Write dimension version info to export yaml

And use that version on import.

python/lsst/daf/butler/_butler.py

@@ -2044,7 +2044,7 @@ def export(
             filename = os.path.join(directory, filename)
         BackendClass = get_class_of(self._config["repo_transfer_formats"][format]["export"])
         with open(filename, "w") as stream:
-            backend = BackendClass(stream)
+            backend = BackendClass(stream, universe=self.registry.dimensions)
             try:
                 helper = RepoExportContext(
                     self.registry, self.datastore, backend=backend, directory=directory, transfer=transfer

python/lsst/daf/butler/core/dimensions/_universe.py

@@ -182,6 +182,30 @@ def __new__(
 
         return self
 
+    @property
+    def version(self) -> int:
+        """The version number of this universe.
+
+        Returns
+        -------
+        version : `int`
+            An integer representing the version number of this universe.
+            Uniquely defined when combined with the `namespace`.
+        """
+        return self._version
+
+    @property
+    def namespace(self) -> str:
+        """The namespace associated with this universe.
+
+        Returns
+        -------
+        namespace : `str`
+            The namespace. When combined with the `version` can uniquely
+            define this universe.
+        """
+        return self._namespace
+
     def __repr__(self) -> str:
         return f"DimensionUniverse({self._version}, {self._namespace})"
 
@@ -495,4 +519,4 @@ def __deepcopy__(self, memo: dict) -> DimensionUniverse:
 
     _version: int
 
-    _namespace: Optional[str]
+    _namespace: str

python/lsst/daf/butler/transfers/_yaml.py

@@ -43,6 +43,7 @@
     Datastore,
     DimensionElement,
     DimensionRecord,
+    DimensionUniverse,
     FileDataset,
     Timespan,
 )
@@ -94,8 +95,9 @@ class YamlRepoExportBackend(RepoExportBackend):
         A writeable file-like object.
     """
 
-    def __init__(self, stream: IO):
+    def __init__(self, stream: IO, universe: DimensionUniverse):
         self.stream = stream
+        self.universe = universe
         self.data: List[Dict[str, Any]] = []
 
     def saveDimensionData(self, element: DimensionElement, *data: DimensionRecord) -> None:
@@ -195,6 +197,8 @@ def finish(self) -> None:
             {
                 "description": "Butler Data Repository Export",
                 "version": str(EXPORT_FORMAT_VERSION),
+                "universe_version": self.universe.version,
+                "universe_namespace": self.universe.namespace,
                 "data": self.data,
             },
             stream=self.stream,
@@ -248,13 +252,20 @@ def __init__(self, stream: IO, registry: Registry):
         self.refsByFileId: Dict[DatasetId, DatasetRef] = {}
         self.registry: Registry = registry
 
+        universe_version = wrapper.get("universe_version", 0)
+        universe_namespace = wrapper.get("universe_namespace", "daf_butler")
+
         # If this is data exported before the reorganization of visits
         # and visit systems and that new schema is in use, some filtering
         # will be needed. The entry in the visit dimension record will be
         # silently dropped when visit is created but the
         # visit_system_membership must be constructed.
         migrate_visit_system = False
-        if fileVersion < VersionTuple(1, 0, 2) and "visit_system_membership" in self.registry.dimensions:
+        if (
+            universe_version < 2
+            and universe_namespace == "daf_butler"
+            and "visit_system_membership" in self.registry.dimensions
+        ):
             migrate_visit_system = True
 
         datasetData = []

tests/test_dimensions.py

@@ -128,6 +128,11 @@ def testConfigPresent(self):
         config = self.universe.dimensionConfig
         self.assertIsInstance(config, DimensionConfig)
 
+    def testVersion(self):
+        self.assertEqual(self.universe.namespace, "daf_butler")
+        # Test was added starting at version 2.
+        self.assertGreaterEqual(self.universe.version, 2)
+
     def testConfigRead(self):
         self.assertEqual(
             set(self.universe.getStaticDimensions().names),