'Retrieve the pull request associated with this issue.
:returns: :class:`~github3.pulls.PullRequest`'

def pull_request(self):

    from .. import pulls
    json = None
    pull_request_url = self.pull_request_urls.get(u'url')
    if pull_request_url:
        json = self._json(self._get(pull_request_url), 200)
    return self._instance_or_null(pulls.PullRequest, json)
