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 = "<packet id=\"p1\"><quantity>-2</quantity></packet>"
root = parse(source)
print("Parsed:", root.tag)
print("Missing:", len(root.xpath("missing")))
print("Business rule:", int(root.findtext("quantity")) >= 0)