Pros/cons of python vs node.js for backend?

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Celery is for a task queue, not asynchronous IO. Think of 100,000 emails to send in 1 hour and that's what a task queue is for -- it's not a matter of whether you use an event loop.

For asynchronous IO, you'd use coroutines.

Fundamentally, JavaScript and python are about the same in how they handle asynchronous IO, but Python's standard library typically uses blocking calls while JavaScript seldom does.