Django Rest Framework APIClient not handling exceptions during tests - django

Question

I'm testing an API endpoint that is supposed to raise a ValidationError in a Django model (note that the exception is a Django exception, not DRF, because it's in the model).
from rest_framework.test import APITestCase
class TestMyView(APITestCase):
# ...
def test_bad_request(self):
# ...
response = self.client.post(url, data)
self.assertEqual(response.status_code, status.HTTP_400_BAD_REQUEST)
However, my test errors out with an exception instead of passing. It doesn't even fail getting a 500 instead of 400, it doesn't get there at all. Isn't DRF's APIClient supposed to handle every exception? I've search online but found nothing. I've read that DRF doesn't handle Django's native ValidationError, but still that doesn't explain why I am not even getting a 500. Any idea what I'm doing wrong?
Full stack trace:
E
======================================================================
ERROR: test_cannot_create_duplicate_email (organizations.api.tests.test_contacts.TestContactListCreateView)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/code/organizations/api/tests/test_contacts.py", line 98, in test_cannot_create_duplicate_email
response = self.jsonapi_post(self.url(new_partnership), data)
File "/code/config/tests/base.py", line 166, in jsonapi_post
url, data=json.dumps(data), content_type=content_type)
File "/usr/local/lib/python3.7/site-packages/rest_framework/test.py", line 300, in post
path, data=data, format=format, content_type=content_type, **extra)
File "/usr/local/lib/python3.7/site-packages/rest_framework/test.py", line 213, in post
return self.generic('POST', path, data, content_type, **extra)
File "/usr/local/lib/python3.7/site-packages/rest_framework/test.py", line 238, in generic
method, path, data, content_type, secure, **extra)
File "/usr/local/lib/python3.7/site-packages/django/test/client.py", line 422, in generic
return self.request(**r)
File "/usr/local/lib/python3.7/site-packages/rest_framework/test.py", line 289, in request
return super(APIClient, self).request(**kwargs)
File "/usr/local/lib/python3.7/site-packages/rest_framework/test.py", line 241, in request
request = super(APIRequestFactory, self).request(**kwargs)
File "/usr/local/lib/python3.7/site-packages/django/test/client.py", line 503, in request
raise exc_value
File "/usr/local/lib/python3.7/site-packages/django/core/handlers/exception.py", line 34, in inner
response = get_response(request)
File "/usr/local/lib/python3.7/site-packages/django/core/handlers/base.py", line 115, in _get_response
response = self.process_exception_by_middleware(e, request)
File "/usr/local/lib/python3.7/site-packages/django/core/handlers/base.py", line 113, in _get_response
response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/usr/local/lib/python3.7/site-packages/django/views/decorators/csrf.py", line 54, in wrapped_view
return view_func(*args, **kwargs)
File "/usr/local/lib/python3.7/site-packages/django/views/generic/base.py", line 71, in view
return self.dispatch(request, *args, **kwargs)
File "/usr/local/lib/python3.7/site-packages/rest_framework/views.py", line 495, in dispatch
response = self.handle_exception(exc)
File "/usr/local/lib/python3.7/site-packages/rest_framework/views.py", line 455, in handle_exception
self.raise_uncaught_exception(exc)
File "/usr/local/lib/python3.7/site-packages/rest_framework/views.py", line 492, in dispatch
response = handler(request, *args, **kwargs)
File "/usr/local/lib/python3.7/site-packages/rest_framework/generics.py", line 244, in post
return self.create(request, *args, **kwargs)
File "/usr/local/lib/python3.7/site-packages/rest_framework/mixins.py", line 21, in create
self.perform_create(serializer)
File "/usr/local/lib/python3.7/site-packages/rest_framework/mixins.py", line 26, in perform_create
serializer.save()
File "/usr/local/lib/python3.7/site-packages/rest_framework/serializers.py", line 214, in save
self.instance = self.create(validated_data)
File "/code/organizations/api/serializers.py", line 441, in create
'partnership': self.context['partnership']
File "/usr/local/lib/python3.7/site-packages/rest_framework/serializers.py", line 943, in create
instance = ModelClass._default_manager.create(**validated_data)
File "/usr/local/lib/python3.7/site-packages/django/db/models/manager.py", line 82, in manager_method
return getattr(self.get_queryset(), name)(*args, **kwargs)
File "/usr/local/lib/python3.7/site-packages/django/db/models/query.py", line 422, in create
obj.save(force_insert=True, using=self.db)
File "/code/organizations/models.py", line 278, in save
self.full_clean()
File "/usr/local/lib/python3.7/site-packages/django/db/models/base.py", line 1203, in full_clean
raise ValidationError(errors)
django.core.exceptions.ValidationError: {'__all__': ['Supplier contact emails must be unique per organization.']}

