diff --git a/nova/tests/unit/virt/disk/mount/test_nbd.py b/nova/tests/unit/virt/disk/mount/test_nbd.py
index 0d52c67c2613..8c74f35dab2e 100644
--- a/nova/tests/unit/virt/disk/mount/test_nbd.py
+++ b/nova/tests/unit/virt/disk/mount/test_nbd.py
@@ -51,11 +51,11 @@ def _fake_exists_all_used(path):
     return ORIG_EXISTS(path)
 
 
-def _fake_detect_nbd_devices_none(self):
+def _fake_detect_nbd_devices_none():
     return []
 
 
-def _fake_detect_nbd_devices(self):
+def _fake_detect_nbd_devices():
     return ['nbd0', 'nbd1']
 
 
@@ -63,6 +63,28 @@ def _fake_noop(*args, **kwargs):
     return
 
 
+class NbdTestCaseNoStub(test.NoDBTestCase):
+
+    @mock.patch('os.listdir')
+    def test_detect_nbd_devices(self, list_dir_mock):
+        list_dir_mock.return_value = _fake_detect_nbd_devices()
+        result = nbd.NbdMount._detect_nbd_devices()
+        self.assertIsNotNone(result)
+        self.assertIsInstance(result, list)
+        self.assertEqual(len(list_dir_mock.return_value), len(result))
+        for path in list_dir_mock.return_value:
+            self.assertIn(path, result)
+
+    @mock.patch('os.listdir')
+    def test_detect_nbd_devices_empty(self, list_dir_mock):
+        list_dir_mock.return_value = [
+            "nbdz", "fake0", "not-nbd1"]
+        result = nbd.NbdMount._detect_nbd_devices()
+        self.assertIsNotNone(result)
+        self.assertIsInstance(result, list)
+        self.assertEqual(0, len(result))
+
+
 class NbdTestCase(test.NoDBTestCase):
     def setUp(self):
         super(NbdTestCase, self).setUp()
@@ -272,7 +294,7 @@ class NbdTestCase(test.NoDBTestCase):
         # they cannot choose the same nbd number (see bug 1207422)
 
         tempdir = self.useFixture(fixtures.TempDir()).path
-        free_devices = _fake_detect_nbd_devices(None)[:]
+        free_devices = _fake_detect_nbd_devices()[:]
         chosen_devices = []
 
         def fake_find_unused(self):
diff --git a/nova/virt/disk/mount/nbd.py b/nova/virt/disk/mount/nbd.py
index c8a4f1ddf2c2..f00bc4e028af 100644
--- a/nova/virt/disk/mount/nbd.py
+++ b/nova/virt/disk/mount/nbd.py
@@ -39,9 +39,10 @@ class NbdMount(api.Mount):
     """qemu-nbd support disk images."""
     mode = 'nbd'
 
-    def _detect_nbd_devices(self):
+    @staticmethod
+    def _detect_nbd_devices():
         """Detect nbd device files."""
-        return filter(NBD_DEVICE_RE.match, os.listdir('/sys/block/'))
+        return list(filter(NBD_DEVICE_RE.match, os.listdir('/sys/block/')))
 
     def _find_unused(self, devices):
         for device in devices: