From the description "If the system does not have a unique solution (has infinitely many solutions or is unsolvable), return null (None in python)."

Nice kata. Initially thought this was too hard for 4 kyu. I changed my mind

ah I did it now
Because I only know the cramer's rule way to calculate it which is slow

Have you noticed that the kata is available in 5 languages? Only Python has numpy, how can the others do? Computing a matrix determinant or solving a linear equation isn't something magical, it's just math. You have to do it by yourself to solve this kata, that's it. And don't expect someone to give you the recipe in discussion.

Calculation of a determinant is perfectly in scope of a 4 kyu task :)

how am I supposed to do this if numpy.linalg.det is not permitted

This was fun, thank you! I am particularly happy with the way I achieved numerical stability in Python.

it's certainly too hard for 4kyu

The Python "No solution" test in the sample test cases actually has infinitely many solutions.

    @test.it('No solution')
    def b():
        tester(solve("x+2y=1", "2x=2-4y"), None)

ah, nice. Nobody thought about that so far... x)

Something else?

note: what about sys when it was accessible (I mean, specifically)

'removed more, even if none were a problem. I left one because it's there on purpose.

What about my question above?

done.

Side question: how do you use it exactly, in the previous context?

This kata has too much text handling. No one needs cases like "x+2=y", that's just extra annoyance.