why would a django test fail only when the full test suite is run?

Question

I have a test in Django 1.5 that passes in these conditions:

• when run by itself in isolation
• when the full TestCase is run
• when all of my app's tests are run

But it fails when the full test suite is run with python manage.py test. Why might this be happening?

The aberrant test uses django.test.Client to POST some data to an endpoint, and then the test checks that an object was successfully updated. Could some other app be modifying the test client or the data itself?

I have tried some print debugging and I see all of the data being sent and received as expected. The specific failure is a does-not-exist exception that is raised when I try to fetch the to-be-updated object from the db. Strangely, in the exception handler itself, I can query for all objects of that type and see that the target object does in fact exist.

Edit:

My issue was resolved when I found that I was querying for the target object by id and User and not id and UserProfile, but it's still confusing to me that this would work in some cases but fail in others.

I also found that the test would fail with python manage.py test auth <myapp>

Answer 1

It sounds like your problem does not involve mocks, but I just spent all day debugging an issue with similar symptoms, and your question is the first one that came up when I was searching for a solution, so I wanted to share my solution here, just in case it will prove helpful for others. In my case, the issue was as follows.

I had a single test that would pass in isolation, but fail when run as part of my full test suite. In one of my view functions I was using the Django send_mail() function. In my test, rather than having it send me an email every time I ran my tests, I patched send_mail in my test method:

from mock import patch ...  def test_stuff(self):     ...      with patch('django.core.mail.send_mail') as mocked_send_mail:      ... 

That way, after my view function is called, I can test that send_mail was called with:

self.assertTrue(mocked_send_mail.called) 

This worked fine when running the test on its own, but failed when run with other tests in the suite. The reason this fails is that when it runs as part of the suite other views are called beforehand, causing the views.py file to be loaded, causing send_mail to be imported before I get the chance to patch it. So when send_mail gets called in my view, it is the actual send_mail that gets called, not my patched version. When I run the test alone, the function gets mocked before it is imported, so the patched version ends up getting imported when views.py is loaded. This situation is described in the mock documentation, which I had read a few times before, but now understand quite well after learning the hard way...

The solution was simple: instead of patching django.core.mail.send_mail I just patched the version that had already been imported in my views.py - myapp.views.send_mail. In other words:

with patch('myapp.views.send_mail') as mocked_send_mail: ...