Add skip datastore to ButlerConfig and add tests

python/lsst/daf/butler/_butler.py

@@ -222,7 +222,7 @@ def __init__(
             self.storageClasses = butler.storageClasses
             self._config: ButlerConfig = butler._config
         else:
-            self._config = ButlerConfig(config, searchPaths=searchPaths)
+            self._config = ButlerConfig(config, searchPaths=searchPaths, skip_datastore=skip_datastore)
             try:
                 if "root" in self._config:
                     butlerRoot = self._config["root"]

python/lsst/daf/butler/_butlerConfig.py

@@ -61,12 +61,15 @@ class ButlerConfig(Config):
         than those read from the environment in
         `ConfigSubset.defaultSearchPaths()`.  They are only read if ``other``
         refers to a configuration file or directory.
+    skip_datastore : `bool`, optional
+        If `True` remove the datastore configuration.
     """
 
     def __init__(
         self,
         other: ResourcePathExpression | Config | None = None,
         searchPaths: Sequence[ResourcePathExpression] | None = None,
+        skip_datastore: bool = False,
     ):
         self.configDir: ResourcePath | None = None
 
@@ -155,6 +158,13 @@ def __init__(
         # configuration classes. We ask each of them to apply defaults to
         # the values we have been supplied by the user.
         for configClass in CONFIG_COMPONENT_CLASSES:
+            assert configClass.component is not None, "Config class component cannot be None"
+
+            if skip_datastore and configClass is DatastoreConfig:
+                if configClass.component in butlerConfig:
+                    del butlerConfig[configClass.component]
+                continue
+
             # Only send the parent config if the child
             # config component is present (otherwise it assumes that the
             # keys from other components are part of the child)
@@ -163,7 +173,6 @@ def __init__(
                 localOverrides = butlerConfig
             config = configClass(localOverrides, searchPaths=searchPaths)
             # Re-attach it using the global namespace
-            assert configClass.component is not None, "Config class component cannot be None"
             self.update({configClass.component: config})
             # Remove the key from the butlerConfig since we have already
             # merged that information.

tests/test_butler.py

@@ -76,6 +76,7 @@ def mock_s3(*args: Any, **kwargs: Any) -> Any:  # type: ignore[no-untyped-def]
     FileDataset,
     FileTemplate,
     FileTemplateValidationError,
+    NullDatastore,
     StorageClassFactory,
     ValidationError,
     script,
@@ -2332,6 +2333,54 @@ class ChainedDatastoreTransfers(PosixDatastoreTransfers):
     configFile = os.path.join(TESTDIR, "config/basic/butler-chained.yaml")
 
 
+class NullDatastoreTestCase(unittest.TestCase):
+    """Test that we can fall back to a null datastore."""
+
+    # Need a good config to create the repo.
+    configFile = os.path.join(TESTDIR, "config/basic/butler.yaml")
+
+    @classmethod
+    def setUpClass(cls) -> None:
+        cls.storageClassFactory = StorageClassFactory()
+        cls.storageClassFactory.addFromConfig(cls.configFile)
+
+    def setUp(self) -> None:
+        """Create a new butler root for each test."""
+        self.root = makeTestTempDir(TESTDIR)
+        Butler.makeRepo(self.root, config=Config(self.configFile))
+
+    def tearDown(self) -> None:
+        removeTestTempDir(self.root)
+
+    def test_fallback(self) -> None:
+        # Read the butler config and mess with the datastore section.
+        bad_config = Config(os.path.join(self.root, "butler.yaml"))
+        bad_config["datastore", "cls"] = "lsst.not.a.datastore.Datastore"
+
+        with self.assertRaises(RuntimeError):
+            Butler(bad_config)
+
+        butler = Butler(bad_config, writeable=True, skip_datastore=True)
+        self.assertIsInstance(butler._datastore, NullDatastore)
+
+        # Check that registry is working.
+        butler.registry.registerRun("MYRUN")
+        collections = butler.registry.queryCollections(...)
+        self.assertIn("MYRUN", set(collections))
+
+        # Create a ref.
+        dimensions = butler.dimensions.extract([])
+        storageClass = self.storageClassFactory.getStorageClass("StructuredDataDict")
+        datasetTypeName = "metric"
+        datasetType = DatasetType(datasetTypeName, dimensions, storageClass)
+        butler.registry.registerDatasetType(datasetType)
+        ref = DatasetRef(datasetType, {}, run="MYRUN")
+
+        # Check that datastore will complain.
+        with self.assertRaises(NotImplementedError):
+            butler.get(ref)
+
+
 def setup_module(module: types.ModuleType) -> None:
     """Set up the module for pytest."""
     clean_environment()