Django error on custom session middleware when calling request.session.save on process_response

Question

Traceback (most recent call last):
  File "/home/user/.virtualenvs/env/lib/python3.6/site-packages/django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "/home/user/.virtualenvs/env/lib/python3.6/site-packages/django/utils/deprecation.py", line 93, in __call__
    response = self.process_response(request, response)
  File "/home/user/Desktop/project/my_project/app1/middleware.py", line 98, in process_response
    request.session.save()
  File "/home/user/.virtualenvs/env/lib/python3.6/site-packages/django/contrib/sessions/backends/db.py", line 83, in save
    obj = self.create_model_instance(data)
  File "/home/user/.virtualenvs/env/lib/python3.6/site-packages/django/contrib/sessions/backends/db.py", line 70, in create_model_instance
    session_data=self.encode(data),
  File "/home/user/.virtualenvs/env/lib/python3.6/site-packages/django/contrib/sessions/backends/base.py", line 96, in encode
    serialized = self.serializer().dumps(session_dict)
  File "/home/user/.virtualenvs/env/lib/python3.6/site-packages/django/core/signing.py", line 87, in dumps
    return json.dumps(obj, separators=(',', ':')).encode('latin-1')
  File "/usr/local/lib/python3.6/json/__init__.py", line 238, in dumps
    **kw).encode(obj)
  File "/usr/local/lib/python3.6/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/local/lib/python3.6/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "/usr/local/lib/python3.6/json/encoder.py", line 180, in default
    o.__class__.__name__)
TypeError: Object of type '__proxy__' is not JSON serializable

I've already checked the values in request.session.keys by logging each value and their types. None is a __proxy__ object. Also, it's unlikely that a str is actually a django functional.__proxy__ object or some translation object because it would show. Is it possible to be a django error?

Django==2.1.4 The middleware I'm using

class CustomMiddleware(SessionMiddleware):
   ...

This only happens to one of my view that handles post method and returns a JSON response so it's unlikely that the variable causing this error comes from the view as it would show when calling render_json_response(context)

Answer 1

Thanks for the comments, helped me realize I missed something. I used this snippet for catching the faulty variable.

for key in session.keys():
    try:
        json.dumps(session[key])
    catch Exception as e:
        print(key)