from lxml import etree

class ParserTarget:
    events = []
    close_count = 0
    tus = 0
    def start(self, tag, attrib):
        if tag=="tu": 
            self.tus += 1

    def close(self):
        return None


parser_target = ParserTarget()
parser = etree.XMLParser(target=parser_target)
etree.parse("en-pt.tmx", parser)

print(parser_target.tus)