[KennedyRichard]

Not a diagram tool, but can be used like one, since it converts Python functions into nodes that you can connect to each other and export the resulting layout as images (PNG, SVG, SVG inside an HTML page) or as plain Python code: https://github.com/IndiePython/nodezator

full disclosure: I'm the creator and maintainer of the tool (but this is no advertising, as I released it to the public domain since day 1)