问题：
I need a good explanation (references are a plus) on Python's slice notation.我需要对 Python 的切片符号进行很好的解释（参考文献是加分项）。
To me, this notation needs a bit of picking up.对我来说，这个符号需要一点点掌握。
It looks extremely powerful, but I haven't quite got my head around it.它看起来非常强大，但我还没有完全理解它。