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 = "<exchange region=\"west\">\n  <packet id=\"p1\" available=\"true\"><name>Marigold</name><quantity>12</quantity><category>flower</category></packet>\n  <packet id=\"p2\" available=\"false\"><name>Fenugreek</name><quantity>0</quantity><category>herb</category></packet>\n  <packet id=\"p3\" available=\"true\"><name>Basil</name><quantity>7</quantity><category>herb</category></packet>\n</exchange>"
root = parse(source)
data = source.encode("utf-8")
if len(data) > 4096:
    raise ValueError("Too large")
if root.tag != "exchange" or len(root.findall("packet")) > 10:
    raise ValueError("Unsupported exchange")
print("Accepted", len(root.findall("packet")), "packets")