ContentNotRenderedError: The response content must be rendered before it can be accessed (Django Middleware)

Question

I am creating Django middleware for blocking a user when (s)he gets throttled more than 5 times but I am getting ContentNotRenderedError.

Full error msg:

Traceback (most recent call last):
  File "/home/user/Application/utilities/anaconda3/envs/app/lib/python3.8/site-packages/django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "/home/user/Application/utilities/anaconda3/envs/app/lib/python3.8/site-packages/django/utils/deprecation.py", line 119, in __call__
    response = self.process_response(request, response)
  File "/home/user/Application/utilities/anaconda3/envs/app/lib/python3.8/site-packages/django/middleware/common.py", line 113, in process_response
    response.headers['Content-Length'] = str(len(response.content))
  File "/home/user/Application/utilities/anaconda3/envs/app/lib/python3.8/site-packages/django/template/response.py", line 126, in content
    raise ContentNotRenderedError(
django.template.response.ContentNotRenderedError: The response content must be rendered before it can be accessed.
[22/Mar/2022 11:55:03] "GET /api/v1/userdetail/ HTTP/1.1" 500 84321

Middleware class

class BlockMiddleware:
    
    def __init__(self, get_response):
        self.get_response = get_response
        # some other variables
    
    def __call__(self, request):
        # handle_blocking will return "None" if user can access application else "rest_framework.response.Response" object
        blocking_res = self.handle_blocking(request)
        if blocking_res:
            return blocking_res

        response = self.get_response(request)
        # logic for counting how many throttles have left
        # then
        if throttles_left <= 0:
            return Response(
                    data='User is blocked due to exceeding throttles limit.',
                    status=status.HTTP_403_FORBIDDEN
                )
        else:
            return response

Example return of handle_blocking function:

return Response(
    data='User is blocked, please contact the support team.',
    status=status.HTTP_403_FORBIDDEN
)

It works when I remove the middleware and don't use the Response class(return the self.get_response(request) instead). I am unable to figure out the error. What am I doing wrong?

Answer 1

If you really want to return a Response instance, you need to set some properties before returning it:

from rest_framework.renderers import JSONRenderer

class BlockMiddleware:
    def __init__(self, get_response):
        self.get_response = get_response
    
    def __call__(self, request):
        blocking_res = self.handle_blocking(request)
        if blocking_res:
            return blocking_res

        response = self.get_response(request)
        if throttles_left <= 0:
            response = Response(
               data='User is blocked due to exceeding throttles limit.',
               status=status.HTTP_403_FORBIDDEN
            )
            response.accepted_renderer = JSONRenderer()
            response.accepted_media_type = "application/json"
            response.renderer_context = {}
            response.render()
            return response
        else:
            return response

You need to do the same also in your handle_blocking implementation, for example:

from rest_framework.renderers import JSONRenderer
response = Response(
    data='User is blocked, please contact the support team.',
    status=status.HTTP_403_FORBIDDEN
)
response.accepted_renderer = JSONRenderer()
response.accepted_media_type = "application/json"
response.renderer_context = {}
response.render()
return response

These properties are generally set by the api_view decorator that cannot be used with middlewares, so you must set them manually.

Answer 2

Go for 'reponse.data' if you are using RequestFactory. It worked in my case to fetch respose data.