n[::-1] 在 Python 中是什么意思？

Question

I have a string n = "abc" .

I want to reverse it and found a solution like n[::-1] .

What is the meaning of all 3 arguments?

Answer 1

It means, "start at the end; count down to the beginning, stepping backwards one step at a time."

The slice notation has three parts: start , stop , step :

>>> 'abcdefghijklm'[2:10:3]  # start at 2, go upto 10, count by 3
'cfi'
>>> 'abcdefghijklm'[10:2:-1] # start at 10, go downto 2, count down by 1
'kjihgfed'

If the start and stop aren't specified, it means to go through the entire sequence:

>>> 'abcdefghijklm'[::3]  # beginning to end, counting by 3
'adgjm'
>>> 'abcdefghijklm'[::-3] # end to beginning, counting down by 3
'mjgda'

This is explained nicely in Understanding slice notation , in the Python docs under "extended slicing", and in this blogpost: Python Slice Examples: Start, Stop and Step