How to Unit test with different settings in Django?

You can do anything you like to the UnitTest subclass, including setting and reading instance properties:

from django.conf import settings

class MyTest(unittest.TestCase):
   def setUp(self):
       self.old_setting = settings.NUM_LATEST
       settings.NUM_LATEST = 5 # value tested against in the TestCase

   def tearDown(self):
       settings.NUM_LATEST = self.old_setting

Since the django test cases run single-threaded, however, I'm curious about what else may be modifying the NUM_LATEST value? If that "something else" is triggered by your test routine, then I'm not sure any amount of monkey patching will save the test without invalidating the veracity of the tests itself.

You can pass --settings option when running tests

python manage.py test --settings=mysite.settings_local

EDIT: This answer applies if you want to change settings for a small number of specific tests.

Since Django 1.4, there are ways to override settings during tests: https://docs.djangoproject.com/en/stable/topics/testing/tools/#overriding-settings

TestCase will have a self.settings context manager, and there will also be an @override_settings decorator that can be applied to either a test method or a whole TestCase subclass.

These features did not exist yet in Django 1.3.

If you want to change settings for all your tests, you'll want to create a separate settings file for test, which can load and override settings from your main settings file. There are several good approaches to this in the other answers; I have seen successful variations on both hspander's and dmitrii's approaches.