[tune] Raise an error if Tuner.restore() is called on an instance (r…

…ay-project#39676)

`Tuner.restore()` is used to construct a `Tuner` object from an existing experiment. However, calling `tuner = Tuner(...); tuner.restore()` is a common misuse of the API and does not throw an error.

This PR updates the `Tuner` class to only allow `restore()` to be called on the class, not on an instance of the class.

Signed-off-by: Kai Fricke <kai@anyscale.com>

python/ray/tune/tests/test_tuner.py

@@ -517,6 +517,21 @@ def to_dict(self) -> dict:
     tune.run(trainable, config=CustomConfig())
 
 
+def test_tuner_restore_classmethod():
+    tuner = Tuner("PPO")
+
+    # Calling `tuner.restore()` on an instance should raise an AttributeError
+    with pytest.raises(AttributeError):
+        tuner.restore("/", "PPO")
+
+    # Calling `Tuner.restore()` on the class should work. This will throw a
+    # FileNotFoundError because no checkpoint exists at that location. Since
+    # this happens in the downstream restoration code, this means that the
+    # classmethod check successfully passed.
+    with pytest.raises(FileNotFoundError):
+        tuner = Tuner.restore("/invalid", "PPO")
+
+
 if __name__ == "__main__":
     import sys
 

python/ray/tune/tuner.py

@@ -432,3 +432,12 @@ def get_results(self) -> ResultGrid:
                 string_queue,
             )
             return ray.get(get_results_future)
+
+    def __getattribute__(self, item):
+        if item == "restore":
+            raise AttributeError(
+                "`Tuner.restore()` is a classmethod and cannot be called on an "
+                "instance. Use `tuner = Tuner.restore(...)` to instantiate the "
+                "Tuner instead."
+            )
+        return super().__getattribute__(item)