# documentação Django 2.0.8

# Código fonte para django.contrib.messages.middleware

    from django.conf import settings
    from django.contrib.messages.storage import default_storage
    from django.utils.deprecation import MiddlewareMixin

    class MessageMiddleware(MiddlewareMixin):
        """
        Middleware that handles temporary messages.
        """

        def process_request(self, request):
            request._messages = default_storage(request)

        def process_response(self, request, response):
            """
            Update the storage backend (i.e., save the messages).

            Raise ValueError if not all messages could be stored and DEBUG is True.
            """
            # A higher middleware layer may return a request which does not contain
            # messages storage, so make no assumption that it will be there.
            if hasattr(request, '_messages'):
                unstored_messages = request._messages.update(response)
                if unstored_messages and settings.DEBUG:
                    raise ValueError('Not all temporary messages could be stored.')
            return response