Resolving a self link can erase href information

[gadomski]

As I am implementing stac-utils/stactools#151, I discovered what looks like a bug in link resolution. If the self link is resolved on a Collection STACObject, the target of the link is set to the Collection that object. This erases the string href information, which causes a break when later resolving an Item inside that Collection trying to use that link for other resolutions. I'm not opening a PR yet because I'm not quite sure how to fix it.

I realize that resolving a self link with an already-resolved root is a little weird and might feel artificial, but tThe cast at

pystac/pystac/stac_object.py

Lines 163 to 165 in bb8db04

	self_link = self.get_single_link(pystac.RelType.SELF)
	if self_link:
	return cast(str, self_link.target)

of an attribute that might not be a str feels like something that could be a Real Bug™ later, so I figure this test case is an alright way of hammering that out.

Example

EDIT: Updated with a simpler example.

An example test case is here: https://github.com/gadomski/pystac/blob/4f6b2ee61e365612320e17375fe52eecebb93b50/tests/test_link.py#L88-L98. The test code looks like this:

def test_resolving_self(self) -> None:
    catalog = pystac.Catalog(id="test", description="test desc")
    with TemporaryDirectory() as temporary_directory:
        catalog.normalize_and_save(temporary_directory)
        collection = pystac.Catalog.from_file(
            os.path.join(temporary_directory, "catalog.json")
        )
        link = collection.get_single_link(pystac.RelType.SELF)
        assert link
        link.resolve_stac_object()
        link.get_absolute_href()

The error:

pystac/link.py:161: in get_absolute_href
    href = make_absolute_href(href, self.owner.get_self_href())
pystac/utils.py:237: in make_absolute_href
    parsed_start = safe_urlparse(start_href)
pystac/utils.py:21: in safe_urlparse
    parsed = urlparse(href)
/usr/local/Cellar/python@3.9/3.9.6/Frameworks/Python.framework/Versions/3.9/lib/python3.9/urllib/parse.py:392: in urlparse
    url, scheme, _coerce_result = _coerce_args(url, scheme)
/usr/local/Cellar/python@3.9/3.9.6/Frameworks/Python.framework/Versions/3.9/lib/python3.9/urllib/parse.py:128: in _coerce_args
    return _decode_args(args) + (_encode_result,)
/usr/local/Cellar/python@3.9/3.9.6/Frameworks/Python.framework/Versions/3.9/lib/python3.9/urllib/parse.py:112: in _decode_args
    return tuple(x.decode(encoding, errors) if x else '' for x in args)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

.0 = <tuple_iterator object at 0x10f48ec40>

>   return tuple(x.decode(encoding, errors) if x else '' for x in args)
E   AttributeError: 'Catalog' object has no attribute 'decode'

/usr/local/Cellar/python@3.9/3.9.6/Frameworks/Python.framework/Versions/3.9/lib/python3.9/urllib/parse.py:112: AttributeError