Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

from django.conf import settings

settings.configure(
    DEBUG=True,
    SECRET_KEY='test',
    ROOT_URLCONF=__name__,
    MIDDLEWARE_CLASSES=(
    ),
)
django.setup()

from django.db import models
from rest_framework.routers import DefaultRouter
from rest_framework.serializers import ModelSerializer
from rest_framework.viewsets import ModelViewSet

class Foo(models.Model):
    name = models.CharField(max_length=20)
    class Meta:
        app_label='auth'

class FooSerializer(ModelSerializer):
    class Meta:
        model = Foo
        fields = '__all__'

class FooViewSet(ModelViewSet):
    serializer_class = FooSerializer
    queryset = Foo.objects.all()


router = DefaultRouter()
router.register('^endpoint1', FooViewSet)
before_len = len(router.urls)

router.register('^endpoint2', FooViewSet)
after_len = len(router.urls)

assert before_len != after_len

Expected behavior

Nothing

Actual behavior

AssertionError: urls has not updated

Cause

urls is a property that caches its output in _urls. If a new route is subsequently added then it isn't cleared, so urls will now return an incorrect list. This is just one way it could fail; changing any attribute that affects get_urls will have the same problem.

Relevant Source

Relevant change history:

• Initially created as url_patterns in c785628 (2013-04-05)
• url_patterns changed name to urls in b94da24 (2013-04-25)

It seems that this bug has always been there

Solution

There are 2 solutions I see

[1] Invalidate _urls if something relevant changes. Because url calls get_urls() which can be overridden, this would have to be implemented correctly on every child class (example of where lots of attributes could affect the output: DefaultRouter)

[2] Stop caching results. It's not clear to me why it needs to be cached at all, or why it's even a property -- if it's only going to be used for generating url patterns then it will be invoked once on startup and then never again. This seems a more robust solution but I may be overlooking cases where url is repeatedly invoked.