[Tune] Better error when using checkpoint_freq (#9998)

python/ray/tune/tests/test_function_api.py

@@ -300,6 +300,15 @@ class FunctionApiTest(unittest.TestCase):
         ray.shutdown()
         _register_all()  # re-register the evicted objects
 
+    def testCheckpointError(self):
+        def train(config, checkpoint_dir=False):
+            pass
+
+        with self.assertRaises(ValueError):
+            tune.run(train, checkpoint_freq=1)
+        with self.assertRaises(ValueError):
+            tune.run(train, checkpoint_at_end=True)
+
     def testCheckpointFunctionAtEnd(self):
         def train(config, checkpoint_dir=False):
             for i in range(10):