Fix NamespaceVersioning ignoring DEFAULT_VERSION on non-None namespaces (#7278)

* Fix the case where if the namespace is not None and there's no match,
  NamespaceVersioning always raises NotFound even if DEFAULT_VERSION
  is set or None is in ALLOWED_VERSIONS

* Add test cases

rest_framework/versioning.py

@@ -119,15 +119,16 @@ class NamespaceVersioning(BaseVersioning):
 
     def determine_version(self, request, *args, **kwargs):
         resolver_match = getattr(request, 'resolver_match', None)
-        if resolver_match is None or not resolver_match.namespace:
+        if resolver_match is not None and resolver_match.namespace:
-            return self.default_version
-
             # Allow for possibly nested namespaces.
             possible_versions = resolver_match.namespace.split(':')
             for version in possible_versions:
                 if self.is_allowed_version(version):
                     return version
+
+        if not self.is_allowed_version(self.default_version):
             raise exceptions.NotFound(self.invalid_version_message)
+        return self.default_version
 
     def reverse(self, viewname, args=None, kwargs=None, request=None, format=None, **extra):
         if request.version is not None:

tests/test_versioning.py

@@ -272,7 +272,7 @@ class TestInvalidVersion:
         assert response.status_code == status.HTTP_404_NOT_FOUND
 
 
-class TestAllowedAndDefaultVersion:
+class TestAcceptHeaderAllowedAndDefaultVersion:
     def test_missing_without_default(self):
         scheme = versioning.AcceptHeaderVersioning
         view = AllowedVersionsView.as_view(versioning_class=scheme)
@@ -318,6 +318,97 @@ class TestAllowedAndDefaultVersion:
         assert response.data == {'version': 'v2'}
 
 
+class TestNamespaceAllowedAndDefaultVersion:
+    def test_no_namespace_without_default(self):
+        class FakeResolverMatch:
+            namespace = None
+
+        scheme = versioning.NamespaceVersioning
+        view = AllowedVersionsView.as_view(versioning_class=scheme)
+
+        request = factory.get('/endpoint/')
+        request.resolver_match = FakeResolverMatch
+        response = view(request)
+        assert response.status_code == status.HTTP_404_NOT_FOUND
+
+    def test_no_namespace_with_default(self):
+        class FakeResolverMatch:
+            namespace = None
+
+        scheme = versioning.NamespaceVersioning
+        view = AllowedAndDefaultVersionsView.as_view(versioning_class=scheme)
+
+        request = factory.get('/endpoint/')
+        request.resolver_match = FakeResolverMatch
+        response = view(request)
+        assert response.status_code == status.HTTP_200_OK
+        assert response.data == {'version': 'v2'}
+
+    def test_no_match_without_default(self):
+        class FakeResolverMatch:
+            namespace = 'no_match'
+
+        scheme = versioning.NamespaceVersioning
+        view = AllowedVersionsView.as_view(versioning_class=scheme)
+
+        request = factory.get('/endpoint/')
+        request.resolver_match = FakeResolverMatch
+        response = view(request)
+        assert response.status_code == status.HTTP_404_NOT_FOUND
+
+    def test_no_match_with_default(self):
+        class FakeResolverMatch:
+            namespace = 'no_match'
+
+        scheme = versioning.NamespaceVersioning
+        view = AllowedAndDefaultVersionsView.as_view(versioning_class=scheme)
+
+        request = factory.get('/endpoint/')
+        request.resolver_match = FakeResolverMatch
+        response = view(request)
+        assert response.status_code == status.HTTP_200_OK
+        assert response.data == {'version': 'v2'}
+
+    def test_with_default(self):
+        class FakeResolverMatch:
+            namespace = 'v1'
+
+        scheme = versioning.NamespaceVersioning
+        view = AllowedAndDefaultVersionsView.as_view(versioning_class=scheme)
+
+        request = factory.get('/endpoint/')
+        request.resolver_match = FakeResolverMatch
+        response = view(request)
+        assert response.status_code == status.HTTP_200_OK
+        assert response.data == {'version': 'v1'}
+
+    def test_no_match_without_default_but_none_allowed(self):
+        class FakeResolverMatch:
+            namespace = 'no_match'
+
+        scheme = versioning.NamespaceVersioning
+        view = AllowedWithNoneVersionsView.as_view(versioning_class=scheme)
+
+        request = factory.get('/endpoint/')
+        request.resolver_match = FakeResolverMatch
+        response = view(request)
+        assert response.status_code == status.HTTP_200_OK
+        assert response.data == {'version': None}
+
+    def test_no_match_with_default_and_none_allowed(self):
+        class FakeResolverMatch:
+            namespace = 'no_match'
+
+        scheme = versioning.NamespaceVersioning
+        view = AllowedWithNoneAndDefaultVersionsView.as_view(versioning_class=scheme)
+
+        request = factory.get('/endpoint/')
+        request.resolver_match = FakeResolverMatch
+        response = view(request)
+        assert response.status_code == status.HTTP_200_OK
+        assert response.data == {'version': 'v2'}
+
+
 class TestHyperlinkedRelatedField(URLPatternsTestCase, APITestCase):
     included = [
         path('namespaced/<int:pk>/', dummy_pk_view, name='namespaced'),