["python","list"]
Why does Python start at index -1 (as opposed to 0) when indexing a list from the end?

list = ["a", "b", "c", "d"] print(list[3]) # Number 3 is "d" print(list[-4]) # Number -4 is "a"
To explain it in another way, because -0 is equal to 0, if backward starts from 0, it is ambiguous to the interpreter. If you are confused about -, and looking for another way to index backwards more understandably, you can try ~, it is a mirror of forward: arr = ["a", "b", "c", "d"] print(arr[~0]) # d print(arr[~1]) # c The typical usages for ~ are like "swap mirror node" or "find median in a sort list": """swap mirror node""" def reverse(arr: List[int]) -> None: for i in range(len(arr) // 2): arr[i], arr[~i] = arr[~i], arr[i] """find median in a sort list""" def median(arr: List[float]) -> float: mid = len(arr) // 2 return (arr[mid] + arr[~mid]) / 2 """deal with mirror pairs""" # verify the number is strobogrammatic, strobogrammatic number looks the same when rotated 180 degrees def is_strobogrammatic(num: str) -> bool: return all(num[i] + num[~i] in '696 00 11 88' for i in range(len(num) // 2 + 1)) ~ actually is a math trick of inverse code and complement code, and it is more easy to understand in some situations. Discussion about whether should use python tricks like ~: In my opinion, if it is a code maintained by yourself, you can use any trick to avoid potential bug or achieve goal easier, because of maybe a high readability and usability. But in team work, avoid using 'too clever' code, may bring troubles to your co-workers. For example, here is one concise code from Stefan Pochmann to solve this problem. I learned a lot from his code. But some are just for fun, too hackish to use. # a strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down) # find all strobogrammatic numbers that are of length = n def findStrobogrammatic(self, n): nums = n % 2 * list('018') or [''] while n > 1: n -= 2 # n < 2 is so genius here nums = [a + num + b for a, b in '00 11 88 69 96'.split()[n < 2:] for num in nums] return nums I have summarized python tricks like this, in case you are interested.