Serialisable transaction inside the database.py

function purchase(productId):
  BEGIN TRANSACTION (ISOLATION = SERIALIZABLE)

  stock = query("SELECT stock FROM products WHERE id = ?", productId)

  if stock <= 0:
    ROLLBACK
    return "Out of stock"

  exec("UPDATE products SET stock = stock - 1 WHERE id = ?", productId)

  COMMIT
  return "OK"