# 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.

From: stackoverflow.com/q/55684960

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