`CatalogExt.has()` throws KeyError on unknown extension

[soxofaan]

import pystac

c = pystac.Collection(id="foo", description="bar", extent=None)
c.ext.has("meh")

throws

KeyError: "Extension 'meh' is not a valid extension. Options are ['classification', 'cube', 'eo', 'file', 'grid', 'item_assets', 'mgrs', 'pc', 'proj', 'raster', 'sar', 'sat', 'sci', 'storage', 'table', 'timestamps', 'version', 'view', 'xarray']"

Is it the intended design to raise an exception here? As a user, I expect has() to return false if the specified extension is not available for consumption.

[jsignell]

I think that behavior is intentional but open to suggestions on how it could be improved. That error is meant to say: there is no extension named "meh" this is as opposed to the False condition which would indicate that the extension in question has not been added to the specified collection.

[soxofaan]

To give some context about my expected behavior: As a user or even more strongly as a library developer I want to build on pystac and write something like

if collection.ext.has("foo"):
    # do something with "foo" extension related metadata

Without worrying about whether/how "foo" is defined as an extension in some other place. Now I have to wrap that in try-catch boilerplate

Or is there a cleaner way to first dynamically check if the "foo" extension exists? Something like

if pystac.ext.has("foo") and collection.ext.has("foo"):
     # ...

But then again, from user standpoint this looks a bit redundant and boilerplaty

[gadomski]

We don't currently have a graceful mechanism to register third party extensions into the ext attribute of catalog/collection/item/asset, so pystac.extensions.ext.EXTENSION_NAMES list is the exhaustive list of supported extensions — unless you're doing something wild, that list should be fixed for your current pystac version.

Or is there a cleaner way to first dynamically check if the "foo" extension exists?

Yup:

from pystac.extensions.ext import EXTENSION_NAMES
if "foo" in EXTENSION_NAMES and collection.ext.has("foo"):
    ...

[soxofaan]

EXTENSION_NAMES is apparently a typing.Literal object, and on Python 3.9 (which we still want to support) that does not work:

from pystac.extensions.ext import EXTENSION_NAMES
"foo" in EXTENSION_NAMES

gives

TypeError: Parameters to generic types must be types. Got 0.

(It only seems to work on python 3.11 or higher)

[gadomski]

Ah gotcha ... For <= 3.10 you could probably do this instead:

from pystac.extensions.ext import EXTENSION_NAMES_MAPPING
"foo" in EXTENSION_NAMES_MAPPING

[jsignell]

I am fine with changing the behavior of has if that makes more sense. Do you think that has is clearer than just using hasattr though?

if hasattr(c.ext, "meh"):
    ...

[gadomski]

I am fine with changing the behavior of has if that makes more sense.

I don't think it does — IMO "does this thing implement this known extension" needs a different behavior than "we don't know what that extension is".

[jsignell]

I'm going to go ahead and close this, but @soxofaan if you think the error message could be clearer please open a pull request.