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"

itemprop="text">

class="normal">Answer

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.

/>

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 href="https://stackoverflow.com/users/1672429/stefan-pochmann">Stefan
Pochmann to solve href="https://leetcode.com/problems/strobogrammatic-number-ii/" rel="noreferrer">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.