Simpler extension interface

[jsignell]

I just started interacting with the extensions in pystac and I was surprised by the API. As an xarray user I was expecting to just do item.proj. I think we can implement that by copying the _CachedAccessor code from xarray (this seems to be what pandas has done as well)

Getting extension and setting fields on it

Existing API

from pystac.extensions.proj import ProjectionExtension
import pystac 

PROJ_EXT_EXAMPLE_URL = "https://raw.githubusercontent.com/stac-extensions/projection/v1.1.0/examples/item.json"
item = pystac.Item.from_file(PROJ_EXT_EXAMPLE_URL)

proj_ext = ProjectExtension.ext(item)
proj_ext.proj

Proposed API

import pystac 

PROJ_EXT_EXAMPLE_URL = "https://raw.githubusercontent.com/stac-extensions/projection/v1.1.0/examples/item.json"
item = pystac.Item.from_file(PROJ_EXT_EXAMPLE_URL)

item.proj.epsg

Adding extension to item

I'm not sure what adding new extensions to existing items should look like.

Existing API

from pystac.extensions.eo import EOExtension

eo_ext = EOExtension.add_to(item)
eo_ext.cloud_cover = 0.7

Options

In the most magical world (probably too magical) just setting an extension field on the item could add the extension to the item:

item.eo.cloud_cover = 0.7

Another option would be to preserver the existing explicit add_to and remove_from functionality but maybe with the functions defined on the STACObject class so you could do:

item.add_extension("eo")  
item.eo.cloud_cover = 0.7

Either way, I think trying to set the field directly: item.proj = ProjectionExtension() should fail.

xref: #448 where extensions are also being discussed.

[jsignell]

I did a little implementation (no types yet) just to get a sense of what this feels like and I am pretty sure I like it in the get case.

[gadomski]

A couple of comments/questions.

Extension names as enum

item.add_extension("eo")

This should probably be an enum, e.g.:

class Extension(enum):  # Extensions would then inherit from a `BaseExtension` or `AbstractExtension` or something
    Eo = "eo"
    Projection = "projection"
    ...

Bulk update

Either way, I think trying to set the field directly: item.proj = ProjectionExtension() should fail.

I think having some sort of interface for bulk-updating an extension's values is important. It can be useful to store constant values as dictionaries (e.g. this example for the classification extension).

I'm not sure what that would look like, maybe:

item.proj.update(epsg=4326, shape=[42, 16])

Assets

Would assets have a similar interface? E.g.:

asset = item.assets["data"]
asset.raster.bands[0].nodata = 42

Sub structures

Many extensions have sub-structures, e.g. raster:bands. Would those still be their own standalone data structures? E.g.:

asset.raster.bands = [RasterBand(nodata=42, data_type="int16")]

[jsignell]

I was definitely approaching this more from the perspective of someone reading rather than writing STAC. I just spent some time looking over stactools/sentinel2 to get a sense of what the write patterns look like. So with that in mind I am going to try to answer Pete's questions. But if there is just no appetite for this type of change, feel free to close this ticket.

Extension names as enum

item.add_extension("eo")

This should probably be an enum, e.g.:

I am leaning more towards the very magic solution of not requiring these at all. So how you add an extension to an item is just by starting to write fields on it.

item.eo.cloud_cover = 0.7

Bulk update

Either way, I think trying to set the field directly: item.proj = ProjectionExtension() should fail.

I think having some sort of interface for bulk-updating an extension's values is important. It can be useful to store constant values as dictionaries (e.g. this example for the classification extension).

I'm not sure what that would look like, maybe:

item.proj.update(epsg=4326, shape=[42, 16])

I think that is pretty much right and already implemented in *Extension.apply. We can add enforcement of this method in the base class.

Assets

Would assets have a similar interface? E.g.:

asset = item.assets["data"]
asset.raster.bands[0].nodata = 42

Yes I think all pystac objects would have this interface.

Sub structures

Many extensions have sub-structures, e.g. raster:bands. Would those still be their own standalone data structures? E.g.:

asset.raster.bands = [RasterBand(nodata=42, data_type="int16")]

I think those would still be standalone structures yeah.

