Common agreement on loading CF non-compliant NetCDF files

[trexfeathers]

Iris needs a public statement on how it handles NetCDF files that deviate from the CF conventions. This will serve multiple benefits:

• More certainty when discussing if/how Iris should load a particular file.
• Clearer direction when developing the codebase.
• Set user expectations.

Writing this statement will involve making some difficult decisions. A working group is tackling this now: @tkknight, @bjlittle, @lbdreyer, @pp-mo, @trexfeathers, @stephenworsley, @ESadek-MO, @scottrobinson02, @HGWright

Factors at play

• More CF compliance means smoother collaboration between institutions, and Iris can play a part in raising awareness.
• CF evolves over time, so may develop 'opinions' on things that previously didn't matter and invalidate older files.
• The available tooling can make it difficult to address non-compliances in a file.
• UX - being strict/verbose about CF compliance makes the user experience more awkward.
• Iris has a place in the scientific Python community - people choose Iris / Xarray / raw netCDF4 / something else / for different purposes, and CF handling plays a part in that.
• Continuing to work in the face of CF non-compliances could need more defensive code.

Items affected

(please edit if you know of others)

• Failing to load cubes due to cell_method issues #5119
• Cellmethod tolerance #5126
• What should happen to non-loaded coordinates etcetera? #5068
• Failing to load a NetCDF dataset #5067
• Allow exemptions to coordinate axis guessing on save #5003
• Warn or error rather than returning None from name check in netcdf #4495
• loading cubes with differing units for time and time_bounds #1801
• Loading not possible with non-standard netcdf variable names #5171
• Case insensitive lenient special name support? #4453
• Handling of Standard name aliases #5257

### Tasks
- [ ] https://github.com/SciTools/iris/issues/5068
- [ ] https://github.com/SciTools/iris/issues/5119

[trexfeathers]

Summary from working group conversations

2023-02-02, 2023-02-14, 2023-03-22

Note this issue is not intended as a debate, hence why it is not posted as a discussion. The below conversations took place in real time, with a group deliberately sized to aid decision making.

Outcome - our ideal implementation

When loading NetCDF files, Iris will load all CF-compliant elements. A container of non-compliant variables and attributes will be attached to the Cube(s).

Encourage users:

If this causes you problems, please reach out to us to see if we can collaborate on a solution.

Implementation considerations

• How to contain things that can't be represented properly?
• Associate things with Cubes or isolated in own list?
• Activate behaviour with a FUTURE flag?

Working group summary comments

• @trexfeathers: embrace imperfection, skipping non-compliances sounds good if warnings work.
• @stephenworsley: CF compliance is a good aim, but can't always be expected.
• @pp-mo: CF offers optional ways of doing things, Iris ought to do its best, but not insist. Discourage 'bad CF'.
• @bjlittle: KISS. Make users' lives simple, don't be awkward.
• @lbdreyer: we'll always break someone's workflow. Need a plan to help those who are left behind.
• @scottrobinson02: spirit of compromise. Accept that going in.
• @tkknight: KISS. Informative messages when things don't work.
• @HGWright: if we can do something we should do something. Don't throw toys from pram. Make our actions clear.
• @ESadek-MO: no easy solution, communicate well, focus on warnings.

Discussion topics

Encouraging compliance in the community

• We know examples where Iris' strictness has resulted in more compliant - more interoperable - files.
• CF is a convention, not a standard.
• CF is the only available convention and is therefore used for anyone looking for help making files interoperable.
• Iris' scope is wider than CF, and Iris doesn't implement all of CF.
  Need to avoid inventing our own rules.
• CF's longevity is relevant.

Files changing from acceptable to unacceptable

• While CF is intended to be backwards compatible, checks (within Iris, cf-checker, whatever) are not a complete implementation and may evolve over time, invalidating previously acceptable files.

Ease of massaging files to be compliant

• Always going to be somewhat difficult.
• If Iris can't cope with non-CF, then users forced onto another tool.
  • Could edit the file directly using ncedit or NetCDF4, but this can be challenging, and editing a copy may be unrealistic.
  • All the rich tools (Iris, Xarray, cf-python) have their own opinions.
  • ncdata has the potential to make this much easier.
• Should Iris include a non-CF layer, lower than a Cube, to help with fixing?

User experience (UX)

• Cannot be underestimated.
• Undesirable to flatly refuse to load.
• Need clarity on what Iris expects.
• Need user education.
• Warnings are an opportunity to encourage compliance and help, without 'being awkward'
  • Really important to not ruin UX with even more warnings.
  • Classify warnings? Allowing users granularity for what the care about / ignore?
• CF brings some inevitable complexity, some user effort required.
• Compromises are necessary.

Iris' place in the world

• Interoperability allows using other, more tolerant tools.
• Learning/adopting other tools is nevertheless not as good as getting everything from one place.
• We should aim to avoid duplication within the geoscience community.

Ease of software development

• Defensive code takes extra effort.
• Iris could be written to work with things it doesn't explicitly understand.
• API changes could make things easier:
  • Interchange between Cube and _DimensionalMetadata.
  • Easier construction of Cubes from scratch.
• Might be easier to include user-level fixing tools in Iris, rather than making Iris cope better.

Preferred approaches

Determined via voting.

1. Iris only loads CF compliant parts of file, skipping non-compliant (maybe raises warning?).
2. Iris allows the user to configure how it will interpret malformed file.