from lxml import etree as E

class DenyExternal(E.Resolver):
    def resolve(self, url, public_id, context):
        raise ValueError("External resources are disabled")

def parse(text):
    if "<!DOCTYPE" in text.upper() or "<!ENTITY" in text.upper():
        raise ValueError("Declarations are disabled for untrusted XML")
    parser = E.XMLParser(resolve_entities=False, load_dtd=False,
                         no_network=True, huge_tree=False)
    parser.resolvers.add(DenyExternal())
    return E.fromstring(text.encode("utf-8"), parser)

source = "<rdf:RDF xmlns:rdf=\"http://www.w3.org/1999/02/22-rdf-syntax-ns#\" xmlns:seed=\"urn:bhylu:seed:\"><rdf:Description rdf:about=\"urn:packet:p1\"><seed:category rdf:resource=\"urn:category:flower\"/></rdf:Description></rdf:RDF>"
root = parse(source)
ns = {"rdf": "http://www.w3.org/1999/02/22-rdf-syntax-ns#", "seed": "urn:bhylu:seed:"}
print(root.xpath("string(rdf:Description/@rdf:about)", namespaces=ns))
print(root.xpath("string(rdf:Description/seed:category/@rdf:resource)", namespaces=ns))