def leave_room(room, sid=None, namespace=None):
    """Leave a room.

    This function removes the user from a room, under the current namespace.
    The user and the namespace are obtained from the event context. Example::

        @socketio.on('leave')
        def on_leave(data):
            username = session['username']
            room = data['room']
            leave_room(room)
            send(username + ' has left the room.', room=room)

    :param room: The name of the room to leave.
    :param sid: The session id of the client. If not provided, the client is
                obtained from the request context.
    :param namespace: The namespace for the room. If not provided, the
                      namespace is obtained from the request context.
    """
    socketio = flask.current_app.extensions['socketio']
    sid = sid or flask.request.sid
    namespace = namespace or flask.request.namespace
    socketio.server.leave_room(sid, room, namespace=namespace)