fix URLPathVersioning reverse fallback

rest_framework/versioning.py

@@ -81,8 +81,10 @@ def determine_version(self, request, *args, **kwargs):
 
     def reverse(self, viewname, args=None, kwargs=None, request=None, format=None, **extra):
         if request.version is not None:
-            kwargs = {} if (kwargs is None) else kwargs
-            kwargs[self.version_param] = request.version
+            kwargs = {
+                self.version_param: request.version,
+                **(kwargs or {})
+            }
 
         return super().reverse(
             viewname, args, kwargs, request, format, **extra

tests/test_versioning.py

@@ -152,6 +152,8 @@ class TestURLReversing(URLPatternsTestCase, APITestCase):
         path('v1/', include((included, 'v1'), namespace='v1')),
         path('another/', dummy_view, name='another'),
         re_path(r'^(?P<version>[v1|v2]+)/another/$', dummy_view, name='another'),
+        re_path(r'^(?P<foo>.+)/unversioned/$', dummy_view, name='unversioned'),
+
     ]
 
     def test_reverse_unversioned(self):
@@ -198,6 +200,14 @@ def test_reverse_url_path_versioning(self):
         response = view(request)
         assert response.data == {'url': 'http://testserver/another/'}
 
+        # Test fallback when kwargs is not None
+        request = factory.get('/v1/endpoint/')
+        request.versioning_scheme = scheme()
+        request.version = 'v1'
+
+        reversed_url = reverse('unversioned', request=request, kwargs={'foo': 'bar'})
+        assert reversed_url == 'http://testserver/bar/unversioned/'
+
     def test_reverse_namespace_versioning(self):
         class FakeResolverMatch:
             namespace = 'v1'