SymPy is great!

I have used it to assist me in some formal proofs. (I hope to publish that later.)

Note that in my case, I hit a couple of bugs. See the GitHub issue tracker (https://github.com/sympy/sympy/issues). The bugs are easy to detect when you just get an exception (which was often the case for me). However, you can also simply get an incorrect result, which can be more tricky to detect. So be careful and maybe don't trust the results right away without double checking them in some other way.