Add experimental exception handling in client/server

Demonstrate that MissingDatasetTypeError can be caught in
the server and raised again in the client.

python/lsst/daf/butler/remote_butler/_remote_butler.py

@@ -49,7 +49,7 @@
 from .._timespan import Timespan
 from ..datastore import DatasetRefURIs
 from ..dimensions import DataCoordinate, DataId, DimensionConfig, DimensionUniverse, SerializedDataCoordinate
-from ..registry import NoDefaultCollectionError, Registry, RegistryDefaults
+from ..registry import MissingDatasetTypeError, NoDefaultCollectionError, Registry, RegistryDefaults
 from ..registry.wildcards import CollectionWildcard
 from ..transfers import RepoExportContext
 from ._config import RemoteButlerConfigModel
@@ -216,6 +216,10 @@ def get_dataset_type(self, name: str) -> DatasetType:
         # and only go to the server if the dataset type is not known.
         path = f"dataset_type/{name}"
         response = self._client.get(self._get_url(path))
+        if response.status_code != httpx.codes.OK:
+            content = response.json()
+            if content["exception"] == "MissingDatasetTypeError":
+                raise MissingDatasetTypeError(content["detail"])
         response.raise_for_status()
         return DatasetType.from_simple(SerializedDatasetType(**response.json()), universe=self.dimensions)
 

python/lsst/daf/butler/remote_butler/server/_server.py

@@ -34,11 +34,13 @@
 from functools import cache
 from typing import Any
 
-from fastapi import Depends, FastAPI
+from fastapi import Depends, FastAPI, Request
 from fastapi.middleware.gzip import GZipMiddleware
+from fastapi.responses import JSONResponse
 from lsst.daf.butler import (
     Butler,
     DataCoordinate,
+    MissingDatasetTypeError,
     SerializedDataCoordinate,
     SerializedDatasetRef,
     SerializedDatasetType,
@@ -55,6 +57,17 @@
 app.add_middleware(GZipMiddleware, minimum_size=1000)
 
 
+@app.exception_handler(MissingDatasetTypeError)
+def missing_dataset_type_exception_handler(request: Request, exc: MissingDatasetTypeError) -> JSONResponse:
+    # Remove the double quotes around the string form. These confuse
+    # the JSON serialization when single quotes are in the message.
+    message = str(exc).strip('"')
+    return JSONResponse(
+        status_code=404,
+        content={"detail": message, "exception": "MissingDatasetTypeError"},
+    )
+
+
 @cache
 def _make_global_butler() -> Butler:
     return Butler.from_config(BUTLER_ROOT)

tests/test_server.py

@@ -38,7 +38,7 @@
     TestClient = None
     app = None
 
-from lsst.daf.butler import Butler, DataCoordinate, DatasetRef, StorageClassFactory
+from lsst.daf.butler import Butler, DataCoordinate, DatasetRef, MissingDatasetTypeError, StorageClassFactory
 from lsst.daf.butler.tests import DatastoreMock
 from lsst.daf.butler.tests.utils import MetricTestRepo, makeTestTempDir, removeTestTempDir
 
@@ -107,6 +107,9 @@ def test_get_dataset_type(self):
         bias_type = self.butler.get_dataset_type("bias")
         self.assertEqual(bias_type.name, "bias")
 
+        with self.assertRaises(MissingDatasetTypeError):
+            self.butler.get_dataset_type("not_bias")
+
     def test_find_dataset(self):
         storage_class = self.storageClassFactory.getStorageClass("Exposure")