Seriously, that's not what they were made for. Don't use them.
Messages provide a method to pass data from the Python domain into the flow graph, or C++ domain, and vice versa. This is useful if you have a flow graph that does some heavy lifting signal processing, and have some simple Python-based post-processing. Some of the graphical sinks use this feature; the data is generated in C++ and then passed to Python which in turn runs the display code.
注：Using Message Sinks and piping data to Python（原文出处，翻译整理仅供参考!）