Change sensor attribute to lowercase in FCI L2 NetCDF reader

[strandgren]

Following a discussion on slack (https://pytroll.slack.com/archives/CK9TPLCG0/p1737127470400239 and https://pytroll.slack.com/archives/C0LNH7LMB/p1737561433643359), this PR changes the sensor name dataset attribute in the FCI L2 NetCDF reader (fci_l2_nc) from uppercase to lowercase. This is needed in order to identify instrument specific composites and enhancements. I also modified the tests to mimic the real files where there sensor and platform information are stored in uppercase in the file metadata, such that we can also check that the sensor attribute is still returned with lowercase.

As discussed on slack, it could be worthwhile updating the documentation as well. However, I'm a bit confused and therefore not sure what and how to update. In the documentation for adding custom readers (https://satpy.readthedocs.io/en/v0.53.0/dev_guide/custom_reader.html) it says that the sensor(s) listed in the yaml file, "must be all lowercase letters for full support throughout in Satpy". However, this was already the case before the PR:

reader:
  name: fci_l2_nc
  short_name: FCI L2 NetCDF4
  long_name: MTG FCI L2 data in netCDF4 format
  description: Reader for EUMETSAT MTG FCI L2 files in NetCDF4 format.
  status: Alpha
  supports_fsspec: false
  sensors: [fci]
  reader: !!python/name:satpy.readers.yaml_reader.GEOFlippableFileYAMLReader

Hence, it seems like this is not enough and that the sensor attribute of the dataset also has to be in lowercase, which is not mentioned here: https://satpy.readthedocs.io/en/stable/reading.html#dataset-metadata (sensor is not even listed as mandatory metadata/attribute). Hence, my question is whether there is any link between the sensor(s) listed in the yaml file and the sensor attribute added as metadata to a dataset? To me it seems like it's not the case, and if so we might want to add a note under the dataset metadata (https://satpy.readthedocs.io/en/stable/reading.html#dataset-metadata), similar to what we have for the reader yaml-file, e.g. something like this:

"sensor - name of the sensor. For full support throughout in Satpy this must be all lowercase letters."

• Tests adapted
• Fully documented

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.11%. Comparing base (01237e2) to head (91e758a).
Report is 65 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3048   +/-   ##
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  Coverage   96.11%   96.11%           
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  Files         383      383           
  Lines       55673    55673           
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  Hits        53511    53511           
  Misses       2162     2162           

Flag	Coverage Δ
behaviourtests	3.89% <0.00%> (ø)
unittests	96.21% <100.00%> (ø)

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[coveralls]

Pull Request Test Coverage Report for Build 13131140073

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 13 of 13 (100.0%) changed or added relevant lines in 2 files are covered.
• 8 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.01%) to 96.222%

---

Files with Coverage Reduction	New Missed Lines	%
satpy/tests/utils.py	2	93.16%
satpy/tests/reader_tests/gms/test_gms5_vissr_l1b.py	3	98.67%
satpy/tests/reader_tests/gms/test_gms5_vissr_navigation.py	3	97.18%

Totals
Change from base Build 13089264056:	-0.01%
Covered Lines:	53758
Relevant Lines:	55869

---

💛 - Coveralls

[djhoese]

My guess is that the documentation about the YAML sensor was written when it was assumed that file handlers and the base reader was using that YAML sensor. As more readers were developed I think this quickly became not true as most could get the sensor information from the file content.

I think your suggestion of adding the sensor information to the metadata section would be good. I think we should leave the sensor information about the YAML as that is still sometimes used I think with find_files_and_readers. Your suggestion was:

sensor - name of the sensor. For full support throughout in Satpy this must be all lowercase letters.

Small grammar fix: remove the "in" before Satpy.

I wonder if we should add something about needing it to be consistent across readers? Also there is support for multiple sensors as a set object. Maybe:

sensor - name of the sensor that recorded the data. For full support through Satpy this should be all lowercase. If the data is the result of observations from multiple sensors a set object can be used to specify more than one sensor name.

[strandgren]

I wonder if we should add something about needing it to be consistent across readers? Also there is support for multiple sensors as a set object. Maybe:

sensor - name of the sensor that recorded the data. For full support through Satpy this should be all lowercase. If the data is the result of observations from multiple sensors a set object can be used to specify more than one sensor name.

I like your suggestion, I've added it to the docs now. The docs build fails for the PR though, however it also failed before I edited the docs file, so I think it's something unrelated?

[djhoese]

Thanks! The RTD failure was the one that should be fixed in main so I've merged this.