I am looking for a practical visual programming environment based on Python. My primary application is algorithm development for processing remote-sensing imagery. I was initially inspired by LabVIEW from National Instruments, but that is more geared towards laboratory measurements and simulations. I write a lot of prototype code in Python and do a lot of interactive analysis with IPython.

Does there exist a visual framework where a "program" is represented by connected nodes which each read data, do some work, and output data to the next node? I would like to use Python to write the code residing in each node.

So far the best I've seen is Orange http://www.ailab.si/orange/, but it does not have the ability to start/stop individual nodes.