My team has been working on a Python library called Loman that represents computations as graphs. We've open-sourced it [1][2]. One of our aims is to make it as natural as possible to use graph-based programming, and within an already-familiar programming language. Be interested to know what you think.
[1] https://github.com/janusassetallocation/loman [2] http://loman.readthedocs.io/en/latest/user/intro.html