def __init__(self, src, dst, callback):
        '''
        :param src: the source node
        :param dst: the destination node
        :param callback: the function to call after sending src and before sending dst
        '''
        self.src = src
        self.dst = dst
        self.callback = callback
def __init__ ( self , src , dst , callback ) : self . src = src self . dst = dst self . callback = callback
:param src: the source node
        :param dst: the destination node
        :param callback: the function to call after sending src and before sending dst