[Tutor] s.insert(i, x) explanation in docs for Python 3.4 confusing to me

[Peter Otten]

Steven D'Aprano wrote:

> But slices are slightly different. When you provide two indexes in a 
> slice, they mark the gaps BETWEEN items:

The other explanation that Python uses half-open intervals works for me. 

> Now, what happens with *negative* indexes?
> 
> mylist = [ 100, 200, 300, 400, 500 ]
> indexes:  ^    ^    ^    ^    ^   ^
>           -6   -5   -4   -3   -2  -1
> 
> mylist[-5:-2] will be [200, 300, 400]. Easy.

>>> mylist = [ 100, 200, 300, 400, 500 ]
>>> mylist[-5:-2]
[100, 200, 300]

Off by one, you picked the wrong gaps.

Slightly related is a problem that comes up in practice; you cannot specify 
"including the last item" with negative indices:

>>> for i in reversed(range(len(mylist))):
...     print(mylist[:-i])
... 
[100]
[100, 200]
[100, 200, 300]
[100, 200, 300, 400]
[]

A simple fix is

>>> for i in reversed(range(len(mylist))):
...     print(mylist[:-i or None])
... 
[100]
[100, 200]
[100, 200, 300]
[100, 200, 300, 400]
[100, 200, 300, 400, 500]

The hard part is to remember to test whenever a negative index is 
calculated.