Fix prior deserialization for priors with buffered attributes (#5167)

ax/storage/json_store/decoders.py

@@ -54,9 +54,9 @@
 from botorch.models.transforms.input import ChainedInputTransform, InputTransform
 from botorch.models.transforms.outcome import ChainedOutcomeTransform, OutcomeTransform
 from botorch.utils.types import _DefaultType, DEFAULT
+from gpytorch.priors import Prior
 from gpytorch.priors.utils import BUFFERED_PREFIX
 from pyre_extensions import assert_is_instance
-from torch.distributions.transformed_distribution import TransformedDistribution
 
 logger: logging.Logger = get_logger(__name__)
 
@@ -369,8 +369,11 @@ def botorch_component_from_json(botorch_class: type[T], json: dict[str, Any]) ->
                 for k, v in state_dict.items()
             }
         )
-    if issubclass(botorch_class, TransformedDistribution):
-        # Extract the buffered attributes for transformed priors.
+    if issubclass(botorch_class, Prior):
+        # Extract the buffered attributes for priors. Some priors (e.g.
+        # BetaPrior, LogNormalPrior) store parameters with BUFFERED_PREFIX
+        # because their underlying distribution uses @property descriptors
+        # that cannot be deleted by _bufferize_attributes.
         for k in list(state_dict.keys()):
             if k.startswith(BUFFERED_PREFIX):
                 state_dict[k[len(BUFFERED_PREFIX) :]] = state_dict.pop(k)

ax/storage/json_store/tests/test_json_store.py

@@ -1069,6 +1069,30 @@ def test_BadStateDict(self) -> None:
             del expected_json["state_dict"]["lower_bound"]
             botorch_component_from_json(interval.__class__, expected_json)
 
+    def test_prior_roundtrip_serialization(self) -> None:
+        """Test encode/decode roundtrip for priors with buffered attributes.
+
+        Priors whose underlying distribution uses @property descriptors
+        (e.g. BetaPrior via Dirichlet, LogNormalPrior via TransformedDistribution)
+        store state_dict keys with BUFFERED_PREFIX. The decoder must strip
+        the prefix to match __init__ arg names.
+        """
+        from botorch.models.utils.priors import BetaPrior
+        from gpytorch.priors.torch_priors import GammaPrior, LogNormalPrior, NormalPrior
+
+        priors = [
+            ("BetaPrior", BetaPrior(concentration1=2.5, concentration0=1.5)),
+            ("GammaPrior", GammaPrior(concentration=2.0, rate=1.0)),
+            ("NormalPrior", NormalPrior(loc=0.0, scale=1.0)),
+            ("LogNormalPrior", LogNormalPrior(loc=0.0, scale=1.0)),
+        ]
+        for name, prior in priors:
+            with self.subTest(prior=name):
+                encoded = botorch_component_to_dict(prior)
+                decoded = botorch_component_from_json(prior.__class__, encoded)
+                self.assertIsInstance(decoded, prior.__class__)
+                self.assertEqual(decoded.state_dict(), prior.state_dict())
+
     def test_observation_features_backward_compatibility(self) -> None:
         json = {
             "__type": "ObservationFeatures",