[air] Raise error on path-like access for Checkpoints (#26970)

Calling e.g. `os.path.exists(checkpoint)` currently raises an TypeError, but we should make it more explicit and guide users towards the correct API.

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

python/ray/air/checkpoint.py

@@ -627,6 +627,12 @@ class Checkpoint:
     def __setstate__(self, state):
         self.__dict__.update(state)
 
+    def __fspath__(self):
+        raise TypeError(
+            "You cannot use `air.Checkpoint` objects directly as paths. "
+            "Use `Checkpoint.to_directory()` or `Checkpoint.as_directory()` instead."
+        )
+
     def get_preprocessor(self) -> Optional["Preprocessor"]:
         """Return the saved preprocessor, if one exists."""
 

python/ray/air/tests/test_checkpoints.py

@@ -544,6 +544,12 @@ class PreprocessorCheckpointTest(unittest.TestCase):
             preprocessor = checkpoint.get_preprocessor()
             assert preprocessor.multiplier == 1
 
+    def testAttrPath(self):
+        with tempfile.TemporaryDirectory() as tmpdir:
+            checkpoint = Checkpoint.from_directory(tmpdir)
+            with self.assertRaises(TypeError):
+                os.path.exists(checkpoint)
+
 
 if __name__ == "__main__":
     import pytest