[gadomski]

Cool, I'm on board with your suggestions (with my one comment below). How do you propose handling breaking changes to extensions? Currently we're (you're) building support for non-breaking changes, but how will pystac handle a world where there's (e.g.) two versions of proj in the wild that aren't directly compatible from a data structure sense?

I am leaning more towards the very magic solution of not requiring these at all. So how you add an extension to an item is just by starting to write fields on it

We should retain some way to check to see if an extension is present on an object. Maybe something like this?

assert not item.has_extension("eo")
item.eo.cloud_cover = 42
assert item.has_extension("eo")
assert item.extension_url("eo") == "https://stac-extensions.github.io/eo/v1.1.0/schema.json"

[jsignell]

This is what I was imagining:

assert item.eo is None  # or maybe it raises an error
item.eo.cloud_cover = 42
assert item.eo is not None

Too weird?

How do you propose handling breaking changes to extensions? Currently we're (you're) building support for non-breaking changes, but how will pystac handle a world where there's (e.g.) two versions of proj in the wild that aren't directly compatible from a data structure sense?

I haven't really thought that through. It feels like a separate issue. My original thinking was that if extensions had a simpler interface we could have a separate class per schema version (Option 2 from #448) and it would be less disruptive for the user since they don't really interact with the class directly. From what I've seen so far in new schema versions it seems like newer classes could inherit from older ones and then older ones could be retired on some timeline and when people try to use older one they could get an error and a suggestion to use stac-migrate

[gadomski]

Too weird?

How would we check (e.g.) the version number/url? I think it's a good thing to inspect w/o having to dig into stac_extensions.

My original thinking was that if extensions had a simpler interface we could have a separate class per schema version

👍🏼 yup, that all makes sense, just wanted to make sure we at least were thinking about it as a part of this redesign (even if we're not fully implementing).

[jsignell]

Too weird?

How would we check (e.g.) the version number/url? I think it's a good thing to inspect w/o having to dig into stac_extensions.

Once an extension has been added to an object you can access all properties and methods on the extension class via the namespace like item.eo.get_schema_uri()

[gadomski]

Once an extension has been added to an object you can access all properties and methods on the extension class via the namespace like item.eo.get_schema_uri()

That does get a little weird with our recent support for multiple schema uris (#1091). item.proj.get_schema_uri() would return the latest version, but the underlying item might have an older version?

[jsignell]

I think this whole idea lives in the world where every version of the schema has its own class. So get_schema_uri() (or possibly it's an attr schema_uri) would always be correct.

[gadomski]

I think this whole idea lives in the world where every version of the schema has its own class. So get_schema_uri() (or possibly it's an attr schema_uri) would always be correct.

Even if there's no changes to the data structure? And how do we handle versions we don't have a data structure for (e.g. a new proj version that hasn't been ported to PySTAC yet).

Sorry for the slew of questions :-)

[jsignell]

I feel like I'm missing something. What is the current mechanism for getting the version of an extension?

Even if there's no changes to the data structure?

Why would there be a release with no changes? But yes I think there would be a new class for every version. Even if it's just

class ProjectionExtension(ProjectionExtension_1_0_0):
    schema_uri = "https://stac-extensions.github.io/projection/v1.1.0/schema.json"

And how do we handle versions we don't have a data structure for (e.g. a new proj version that hasn't been ported to PySTAC yet).

I don't have a clear idea of how to handle this. My first inclination is to bake in an assumption about the schema_uri structure and do a match, but I'm not sure if that is a good idea or not.

[gadomski]

What is the current mechanism for getting the version of an extension?

Right now, there isn't one other than get_schema_uri(). But as we expand to support multiple versions, IMO it would be a good idea to let a user inspect what version their object has.

Why would there be a release with no changes?

proj is an example. We relaxed the epsg requirement, and updated some underlying jsonschema versions. None of that required changes to the pystac code base other than the extension schema url.

My first inclination is to bake in an assumption about the schema_uri structure and do a match, but I'm not sure if that is a good idea or not.

This has been my instinct as well. I haven't built anything, but my idea was something like:

• If it's a MINOR change, any unrecognized fields (i.e. attribute adds, say if we add proj:authority) get auto-added as attributes to the extension object.
• If it's a MAJOR change, a warning is issued, but all attributes are still available on the object.

That way we can avoid breaking the ecosystem if pystac is slow for adding support for new extension versions.