`anndata` `0.2.0` on-disk format

[ilan-gold]

Please describe your wishes and possible alternatives to achieve the desired result.

A few things should probably go into a 0.2.0 version of the anndata spec:

• Writing to H5AD has changed in v0.12.1 #2052 brings up hard-specifying multi-dimensionalness of xxxm
• X should probably be disallowed after feat: Unify X and layers #1707
• uns's restriction should be loosened to allow for JSON objects (or simply JSON objects should be considered "anndata-encoded") Allow storing heterogeneous lists in .uns (as JSON or so) #1979
• An escape character key for sub-keys of elements like obs/foo/bar - see (fix): disallow forward slash writes #1940

[flying-sheep]

Is this about changing the format in ways that aren’t covered in “encoding” changes?

OtherwiseApart from that maybe we should add #1940?

[ilan-gold]

I think you have that right if I understand you correctly, I'm talking about https://anndata.readthedocs.io/en/latest/fileformat-prose.html#anndata-specification-v0-1-0 specifically. So #1940 should probably count as we would want to specify the escape character somehow, although the "Element Specification" is not versioned and that would be the most reasonable option since making it a global setting seems wrong somehow. Perhaps it is something that could be specified as part of the anndata spec implicitly i.e., we state in this document that the escape character is XYZ and it does not need to be stored explicitly.