Fix SaveHDF5 operator to allow volume=None.

.github/workflows/wheels.yml

@@ -4,14 +4,14 @@ name:  Test Binary Wheels
 # on: workflow_dispatch
 # FIXME:  Remove these lines once this workflow merged
 # to main branch.
-on:
-  push:
-    branches:
-      - master
-  pull_request:
-    branches:
-      - master
-      - toast3
+# on:
+#   push:
+#     branches:
+#       - master
+#   pull_request:
+#     branches:
+#       - master
+#       - toast3
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}

src/toast/ops/save_hdf5.py

@@ -148,7 +148,11 @@ class SaveHDF5(Operator):
 
     API = Int(0, help="Internal interface version for this operator")
 
-    volume = Unicode("toast_out_hdf5", help="Top-level directory for the data volume")
+    volume = Unicode(
+        "toast_out_hdf5",
+        allow_none=True,
+        help="Top-level directory for the data volume",
+    )
 
     # FIXME:  We should add a filtering mechanism here to dump a subset of
     # observations and / or detectors, as well as figure out subdirectory organization.
@@ -196,6 +200,11 @@ def __init__(self, **kwargs):
     def _exec(self, data, detectors=None, **kwargs):
         log = Logger.get()
 
+        if self.volume is None:
+            msg = "You must set the volume trait prior to calling exec()"
+            log.error(msg)
+            raise RuntimeError(msg)
+
         # One process creates the top directory
         if data.comm.world_rank == 0:
             os.makedirs(self.volume, exist_ok=True)