PyPy.js literally starts with PyPy compiled to asm.js, so you intrinsically get relatively comprehensive Python compatibility right out of the box. Brython, as

PyPy.js literally starts with PyPy compiled to asm.js, so you intrinsically get relatively comprehensive Python compatibility right out of the box. Brython, as I understand it, is an independent attempt at implementing a subset of Python in JavaScript. Edit: rfk addresses this directly around 46:00 in the interview. Paraphrased: "The big trade-off Brython does to get good performance is the number model. Things there map pretty directly to JavaScript. But if you overflow or do something different, you suddenly won't get Python's semantics. If you're writing new code for Brython, it's a good set of trade-offs, and in some ways you get tighter integration with the browser and the DOM. But if you're trying to take some existing code off the shelf and run it... you're more likely to run into issues."