> What makes pybind a "flagship library"?

NumPy, SciPy, and friends are, individually and collectively flagship libraries, and depend on pibind11.

> If it was very widely used this bug probably would've been caught by people testing the alpha/beta/rc versions of 3.9.

They are very widely used, but “probably” means “do enough releases and you will find exceptions”. Welcome to an exception.

People using numpy are not in the fringe. If this wasn’t triaged during prereleases then it’s either that it doesn’t always cause problems, or there is an issue in the diversity of testers.

the parent was probably talking about NumPy/SciPy/Tensorflow/PyTorch, which I would also label as flagship libraries...

I personally don't like Python (I prefer Julia/Ruby/Rust), but all the new advances in machine learning are implemented in Tensorflow and PyTorch, so I came back to Python just because of these libraries and infrastructure on top of them.

I was just using the expit function from scipy yesterday by the way, because that's where it was implemented.

> If it was very widely used this bug probably would've been caught

As the top-level comment says, it rarely causes crashes.