':calls: `GET /repos/:owner/:repo/:archive_format/:ref <http://developer.github.com/v3/repos/contents>`_
:param archive_format: string
:param ref: string
:rtype: string'

def get_archive_link(self, archive_format, ref=github.GithubObject.NotSet):

    assert isinstance(archive_format, (str, unicode)), archive_format
    assert ((ref is github.GithubObject.NotSet) or isinstance(ref, (str, unicode))), ref
    url = ((self.url + '/') + archive_format)
    if (ref is not github.GithubObject.NotSet):
        url += ('/' + ref)
    (headers, data) = self._requester.requestJsonAndCheck('GET', url)
    return headers['location']
