Django's override_settings decorator doesn't work

I'm working on open source cms based on django and I have a problem with the unit-tests. Here's a function I want to test:

from appname.conf import settings

def can_read(article, user):
    if callable(settings.CAN_READ):
        do_something

where appname.conf.settings is:

from django.conf import settings as django_settings

CAN_READ = getattr(django_settings, 'APPNAME_CAN_READ', None)

For the tests I need to change the settings for the can_read,

@override_settings(APPNAME_CAN_READ=lambda *args: False)
def test_user_dont_have_permission(self):

    m = Model.objects.create()
    u = User.objects.create(username='Noman', password='pass')

    output = can_read(m, u)
    self.assertFalse(output)

but somehow built-in override_settings decorator does not work. But if to import django.conf.settings in the same module where is can_read pleced, it has correct values. App also using runtests.py wich has settings config as:

settings.configure(
DEBUG=True,
AUTH_USER_MODEL='testdata.CustomUser',
DATABASES={
    'default': {
        'ENGINE': 'django.db.backends.sqlite3',
    }
},
SITE_ID=1,
ROOT_URLCONF='app.tests.testdata.urls',
INSTALLED_APPS=[
    'appname.tests.testdata',
    'django.contrib.auth',
    'django.contrib.sites',
    'appname',
    etc...
] + (['south'] if django.VERSION < (1, 7) else []),
MIDDLEWARE_CLASSES=[
    'django.middleware.common.CommonMiddleware',
    'django.contrib.sessions.middleware.SessionMiddleware',
    'django.middleware.csrf.CsrfViewMiddleware',
    'django.contrib.auth.middleware.AuthenticationMiddleware',
    'django.contrib.messages.middleware.MessageMiddleware',
],
TEMPLATE_CONTEXT_PROCESSORS=(
    "django.core.context_processors.i18n",
    "django.core.context_processors.media",
    "django.core.context_processors.request",
    "django.core.context_processors.static",
    "django.core.context_processors.tz",
),
USE_TZ=True,
SOUTH_TESTS_MIGRATE=True,
SECRET_KEY = 'somekey',
)