Relocate pexConfig formatter to here from daf_butler and test it

python/lsst/obs/base/formatters/pexConfig.py

@@ -0,0 +1,98 @@
+# 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/>.
+
+from __future__ import annotations
+
+__all__ = ("PexConfigFormatter", )
+
+import os.path
+
+from typing import (
+    Any,
+    Optional,
+    Type,
+)
+
+from lsst.utils import doImport
+from lsst.daf.butler.formatters.file import FileFormatter
+
+
+class PexConfigFormatter(FileFormatter):
+    """Interface for reading and writing pex.config.Config objects from disk.
+    """
+    extension = ".py"
+
+    def _readFile(self, path: str, pytype: Optional[Type[Any]] = None) -> Any:
+        """Read a pex.config.Config instance from the given file.
+
+        Parameters
+        ----------
+        path : `str`
+            Path to use to open the file.
+        pytype : `type`
+            Class to use to read the config file.
+
+        Returns
+        -------
+        data : `lsst.pex.config.Config`
+            Instance of class ``pytype`` read from config file. `None`
+            if the file could not be opened.
+        """
+        if pytype is None:
+            raise RuntimeError("A python type is always required for reading pex_config Config files")
+
+        if not os.path.exists(path):
+            return None
+        instance = pytype()
+        # Configs can only be loaded if you use the correct derived type,
+        # but we can't really store the correct derive type in the StorageClass
+        # because that'd be a huge proliferation of StorageClasses.
+        # Instead, we use a bit of a hack: try to load using the base-class
+        # Config, inspect the exception message to obtain the class we should
+        # have used, import that, and try it instead.
+        # TODO: clean this up, somehow.
+        try:
+            instance.load(path)
+            return instance
+        except AssertionError as err:
+            msg = str(err)
+            if not msg.startswith("config is of type"):
+                raise RuntimeError("Unexpected assertion; cannot infer Config class type.") from err
+            actualPyTypeStr = msg.split()[-1]
+        actualPyType = doImport(actualPyTypeStr)
+        instance = actualPyType()
+        instance.load(path)
+        return instance
+
+    def _writeFile(self, inMemoryDataset: Any) -> None:
+        """Write the in memory dataset to file on disk.
+
+        Parameters
+        ----------
+        inMemoryDataset : `object`
+            Object to serialize.
+
+        Raises
+        ------
+        Exception
+            The file could not be written.
+        """
+        inMemoryDataset.save(self.fileDescriptor.location.path)

tests/test_butlerFits.py

@@ -30,6 +30,7 @@
 
 import lsst.utils.tests
 
+import lsst.pex.config
 import lsst.afw.image
 from lsst.afw.image import LOCAL
 from lsst.geom import Box2I, Point2I
@@ -80,6 +81,12 @@
 READ_COMPONENTS = {"bbox", "xy0", "dimensions"}
 
 
+class SimpleConfig(lsst.pex.config.Config):
+    """Config to use in tests for butler put/get"""
+    i = lsst.pex.config.Field("integer test", int)
+    c = lsst.pex.config.Field("string", str)
+
+
 class ButlerFitsTests(DatasetTestHelper, lsst.utils.tests.TestCase):
 
     @classmethod
@@ -117,7 +124,8 @@ def setUpClass(cls):
         # And some dataset types that have no dimensions for easy testing
         for datasetTypeName, storageClassName in (("ps", "PropertySet"),
                                                   ("pl", "PropertyList"),
-                                                  ("pkg", "Packages")
+                                                  ("pkg", "Packages"),
+                                                  ("config", "Config"),
                                                   ):
             storageClass = cls.storageClassFactory.getStorageClass(storageClassName)
             addDatasetType(cls.creatorButler, datasetTypeName, {}, storageClass)
@@ -191,6 +199,15 @@ def testFundamentalTypes(self) -> None:
         pkg = Packages.fromSystem()
         self.runFundamentalTypeTest("pkg", pkg)
 
+    def testPexConfig(self) -> None:
+        """Test that we can put and get pex_config Configs"""
+        c = SimpleConfig(i=10, c="hello")
+        self.assertEqual(c.i, 10)
+        ref = self.butler.put(c, "config")
+        butler_c = self.butler.get(ref)
+        self.assertEqual(c, butler_c)
+        self.assertIsInstance(butler_c, SimpleConfig)
+
     def testFitsCatalog(self) -> None:
         """Test reading of a FITS catalog"""
         catalog = self.makeExampleCatalog()