Fix BNode.skolemize() returning a URIRef instead of an RDFLibGenid.

[edmondchuc]

Fixes #1404

Proposed Changes

• Correctly return RDFLibGenid instead of URIRef as required by Graph.de_skolemize().
• This fixes skolemize/de-skolemize round-trip which was previously completely broken.
• Add tests.

Why RDFLibGenid and not Genid? See URIRef.de_skolemize().

rdflib/rdflib/term.py

Lines 308 to 328 in 327de4e

	def de_skolemize(self):
	"""Create a Blank Node from a skolem URI, in accordance
	with http://www.w3.org/TR/rdf11-concepts/#section-skolemization.
	This function accepts only rdflib type skolemization, to provide
	a round-tripping within the system.
	.. versionadded:: 4.0
	"""
	if isinstance(self, RDFLibGenid):
	parsed_uri = urlparse("%s" % self)
	return BNode(value=parsed_uri.path[len(rdflib_skolem_genid) :])
	elif isinstance(self, Genid):
	bnode_id = "%s" % self
	if bnode_id in skolems:
	return skolems[bnode_id]
	else:
	retval = BNode()
	skolems[bnode_id] = retval
	return retval
	else:
	raise Exception("<%s> is not a skolem URI" % self)

Essentially, RDFLibGenid will correctly grab the original blank node ID before skolemization.

[edmondchuc]

Let me add some tests for the skolemization/de-skolemization round-trip.

[ghost]

Looks good to me, fwiw.

[nicholascar]

Excellent testing there @edmondchuc!