You realize your promise code is ugly af. All your code is nested within other functions and chained with dots. It's not clear.

For imperative code the goal is to write code that is just as clear as writing a single threaded algorithm: a simple list of steps. Promises and callbacks DO NOT ACHIEVE THIS GOAL.

That stackoverflow post showed a python solution that's infinitely cleaner than js. Python skipped callbacks and promise chains and jumped directly to the best way to do async operations... async/await.

You want to shove a parallel set in an async chain of operations? Here's the way to do serial and parallel async operations in Python:

    async def main():
        # run them sequentially (but the loop can do other stuff in the meanwhile)
        body1 = await get_body('http://httpbin.org/ip')
        print body1
        body2 = await get_body('http://httpbin.org/user-agent')
        print body2


        # run them in parallel
        task1 = get_body('http://httpbin.org/ip')
        task2 = get_body('http://httpbin.org/user-agent')
    
        for body in await asyncio.gather(task1, task2):
            print(body)

You see that code? Way more clearer way more understandable than your promise chain. Python has it's problems but your promise code is not powerful at all, it leads to over complicated code and just slightly better than callbacks.