How to add custom error codes to Django Rest Framework

Question

I am putting together an API with Django Rest Framework. I want to customise my error handling. I read quite a bit (link1, link2, link3) about custom error handling but can't find something that suits my needs.

Basically, I'd like to change the structure of my error messages to get something like this :

{
  "error": True,
  "errors": [
    {
      "message": "Field %s does not exist",
      "code": 1050
    }
  ]
}

Instead of :

{"detail":"Field does not exist"}

I already have a custom ExceptionMiddleware to catch the 500 errors and return a JSON, but I have no power on all the other errors.

Code of the ExceptionMiddleware:

class ExceptionMiddleware(object):

    def process_exception(self, request, exception):

        if request.user.is_staff:
            detail = exception.message
        else:
            detail = 'Something went wrong, please contact a staff member.'

        return HttpResponse('{"detail":"%s"}'%detail, content_type="application/json", status=500)

From Django doc :

Note that the exception handler will only be called for responses generated by raised exceptions. It will not be used for any responses returned directly by the view, such as the HTTP_400_BAD_REQUEST responses that are returned by the generic views when serializer validation fails.

This is exactly what I am trying to achieve, customise those 400 errors.

Thanks a lot,

Answer 1

I know this is a bit late, (better late than never).

If you have a structured error message, then try this by inheriting the Exception class

from rest_framework.serializers import ValidationError
from rest_framework import status


class CustomAPIException(ValidationError):
    status_code = status.HTTP_400_BAD_REQUEST
    default_code = 'error'

    def __init__(self, detail, status_code=None):
        self.detail = detail
        if status_code is not None:
            self.status_code = status_code

and the usage will be like this:

if some_condition:
    error_msg = {
        "error": True,
        "errors": [
            {
                "message": "Field %s does not exist"%('my_test_field'),
                "code": 1050
            }
        ]
    }
    raise CustomAPIException(error_msg)

Reference : How to override exception messages in django rest framework

Answer 2

This is my custom exception handler:

def api_exception_handler(exception, context):

    if isinstance(exception, exceptions.APIException):

        headers = {}

        if getattr(exception, 'auth_header', None):
            headers['WWW-Authenticate'] = exception.auth_header

        if getattr(exception, 'wait', None):
            headers['Retry-After'] = '%d' % exception.wait

        data = exception.get_full_details()
        set_rollback()

        return Response(data, status=exception.status_code, headers=headers)

    return exception_handler(exception, context)

It represents APIException errors in a format like this:

{
    "field_name": [
        {
            "message": "Error message",
            "code": "error_code"
        },
        {
            "message": "Another error",
            "code": "another_error"
        }
    ]
}

Django Rest Framework reference documentation:
http://www.django-rest-framework.org/api-guide/exceptions/#custom-exception-handling

Answer 3

The exception handler is indeed what you're looking for. The current mixing do raise an exception in case of failed validation (https://github.com/tomchristie/django-rest-framework/blob/master/rest_framework/mixins.py).

Note that the exception handler will only be called for responses generated by raised exceptions. It will not be used for any responses returned directly by the view, such as the HTTP_400_BAD_REQUEST responses that are returned by the generic views when serializer validation fails.

I think this part doesn't hold any longer and should be rephrased by removing the "generic" word.

Answer 4

Alright so based on Linovia's answer, here is my custom handler :

def custom_exception_handler(exc, context):
    response = exception_handler(exc, context)
    if response is not None:
        errors = []
        for msg in response.data.values():
            errors.append({'message': msg[0], 'error': get_error_message()})

        response.data = {"errors": errors}

    return response