def assign_issue(self, issue, assignee):
        """Assign an issue to a user. None will set it to unassigned. -1 will set it to Automatic.

        :param issue: the issue ID or key to assign
        :type issue: int or str
        :param assignee: the user to assign the issue to
        :type assignee: str

        :rtype: bool
        """
        url = self._options['server'] + \
            '/rest/api/latest/issue/' + str(issue) + '/assignee'
        payload = {'name': assignee}
        r = self._session.put(
            url, data=json.dumps(payload))
        raise_on_error(r)
        return True