I love node.js, socket.io, the templating engines, etc: as a web framework, it's amazing.
A lot of my back-end work is with NLP, Machine Learning, and Data Mining, for which there exist hundreds of rock-solid Python libraries, but no Javascript libraries. If I were using Django, I'd just import the libraries and chug away.
What's the recommended approach for handling these complex tasks with node.js? Should I stick with Python web frameworks, is there a convention to dealing with these libraries, or some solution I'm missing?
I think you have answered your own question. Python has greater maturity and by your own admission has the libraries you require. Could you narrow down your requirements a bit more?
Complex server logic and node.js