Why does slicing in python stop at -1?

0 | 1 | 2 | 3 | 4

a | b | c | d | e

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[4:-1:-1]

 4 -> e -1 -> e -1 -> step back 

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=> nothing between index 4 -> -1 with a step back

Python list syntax is:

list[start:stop:step]

In the first example: start=0, stop=5, step=1. This is equivalent to the C style for loop:

for (i=0; i<5; i++)

In your second example, start=4, stop=-1=4, step = -1. This is equivalent to the C style for loop:

for (i=4; i>4; i--)

Since the start and stop conditions are the same, your result is empty.

Since -1 is a negative number, which as a start/stop is a special case in python list slicing that means "len(list)-1", you can't actually do the operation you want:

for (i=4; i>-1; i--)

So instead your best bet is "abcde"[::-1] which will correctly infer the end, and the start, from the direction.

If you need to use a number for the stop position, for example, you are using a variable to represent the stop index, then you can use the negative value equal to -len(obj) - 1 where obj is the object you want to reverse as the position before the first character:

>>> 'abcde'[4:-6:-1] 'edcba' 

or,

>>> 'abcde'[-1:-6:-1] 'edcba' 

You are slicing from the last index (4) to index 4 which results in an empty string. In other words, Python is doing the following:

"abcde"[4:4:-1] # which is equal to "abcde"[4:4] # and evaluates to "" 

If you slice from the last index (4) to the first index (0) with a stride of -1 you can reverse it. Thus the following code would work:

"abcde"[-1:0:-1] 

Which in actual practice can be abbreviated to:

"abcde"[::-1]