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

Question

list = ["a", "b", "c", "d"] print(list[3]) # Number 3 is "d"  print(list[-4]) # Number -4 is "a" 

Answer 1

To explain it in another way, because -0 is equal to 0, if backward starts from 0, it is ambiguous to the interpreter.

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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.

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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.