[RLlib] Add error message if Checkpointable.from_checkpoint() is ca…

…lled on wrong class. (ray-project#46676)

rllib/utils/checkpoints.py

@@ -410,8 +410,22 @@ def from_checkpoint(
  # Get the class constructor to call.
  with open(path / cls.CLASS_AND_CTOR_ARGS_FILE_NAME, "rb") as f:
  ctor_info = pickle.load(f)
+ ctor = ctor_info["class"]
+
+ # Check, whether the constructor actually goes together with `cls`.
+ if not issubclass(ctor, cls):
+ raise ValueError(
+ f"The class ({ctor}) stored in checkpoint ({path}) does not seem to be "
+ f"a subclass of `cls` ({cls})!"
+ )
+ elif not issubclass(ctor, Checkpointable):
+ raise ValueError(
+ f"The class ({ctor}) stored in checkpoint ({path}) does not seem to be "
+ "an implementer of the `Checkpointable` API!"
+ )
+
  # Construct an initial object.
- obj = ctor_info["class"](
+ obj = ctor(
  *ctor_info["ctor_args_and_kwargs"][0],
  **ctor_info["ctor_args_and_kwargs"][1],
  )