't do ETag processing in "
                "Django 2.1.",
                RemovedInDjango21Warning
            )
              if not response.has_header('ETag'):
                  set_response_etag(response)
  
              if response.has_header('ETag'):
                  return get_conditional_response(
keep keep keep add keep keep keep keep keep
 Fixed #26447 -- Deprecated settings.USE_ETAGS in favor of ConditionalGetMiddleware. 
django/middleware/common.py