--- a/tests/test.py	Fri May 03 09:46:43 2019 +0200
+++ b/tests/test.py	Fri May 03 10:42:54 2019 +0200
@@ -344,6 +344,32 @@
 
         self.assertRaises(KeyError, _g)
 
+    def test04_determine_mode(self):
+        configmix.clear_assoc()
+        configmix.set_assoc("*.conf", configmix.try_determine_filemode)
+        cfg = configmix.load(os.path.join(TESTDATADIR, "conf30.conf"))
+        self.assertEqual(u("new value"), cfg.getvar_s("key-new"))
+
+        self.assertRaises(KeyError, cfg.getvar_s, "key7")
+
+    def test05_try_determine_mode_none(self):
+        configmix.clear_assoc()
+        configmix.set_assoc("*.conf", configmix.try_determine_filemode)
+
+        def _ld():
+            return configmix.load(os.path.join(TESTDATADIR, "no-mode.conf"))
+
+        self.assertRaises(ValueError, _ld)
+
+    def test06_try_determine_mode_unknown(self):
+        configmix.clear_assoc()
+        configmix.set_assoc("*.conf", configmix.try_determine_filemode)
+
+        def _ld():
+            return configmix.load(os.path.join(TESTDATADIR, "unknown-mode.conf"))
+
+        self.assertRaises(KeyError, _ld)
+
 
 if __name__ == "__main__":
     unittest.main()