django lazy translation sometimes appears in response.data in unit tests - django

Question

I'm writing unit tests for my django api written with django-rest-framework, and I'm encountering seemingly inconsistent response data from calls that generate 400_BAD_REQUEST.
When I make a request that fails because it references an invalid primary key, repr(response.data) is a string that I can check for the substring "Invalid pk". But when I make a request that fails because it's trying to create a non-unique object, repr(response.data) contains {'name': [<django.utils.functional.__proxy__ object at 0x7f3ccdcb26d0>]} instead of the expected {'name': ['This field must be unique.']}. When I make an actual POST call to the real server, I get the expected 400 response with {'name': ['This field must be unique.']}.
Here's a code sample:
class GroupViewTests(APITestCase):
def test_post_existing_group(self):
"""
Use POST to update a group at /groups
"""
# create a group
# self.group_data returns a valid group dict
data = self.group_data(1)
url = reverse('group-list')
response = self.client.post(url, data, format='json')
self.assertEqual(response.status_code, status.HTTP_201_CREATED)
# create the same group again
response = self.client.post(url, data, format='json')
self.assertEqual(response.status_code, status.HTTP_400_BAD_REQUEST)
# this fails
self.assertIn('must be unique', repr(response.data))
def test_create_group_with_nonexistent_user(self):
"""
Create a group with an invalid userid in it.
"""
url = reverse('group-list')
data = self.group_data(5)
data.update({'users': ['testnonexistentuserid']})
response = self.client.post(url, data, format='json')
self.assertEqual(response.status_code, status.HTTP_400_BAD_REQUEST)
# this succeeds
self.assertIn('Invalid pk', repr(response.data))
I'm using only the default middleware.
From what I've found online, the __proxy__ object is a representation of a lazy translation, which can be rendered as a string by calling unicode(obj) or obj.__unicode__. And indeed, response.data['name'][0].__unicode__ does return the expected value, but calling unicode(response.data) instead of repr(response.data) still returns the object identified by memory address.
Can anyone explain what's going on here, and how to fix it? I can work around the issue for now, but I'd like to know the "real way" to fix the problem. Is it possibly a bug in django or django-rest-framework?

Answer 1

Use response.content instead of response.data.
response.content contains the json output just like your client will receive it. It's also useful if you have custom renderer that change the response because response.data is not rendered, but response.content yes.
E.g :
import json
# we have to parse the output since it no more a dict
data = json.loads(response.content)
# if `response.content` is a byte string, decode it
data = json.loads(response.content.decode('utf-8'))