How well are standalone collections supported in `pystac`?

[raster-blaster]

Hi,

I have a few questions regarding standalone collections in pystac:
1. Can the ProjectionExtension.ext be used directly on a standalone collection, rather than on an item or asset?
2. Is there a recommended or straightforward way to convert a standalone collection into an item using pystac?

These questions are based on a related discussion in the odc-stac repository, and I’d like to understand how well pystac supports these use cases.

Thanks for your insights!

[gadomski]

1. Can the ProjectionExtension.ext be used directly on a standalone collection, rather than on an item or asset?

No, and I think this is in line with the projection extension, which only supports projection fields on assets and item_assets (and implicitly summaries): https://github.com/stac-extensions/projection/blob/f17b5707439e4d6aa5102a9018e5e52984d0d744/json-schema/schema.json#L50-L61.

2. Is there a recommended or straightforward way to convert a standalone collection into an item using pystac?

Not at this time, but PRs are welcome! A Collection.to_item method could make sense, but feels very niche and potentially confusing, as it only really makes sense if the collection doesn't have any items itself. Maybe a Item.from_standalone_collection classmethod could work.