Originally made for a CodeWars kata. I enjoyed the idea, so I decided to turn it into a full function. To use, import SudokuSolver and run the function with the exact same name. It assumes the parameter is a 9x9 2D list. And returns the solution as another list. But if the sudoku is unsolvable, returns 0 in this case. You can also run the file alone, to see the test results of two sudoku puzzles.
Even it took weeks for me to take it to this level, this code is still far from optimal. So I think we can call it sub-optimal. But it's time to move on. I said I made this algorithm for a CodeWars kata, but even though it is fast enough in my opinion, it wasn't fast enough for the kata. Still, I enjoyed every minute working on it.