def add(self, elem, replace=False):
        """Add a requirement.

        :param elem: a string or :class:`Requirement` instance.
        :param replace: if `True`, packages in the set with the same name will
            be removed first.
        """
        if isinstance(elem, text_type):
            elem = Requirement.coerce(elem)
        if replace and elem in self:
            self.remove(elem)
        super(Requirements, self).add(elem)
def update_snapshot_status(self, snapshot_id, status, progress):
        """Update the specified snapshot's status."""
        post_body = {
            'status': status,
            'progress': progress
        }
        post_body = json.dumps({'os-update_snapshot_status': post_body})
        url = 'snapshots/%s/action' % str(snapshot_id)
        resp, body = self.post(url, post_body)
        return resp, body