I modify response.data in Django REST Framework view.py. Why am I getting an AttributeError during testing but not with actual API use?

My Django REST Framework API is working just as expected. In view.py, I modify OrderViewSet, def partial_update, to add another key/value pair to the response.data dict before it is saved to the db. It works without errors when I call the API with Postman.

However, when I run tests for the same functionality it fails and returns:

request.data["submission_date"] = datetime.now()
AttributeError: This QueryDict instance is immutable

Why would I get this error during testing if it is not occurring during actual API use?

View.py

class OrderViewSet(viewsets.ModelViewSet):
    """ Includes custom PATCH functionality """
    queryset = Order.objects.all().order_by('-order_date')
    serializer_class = OrderSerializer
    authentication_classes = (authentication.TokenAuthentication,)
    permission_classes = [permissions.IsAuthenticated]

    def partial_update(self, request, *args, **kwargs):
        """ Update status and timestamp fields accordingly """
        status = request.data['status']

        if status == 'submitted':
            request.data["submission_date"] = datetime.now()

        if status == 'packaged':
            request.data["packaged_date"] = datetime.now()

        if status in ['sold', 'canceled', 'abandoned']:
            request.data["finished_date"] = datetime.now()

        return super().partial_update(request, *args, **kwargs)

Test.py

def test_patch_order_status_from_cart_to_submitted(self):
    """ test patching order status from cart to submitted """
    order = sample_order(user=self.user)

    payload = {
        "status": "submitted"
    }

    res = self.client.patch(order_detail_url(order.id), payload)
    self.assertEqual(res.status_code, status.HTTP_200_OK)

    patched_order = Order.objects.get(id=order.id)
    self.assertEqual(patched_order.status, 'submitted')

def test_submitted_timestamp(self):
    """ test that patching order status to submitted also leaves timestamp """
    order = sample_order(user=self.user)

    payload = {
        "status": "submitted"
    }

    self.client.patch(order_detail_url(order.id), payload)

    patched_order = Order.objects.get(id=order.id)
    self.assertNotEqual(patched_order.submission_date, None)

You mean modifying request.data? I also got this error before.

Then I read Django documentation QueryDict, I think the "request.data":

  • is dictionary-like, not dictionary.

  • data type is immutable.

  • copy that before modifying, like:

     req_data = request.data.copy()
 req_data['submission_date'] = datetime.now()

And the reason of error not occurring during actual API use, maybe just the status not in these conditions.

On the other hand, in this case I usually set a modified_at or created_at DateTimeField in models.py, so that will record the submission_date automatically whenever the data is modified or created. I feel that is convenient.

created_at = models.DateTimeField(auto_now_add=True, help_text="submission_date")
modified_at = models.DateTimeField(auto_now=True, help_text="re-submission_date")
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