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 create_snapshot(self, volume_id, **kwargs):
        """
        Creates a new snapshot.
        volume_id(Required): id of the volume.
        force: Create a snapshot even if the volume attached (Default=False)
        display_name: Optional snapshot Name.
        display_description: User friendly snapshot description.
        """
        post_body = {'volume_id': volume_id}
        post_body.update(kwargs)
        post_body = json.dumps({'snapshot': post_body})
        resp, body = self.post('snapshots', post_body)
        body = json.loads(body)
        return resp, body['snapshot']