Add convenience method to get default dimension packers.

doc/changes/DM-39453.feature.md

@@ -0,0 +1 @@
+Add `Instrument.make_default_dimension_packer` to restore simple access to the default data ID packer for an instrument.

python/lsst/pipe/base/_instrument.py

@@ -31,14 +31,13 @@
 
 from lsst.daf.butler import DataCoordinate, DataId, DimensionPacker, DimensionRecord, Formatter
 from lsst.daf.butler.registry import DataIdError
-from lsst.pex.config import RegistryField
+from lsst.pex.config import Config, RegistryField
 from lsst.utils import doImportType
 
 from ._observation_dimension_packer import observation_packer_registry
 
 if TYPE_CHECKING:
     from lsst.daf.butler import Registry
-    from lsst.pex.config import Config
 
 
 class Instrument(metaclass=ABCMeta):
@@ -624,6 +623,53 @@ class to determine the behavior for each data ID encountered.  When the
             doc, default=None, optional=True, on_none=Instrument._make_default_dimension_packer_dispatch
         )
 
+    @staticmethod
+    @final
+    def make_default_dimension_packer(
+        data_id: DataCoordinate, is_exposure: bool | None = None
+    ) -> DimensionPacker:
+        """Return the default dimension packer for the given data ID.
+
+        Parameters
+        ----------
+        data_id : `lsst.daf.butler.DataCoordinate`
+            Data ID that identifies at least the ``instrument`` dimension. Must
+            have dimension records attached.
+        is_exposure : `bool`, optional
+            If `False`, construct a packer for visit+detector data IDs.  If
+            `True`, construct a packer for exposure+detector data IDs.  If
+            `None`, this is determined based on whether ``visit`` or
+            ``exposure`` is present in ``data_id``, with ``visit`` checked
+            first and hence used if both are present.
+
+        Returns
+        -------
+        packer : `lsst.daf.butler.DimensionPacker`
+            Object that packs {visit, detector} or {exposure, detector} data
+            IDs into integers.
+
+        Notes
+        -----
+        When using a dimension packer in task code, using
+        `make_dimension_packer_config_field` to make the packing algorithm
+        configurable is preferred over this method.
+
+        When obtaining a dimension packer to unpack IDs that were packed by
+        task code, it is similarly preferable to load the configuration for
+        that task and the existing packer configuration field there, to ensure
+        any config overrides are respected.  That is sometimes quite difficult,
+        however, and since config overrides for dimension packers are expected
+        to be exceedingly rare, using this simpler method will almost always
+        work.
+        """
+
+        class _DummyConfig(Config):
+            packer = Instrument.make_dimension_packer_config_field()
+
+        config = _DummyConfig()
+
+        return config.packer.apply(data_id, is_exposure=is_exposure)
+
     @staticmethod
     @final
     def _make_default_dimension_packer_dispatch(

tests/test_instrument.py

@@ -208,8 +208,14 @@ def test_dimension_packer_config_defaults(self):
             # Note that we don't need to pass any more than the instrument in
             # the data ID yet, because we're just constructing packers, not
             # calling their pack method.
-            visit_packers=[config.packer.apply(instrument_data_id, is_exposure=False)],
-            exposure_packers=[config.packer.apply(instrument_data_id, is_exposure=True)],
+            visit_packers=[
+                config.packer.apply(instrument_data_id, is_exposure=False),
+                Instrument.make_default_dimension_packer(instrument_data_id, is_exposure=False),
+            ],
+            exposure_packers=[
+                config.packer.apply(instrument_data_id, is_exposure=True),
+                Instrument.make_default_dimension_packer(instrument_data_id, is_exposure=True),
+            ],
             n_detectors=record.detector_max,
             n_visits=record.visit_max,
             n_exposures=record.exposure_max,