(And this one popped in Google as second when I just searched; https://github.com/Mathics3/mathics-core)
Unfortunately, SageMath is not directly usable as a Python package.
That's where passagemath [0] comes in, making the rich ecosystem of SageMath available to Python devs, one package at a time.
Maybe i’m just missing something. But it looks like nobody is really using it except for some very specific math research which has grown from within that ecosystem from the beginning.
I think one of the basic problems is that the core language is just not very performant on modern cpus, so not the best tool for real-world applications.
Again- maybe i’m missing something?
This is why its not particularly problematic that it is closed source. Most people I've worked with who use it produce mathematical results with it that are fully checkable by hand.
You could do stuff other than theoretical physics research with Mathematica, though. I has a lot of functionality and I always felt that I used only a tiny fraction of it.