Specifically:

1. In writeFits add a new primary header key "STAMPCLS" (8 characters is simplest) with the full name of the class being serialized.
2. This name could come from lsst.daf.butler.core.utils.getFullTypeName (which means it probably should be moved to lsst.utils at some point)
3. In readFits read the primary header, look for STAMPCLS, if it's there call lsst.utils.doImport on the class string.
4. Use the return value from doImport to override the `cls` variable used for the class method.

Not sure you want to bypass the reading of the header if your cls variable is already a subclass of StampsBase (implying that someone is explicitly asking for a class and not using the base class to do it).

Still seeing this error:

`KeyError: "Unable to find item in registry with keys: donutStamps ({'instrument': 'LSSTCam-PhoSim'}), band+instrument+detector+physical_filter+exposure ({'instrument': 'LSSTCam-PhoSim'}), StampsBase ({'instrument': 'LSSTCam-PhoSim'}), donutStamps, band+instrument+detector+physical_filter+exposure, StampsBase"`

Is there a step we are missing?

My changes are pushed to github here: https://github.com/lsst/meas_algorithms/compare/tickets/DM-29412

I can take a look at that meas_algorithms change if you make a pull request.

Have you got more context for that KeyError you are getting?

Yeah, I have a full traceback here:

Process task-5:
Traceback (most recent call last):
File "/astro/store/epyc/users/brycek/Commissioning/aos/clean_setup/daf_butler/python/lsst/daf/butler/datastores/fileDatastore.py", line 659, in _determine_put_formatter_location
formatter = self.formatterFactory.getFormatter(ref,
File "/astro/store/epyc/users/brycek/Commissioning/aos/clean_setup/daf_butler/python/lsst/daf/butler/core/formatter.py", line 751, in getFormatter
_, formatter = self.getFormatterWithMatch(entity, *args, **kwargs)
File "/astro/store/epyc/users/brycek/Commissioning/aos/clean_setup/daf_butler/python/lsst/daf/butler/core/formatter.py", line 724, in getFormatterWithMatch
matchKey, formatter = self._mappingFactory.getFromRegistryWithMatch(names, *args, **kwargs)
File "/astro/store/epyc/users/brycek/Commissioning/aos/clean_setup/daf_butler/python/lsst/daf/butler/core/mappingFactory.py", line 188, in getFromRegistryWithMatch
key, cls, registry_kwargs = self.getClassFromRegistryWithMatch(targetClasses)
File "/astro/store/epyc/users/brycek/Commissioning/aos/clean_setup/daf_butler/python/lsst/daf/butler/core/mappingFactory.py", line 134, in getClassFromRegistryWithMatch
raise KeyError(f"Unable to find item in registry with key{plural}: {msg}")
KeyError: "Unable to find item in registry with keys: donutStamps ({'instrument': 'LSSTCam-PhoSim'}), band+instrument+detector+physical_filter+exposure ({'instrument': 'LSSTCam-PhoSim'}), StampsBase ({'instrument': 'LSSTCam-PhoSim'}), donutStamps, band+instrument+detector+physical_filter+exposure, StampsBase"

