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Comment by gshulegaard | Hacker News Reader

> I fail to understand the use of python in a distributed environment while the language has such poor concurrency support

Because it's a distributed environment probably is exactly why. Python has (arguably) great concurrency support apart from Multi-threading.

https://www.youtube.com/watch?v=MCs5OvhV9S4

So if you need concurrency in the context of a single thread, then Python's GIL is a non-starter. But a distributed environment is not likely one of those.

Edit: I should amend concurrency in a single thread to: concurrency in a single thread that is compute gated...since coroutines can give you pseudo concurrency in a single thread provided you're workload has blocking steps like IO or TCP calls.

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In fact it's precisely Python's deficiency in multithreading that lead to it having one of the best ecosystems for every other form of concurrency, like green threads and multiprocess applications.

If you’re doing (data analysis|simulations|Image processing) you can offload computation to numpy, which releases the GIL. This allows nice multicore speedups with python and threading.

The same holds for various CPU intensive standard library functions implemented in C.

The GIL issue is real, but posts like this one confused me for years. Please, don’t exaggerate GIL issues.

Not everything is amenable to numpy and it's pretty easy to make performance worse by throwing numpy at every problem. For example, if your array contains Python objects that are part of an operation, you've likely just introduced a significant performance regression. Worse, there's no way to detect these regressions except to have performance tests. Please, don't understate GIL issues.

Why would you make a NumPy array of objects? Use a list until it makes sense to create an array.

Because you’re using Numpy via an intermediate library like pandas and you have composite data in one column?

Ah. Pandas is the problem. Unfortunately, you need to understand how its features are implemented to use it well. Still, my main trouble with Pandas is unnecessary memory bloat, not compute inefficiency.

That caveat is somewhat true for all programming abstractions, but well-designed interfaces make the more efficient techniques more obvious and beautiful, while the inefficient or risky techniques are made esoteric and ugly.

Pandas isn't the problem, the problem is assuming that "$LIBRARY releases the GIL so things will be fast!". It's a pennywise, pound-foolish approach to performance. Someone will write a function assuming the user is only going to pass in a list of ints and someone else will extend that function to take a list of Tuple[str, int] or something and all of a sudden your program has a difficult to debug performance regression.

In general, the "just rewrite the slow parts in C!" motto is terrible advice because it's unlikely that it will actually make your code appreciably slower, and if it does, it's very likely to be defeated unexpectedly as soon as requirements change. Using FFI to make things faster can work, but only if you've really considered your problem and you're quite sure you can safely predict relevant changes to requirements.

> Someone will write a function assuming the user is only going to pass in a list of ints and someone else will extend that function to take a list of Tuple[str, int]

There are plenty of pitfalls in leaky abstractions. Establishing that fast numeric calculations only work with specific numeric types seems to help.

One thing you seem to be encountering, that I've seen a few times, is that people don't realize NumPy and core Python are almost orthoganal. The best practices for each are nearly opposite. I try to make it clear when I'm switching from one to the other by explaining the performance optimization (broadly) in comments.

Regardless, any function that receives a ``list`` of ints will need to convert to an ndarray if it wants NumPy speed. If the function interface is modified later, I think it's fair to expect the editor to understand why.

> There are plenty of pitfalls in leaky abstractions

Sure, but this is a _massive_ pitfall. It's an optimization that can trivially make your code slower than the naive Python implementation, all due to a leaky abstraction.

> Regardless, any function that receives a ``list`` of ints will need to convert to an ndarray if it wants NumPy speed. If the function interface is modified later, I think it's fair to expect the editor to understand why.

Yeah, that was a toy example. In practice, the scenario was similar except the function called to a third party library that used Numpy under the hood. We introduced a ton of complexity to use this third party library on the grounds that "it will make things fast" instead of the naive list implementation, and the very next sprint we needed to update it such that it became 10X slower than the naive Python implementation.

That's the starkest example, but there have been others and there would have been many more if we didn't have the stark example to point to.

The current slogan is "just use Python; you can always make things fast with Numpy/native code!", but it should be "use Python if you have a deep understanding of how Numpy (or whatever native library you're using) makes things fast such that you can count on that invariant to hold even as your requirements change" or some such.

I have mixed feelings about your conclusion. On one hand I don't want to discourage newbies from using Python. On the other, I enjoy that my expertise is valuable.

It seems reasonable that different parts of the code are appropriate for modification by engineers of differing skills.

Even if your app is IO bound, Python's concurrency is painful. Because it's not statically typed, it's too easy to forget an `await` (causing your program to get a Promise[Foo] when you meant to get a Foo) or to overburden your event loop and such things are difficult to debug (we've had several production outages because of these class of bugs). Never mind the papercuts that come about from dealing with the sync/async dichotomy.

Both problems have built-in debug solutions in recent versions of python. The event loop will literally print out all the un-awaited coroutines when it exits, and you can enable debug on the event loop and have it print out every time a coroutine takes longer than a configurable amount of time.

> The event loop will literally print out all the un-awaited coroutines when it exits

IIRC, I've only ever seen "unawaited coroutine found" (or similar) errors; I've never seen anything that points to a specific unawaited coroutine. In either case, a bug in prod is still many times worse than compile time type error.

> you can enable debug on the event loop and have it print out every time a coroutine takes longer than a configurable amount of time

I don't run my production servers in debug mode, and even when I do manage to find the problem, I have limited options for solving it. Usually it amounts to refactoring out the offending code into a separate process or service.

An extreme counterpoint is a language like Go which

1) Is roughly 100X faster in single-threaded, CPU-bound execution anyway

2) Allows for additional optimizations that simply aren't possible in Python (mostly involving reduced allocations and improved cache coherence)

3) Has a runtime that balances CPU load across all available cores

This isn't a "shit on Python" post; only that concurrency really isn't Python's strong suit (yet).

These are not really an issue in vfx production and other things Python is used for.

It’s a problem for lots of things Python is used for, but maybe not vfx (whatever that is).

They are using it for things it’s good at, for others they use java. So this subthread is largely a waste of time.

Who is “they”? What is your point?

They is Netflix, and other post-production oriented users. You know, what this article and discussion is about?

It's not at all obvious that "they" refers to "netflix and other post-production oriented users", and your argument is a tautology "Python is good at the things that Python is good at". Obviously. The rest of us are debating what those things are or are not.

The subject is well-trodden, there's not much to debate. Python is not good at threading, but works well in multiprocessing situations. Netflix is using it in the later situation, and not the former. Async is unlikely to be a use case either.

> The subject is well-trodden, there's not much to debate

And yet we see the same incorrect information trotted out over and over again.

"(yet)", but then it's still a dynamic language (no typing, not dynamic as in hip). The reason for python is mainly:

- it's easy to learn - we've go numpy

If I were to pick a language to build significant infrastructure with, I wouldn't choose python.

The missing await is a a very common fault indeed, they should have used another keyword like 'not_await' for that scenario to make the decision explicit. Pycharm at least will warn you if you call an awaitable without 'await' and without assigning it to a variable. If you assign it to a variable and pass it into another function that doesn't expect an awaitable, it's up to you to have added sufficient type annotations and run your code through some static checker like mypy. Running python at scale without mypy is kindof doomed to failed to begin with.

And that even with async you're still bound by the GIL

Huh? With async, there's typically only ever 1 thread running, so there's no contention for the GIL.

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