Add gen3 formatter for Filter

This allows an Exposure to be disassembled and assembled without
relying on adding something to metadata.

python/lsst/obs/base/exposureAssembler.py

@@ -219,6 +219,7 @@ def assemble(self, components):
         info.setApCorrMap(components.pop("apCorrMap", None))
         info.setCoaddInputs(components.pop("coaddInputs", None))
         info.setMetadata(components.pop("metadata", None))
+        info.setFilter(components.pop("filter", None))
 
         # If we have some components left over that is a problem
         if components:

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

@@ -0,0 +1,154 @@
+# This file is part of obs_base.
+#
+# 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__ = ("FilterFormatter",)
+
+import yaml
+from lsst.afw.image import Filter
+
+from typing import (
+    Any,
+    Optional,
+    Type,
+)
+
+from lsst.daf.butler.formatters.file import FileFormatter
+
+
+class FilterFormatter(FileFormatter):
+    """Read and write `~lsst.afw.image.Filter` filter information."""
+
+    extension = ".yaml"
+
+    unsupportedParameters = None
+    """This formatter does not support any parameters."""
+
+    def _readFile(self, path: str, pytype: Type[Any] = None) -> Any:
+        """Read a file from the path in YAML format.
+
+        Parameters
+        ----------
+        path : `str`
+            Path to use to open the file.
+        pytype : `class`, optional
+            The type expected to be returned.
+
+        Returns
+        -------
+        data : `object`
+            Either data as Python object read from YAML file, or None
+            if the file could not be opened.
+        """
+        try:
+            with open(path, "rb") as fd:
+                data = self._fromBytes(fd.read(), pytype)
+        except FileNotFoundError:
+            data = None
+
+        return data
+
+    def _fromBytes(self, serializedDataset: bytes, pytype: Optional[Type[Any]] = None) -> Any:
+        """Read the bytes object as a python object.
+
+        Parameters
+        ----------
+        serializedDataset : `bytes`
+            Bytes object to unserialize.
+        pytype : `type`, optional
+            Expected python type to be returned.
+
+        Returns
+        -------
+        inMemoryDataset : `lsst.afw.image.Filter`
+            The requested data as an object.
+        """
+        data = yaml.load(serializedDataset, Loader=yaml.SafeLoader)
+
+        if pytype is None:
+            pytype = Filter
+
+        # This will be a simple dict so we need to convert it to
+        # the Filter type -- just needs the name
+        filter = pytype(data["canonicalName"], force=True)
+
+        return filter
+
+    def _writeFile(self, inMemoryDataset: Any) -> None:
+        """Write the in memory dataset to file on disk.
+
+        Parameters
+        ----------
+        inMemoryDataset : `lsst.afw.image.Filter`
+            Filter to serialize.
+
+        Raises
+        ------
+        Exception
+            Raised if the file could not be written or the dataset could not be
+            serialized.
+        """
+        with open(self.fileDescriptor.location.path, "wb") as fd:
+            fd.write(self._toBytes(inMemoryDataset))
+
+    def _toBytes(self, inMemoryDataset: Any) -> bytes:
+        """Write the in memory dataset to a bytestring.
+
+        Parameters
+        ----------
+        inMemoryDataset : `lsst.afw.image.Filter`
+            Object to serialize.
+
+        Returns
+        -------
+        serializedDataset : `bytes`
+            YAML string encoded to bytes.
+
+        Raises
+        ------
+        Exception
+            Raised if the object could not be serialized.
+        """
+
+        # Convert the Filter to a dict for dumping
+        # Given the singleton situation, only the name is really
+        # needed but it does not hurt to put some detail in the file
+        # to aid debugging.
+        filter = {}
+        filter["canonicalName"] = inMemoryDataset.getCanonicalName()
+        filter["name"] = inMemoryDataset.getName()
+        filter["aliases"] = inMemoryDataset.getAliases()
+
+        # This can fail if the filter is no longer available in the
+        # Singleton
+        try:
+            filterProperty = inMemoryDataset.getFilterProperty()
+        except LookupError:
+            pass
+        else:
+            properties = {}
+            properties["lambdaEff"] = filterProperty.getLambdaEff()
+            properties["lambdaMax"] = filterProperty.getLambdaMax()
+            properties["lambdaMin"] = filterProperty.getLambdaMin()
+            filter["properties"] = properties
+
+        return yaml.dump(filter).encode()

tests/test_butlerFits.py

@@ -75,7 +75,8 @@
 """
 
 # Components present in the test file
-COMPONENTS = {"wcs", "image", "mask", "coaddInputs", "psf", "visitInfo", "variance", "metadata", "photoCalib"}
+COMPONENTS = {"wcs", "image", "mask", "coaddInputs", "psf", "visitInfo", "variance", "metadata", "photoCalib",
+              "filter"}
 
 
 class ButlerFitsTests(DatasetTestHelper, lsst.utils.tests.TestCase):
@@ -245,6 +246,8 @@ def runExposureCompositePutGetTest(self, datasetTypeName: str) -> DatasetRef:
             elif compName == "psf":
                 # Equality for PSF does not work
                 pass
+            elif compName == "filter":
+                self.assertEqual(component.getCanonicalName(), reference.getCanonicalName())
             elif compName == "visitInfo":
                 self.assertEqual(component.getExposureId(), reference.getExposureId(),
                                  f"VisitInfo comparison")