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/astro/store/epyc/projects/lsst_comm/stack_w_2021_07/conda/miniconda3-py38_4.9.2/envs/lsst-scipipe-0.4.1/lib/python3.8/multiprocessing/process.py", line 315, in _bootstrap
self.run()
File "/astro/store/epyc/projects/lsst_comm/stack_w_2021_07/conda/miniconda3-py38_4.9.2/envs/lsst-scipipe-0.4.1/lib/python3.8/multiprocessing/process.py", line 108, in run
self._target(*self._args, **self._kwargs)
File "/astro/store/epyc/projects/lsst_comm/stack_w_2021_07/stack/miniconda3-py38_4.9.2-0.4.1/Linux64/ctrl_mpexec/21.0.0-18-gaa77eba+59b91001fb/python/lsst/ctrl/mpexec/mpGraphExecutor.py", line 127, in _executeJob
quantumExecutor.execute(taskDef, quantum, butler)
File "/astro/store/epyc/projects/lsst_comm/stack_w_2021_07/stack/miniconda3-py38_4.9.2-0.4.1/Linux64/ctrl_mpexec/21.0.0-18-gaa77eba+59b91001fb/python/lsst/ctrl/mpexec/singleQuantumExecutor.py", line 103, in execute
self.runQuantum(task, quantum, taskDef, butler)
File "/astro/store/epyc/projects/lsst_comm/stack_w_2021_07/stack/miniconda3-py38_4.9.2-0.4.1/Linux64/ctrl_mpexec/21.0.0-18-gaa77eba+59b91001fb/python/lsst/ctrl/mpexec/singleQuantumExecutor.py", line 274, in runQuantum
task.runQuantum(butlerQC, inputRefs, outputRefs)
File "/astro/store/epyc/projects/lsst_comm/stack_w_2021_07/stack/miniconda3-py38_4.9.2-0.4.1/Linux64/pipe_base/21.0.0-8-gfbe0b4b+411cd868f8/python/lsst/pipe/base/pipelineTask.py", line 156, in runQuantum
butlerQC.put(outputs, outputRefs)
File "/astro/store/epyc/projects/lsst_comm/stack_w_2021_07/stack/miniconda3-py38_4.9.2-0.4.1/Linux64/pipe_base/21.0.0-8-gfbe0b4b+411cd868f8/python/lsst/pipe/base/butlerQuantumContext.py", line 198, in put
self._put(valuesAttribute, refs)
File "/astro/store/epyc/projects/lsst_comm/stack_w_2021_07/stack/miniconda3-py38_4.9.2-0.4.1/Linux64/pipe_base/21.0.0-8-gfbe0b4b+411cd868f8/python/lsst/pipe/base/butlerQuantumContext.py", line 98, in _put
butler.put(value, ref)
File "/astro/store/epyc/users/brycek/Commissioning/aos/clean_setup/daf_butler/python/lsst/daf/butler/core/utils.py", line 265, in inner
return func(self, *args, **kwargs)
File "/astro/store/epyc/users/brycek/Commissioning/aos/clean_setup/daf_butler/python/lsst/daf/butler/_butler.py", line 851, in put
self.datastore.put(obj, ref)
File "/astro/store/epyc/users/brycek/Commissioning/aos/clean_setup/daf_butler/python/lsst/daf/butler/core/utils.py", line 265, in inner
return func(self, *args, **kwargs)
File "/astro/store/epyc/users/brycek/Commissioning/aos/clean_setup/daf_butler/python/lsst/daf/butler/datastores/fileDatastore.py", line 1461, in put
storedInfo = self._write_in_memory_to_artifact(inMemoryDataset, ref)
File "/astro/store/epyc/users/brycek/Commissioning/aos/clean_setup/daf_butler/python/lsst/daf/butler/datastores/fileDatastore.py", line 952, in _write_in_memory_to_artifact
location, formatter = self._prepare_for_put(inMemoryDataset, ref)
File "/astro/store/epyc/users/brycek/Commissioning/aos/clean_setup/daf_butler/python/lsst/daf/butler/datastores/fileDatastore.py", line 627, in _prepare_for_put
return self._determine_put_formatter_location(ref)
File "/astro/store/epyc/users/brycek/Commissioning/aos/clean_setup/daf_butler/python/lsst/daf/butler/datastores/fileDatastore.py", line 664, in _determine_put_formatter_location
raise DatasetTypeNotSupportedError(f"Unable to find formatter for {ref} in datastore "
lsst.daf.butler.core.exceptions.DatasetTypeNotSupportedError: Unable to find formatter for donutStamps@{instrument: 'LSSTCam-PhoSim', detector: 90, exposure: 9006001, ...}, sc=StampsBase] (id=21) in datastore FileDatastore@<butlerRoot>

In the task I wrote the connections class has this to setup the donut stamps:

donutStamps = connectionTypes.Output(
doc="Donut Postage Stamp Images",
dimensions=("exposure", "detector", "instrument"),
storageClass="StampsBase",
name="donutStamps",
)

you need to add StampsBase to formatters.yaml in butler as well storageClasses.yaml.

This change looks good to me. If you put the jira ticket in review I can formally review it. Please remember to run Jenkins before merging.

Great. Thanks for the help! Running jenkins now.

Looks good to me.