code: def add_cookie_header(self, request):
          
          _debug("add_cookie_header")
          self._cookies_lock.acquire()
          try:
  
              self._policy._now = self._now = int(time.time())
  
              cookies = self._cookies_for_request(request)
  
              attrs = self._cookie_attrs(cookies)
              if attrs:
                  if not request.has_header("Cookie"):
                      request.add_unredirected_header(
                          "Cookie", "; ".join(attrs))
  
              # if necessary, advertise that we know RFC 2965
              if (self._policy.rfc2965 and not self._policy.hide_cookie2 and
                  not request.has_header("Cookie2")):
                  for cookie in cookies:
                      if cookie.version != 1:
                          request.add_unredirected_header("Cookie2", '$Version="1"')
                          break
  
          finally:
              self._cookies_lock.release()
  
          self.clear_expired_cookies()
docs: Add correct Cookie: header to request (urllib.request.Request object).
  
          The Cookie2 header is also added unless policy.hide_cookie2 is true.