DM-16957: Add pickle support to DataId class

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

@@ -339,3 +339,23 @@ def fields(self, element, region=True, metadata=True):
             return entries
         return {k: v for k, v in entries.items()
                 if (metadata or k in self.keys()) and (region or k != "region")}
+
+    def __getnewargs_ex__(self):
+        """Support special pickling for DataId.
+
+        Default unpickling code calls `__new__` without arguments which does
+        not work for this class, need to provide minimal set of arguments to
+        to support logic in `__new__` and to run it without error. Pickle
+        still executes its regular logic to save instance attributes and
+        restore their state (after creating new instance with `__new__`).
+
+        Returns
+        -------
+        args : `tuple`
+            Positional arguments for `__new__`.
+        kwargs : `dict`
+            Keyword arguments for `__new__`.
+        """
+        args = (None,)
+        kwargs = dict(dimensions=self.dimensions)
+        return (args, kwargs)

tests/test_dataid.py

@@ -0,0 +1,73 @@
+# This file is part of daf_butler.
+#
+# Developed for the LSST Data Management System.
+# This product includes software developed by the LSST Project
+# (http://www.lsst.org).
+# See the COPYRIGHT file at the top-level directory of this distribution
+# for details of code ownership.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import unittest
+import pickle
+
+import lsst.utils
+import lsst.utils.tests
+
+from lsst.daf.butler import DataId, DimensionGraph
+
+
+class DataIdTestCase(lsst.utils.tests.TestCase):
+    """Tests for DataId.
+
+    All tests here rely on the content of ``config/dimensions.yaml``, either
+    to test that the definitions there are read in properly or just as generic
+    data for testing various operations.
+    """
+
+    def setUp(self):
+        self.universe = DimensionGraph.fromConfig()
+
+    #
+    # TODO: more tests would be useful
+    #
+
+    def testConstructor(self):
+        """Test for making new instances.
+        """
+
+        dataId = DataId(dict(instrument="DummyInstrument",
+                             detector=1,
+                             visit=2,
+                             physical_filter="i"),
+                        universe=self.universe)
+        self.assertEqual(len(dataId), 4)
+        self.assertCountEqual(dataId.keys(),
+                              ("instrument", "detector", "visit", "physical_filter"))
+
+    def testPickle(self):
+        """Test pickle support.
+        """
+        dataId = DataId(dict(instrument="DummyInstrument",
+                             detector=1,
+                             visit=2,
+                             physical_filter="i"),
+                        universe=self.universe)
+        dataIdOut = pickle.loads(pickle.dumps(dataId))
+        self.assertIsInstance(dataIdOut, DataId)
+        self.assertEqual(dataId, dataIdOut)
+
+
+if __name__ == "__main__":
+    unittest.main()