'Fetch issue from owner/repository.
:param str username: (required), owner of the repository
:param str repository: (required), name of the repository
:param int number: (required), issue number
:return: :class:`Issue <github3.issues.Issue>`'

def issue(self, username, repository, number):

    json = None
    if (username and repository and (int(number) > 0)):
        url = self._build_url(u'repos', username, repository, u'issues', str(number))
        json = self._json(self._get(url), 200)
    return self._instance_or_null(Issue, json)