Answer 1

Question: Isn't DRF's APIClient supposed to handle every exception?
Answer: No. It's a test client, it won't handle any uncaught exceptions, that's how test clients work. Test clients propagate the exception so that the test fails with a "crash" when an exception isn't caught. You can test that exceptions are raised and uncaught with self.assertRaises
Question: The APIView should return HTTP_400_BAD_REQUEST when I raise a ValidationError but the exception isn't caught.
Answer:
You should look at the source code for APIView.
Inside the dispatch() method, all exceptions raised while creating the response object are caught and the method handle_exception() is called.
Your exception is a ValidationError. The crucial lines are:
exception_handler = self.get_exception_handler()
context = self.get_exception_handler_context()
response = exception_handler(exc, context)
if response is None:
self.raise_uncaught_exception(exc)
If you haven't changed settings.EXCEPTION_HANDLER, you get the default DRF exception handler, source code here.
If handles Http404, PermissionDenied and APIException. The APIView itself actually also handles AuthenticationFailed and NotAuthenticated. But not ValidationError. So it returns None and therefore the view raises your ValidationError which stops your test.
You see that in your traceback:
File "/usr/local/lib/python3.7/site-packages/rest_framework/views.py", line 455, in handle_exception
self.raise_uncaught_exception(exc)
You can decide to handle more exceptions than the default ones handled by DRF, you can read this on custom exception handling.
EDIT: You can also raise rest_framework.exceptions.ValidationError instead of the standard Django ValidationError. That is an APIException and therefore will be handled by DRF as a HTTP400_BAD_REQUEST. [1]
Side note: Luckily DRF doesn't catch every single exception! If there's a serious flaw in your code you actually want your code to "crash" and produce an error log and your server to return a HTTP 500. Which is what happens here. The response would be an HTTP 500 if this wasn't the test client.
[1]https://github.com/encode/django-rest-framework/blob/3.9.0/rest_framework/exceptions.py#L142

Answer 2

Something in your code is causing a python error which is halting execution before your POST request can return a valid HTTP response. Your code doesn't even reach the line self.assertEqual(response.status_code, status.HTTP_400_BAD_REQUEST) because there is no response.
If you're calling your tests in the normal way with ./manage.py test then you should see the traceback and be able to narrow down what caused the error.

Answer 3

Django >3.0
Starting with Django 3.0, Client constructor takes raise_request_exception parameter. Set it False and response 500 will returned instead of raising an exception.
Source:
https://docs.djangoproject.com/en/3.2/releases/3.0/#tests
self.client = Client(raise_request_exception=False)
Django <3.0
There is a workaround for this for Django 2.2 and earlier.
TL;DR:
Just mock the store_exc_info method of Client class.
from unittest import mock
class TestMyView(APITestCase):
...
def test_server_error(self):
mock.patch("django.test.client.Client.store_exc_info")
self.client.post("/url/, data)
...
Now, the client will return regular response object with status code = 500.
Explanation:
Django's Test client always raises exceptions, even if they were wrapped in 500 Server error response.
Client connects its method called store_exc_info to the got_request_exception Django's signal (source: https://github.com/django/django/blob/3.2.5/django/test/client.py#L712)
got_request_exception.connect(self.store_exc_info, dispatch_uid=exception_uid)
This method saves an exception info as self.exc_info. This attribute is later tested if its None. If it is, then error is raised. If not, response is normally returned (source: https://github.com/django/django/blob/2.2.24/django/test/client.py#L500)
Mocking the self.store_exc_info makes it do nothing, so exception info is not stored - therefore it's None later :)