def _extract_links(self, response_text, response_url, response_encoding,
    base_url=None):
    if base_url is None:
        base_url = urljoin(response_url, self.base_url
            ) if self.base_url else response_url
clean_url = lambda u: urljoin(base_url, remove_entities(clean_link(u.decode
    (response_encoding))))
clean_text = lambda t: replace_escape_chars(remove_tags(t.decode(
    response_encoding))).strip()
links_text = linkre.findall(response_text)
urlstext = set([(clean_url(url), clean_text(text)) for url, _, text in
    links_text])
return [Link(url, text) for url, text in urlstext]
Как я могу написать свой собственный экстрактор ссылок в scrapy python