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

[boB Stepp]

On Fri, Jan 15, 2016 at 11:32 PM, Cameron Simpson <cs at zip.com.au> wrote:
> On 15Jan2016 23:05, boB Stepp <robertvstepp at gmail.com> wrote:
>>
>> On Fri, Jan 15, 2016 at 10:53 PM, Cameron Simpson <cs at zip.com.au> wrote:
>>>>>>>
>>>>>>> things.insert(-1, 'What the heck?!?')
>>>>>>> things
>>>>
>>>>
>>>> [0, 'Hmm...', 3, 'WhackABunny', 6, 'What the heck?!?', '?']
>>>>
>>>> "...at the index..." to me would mean that 'What the heck?!?' should
>>>> become the last item in the list.  Again, the interpreter help gave
>>>> what I was expecting.
>>>
>>>
>>>
>>> To me it means "insert 'x' so that its index is 'i'".
>>
>>
>> But that's my point!  In my example x (here 'What the heck?!?') is
>> *not* at index i (here, -1).  Instead it winds up at index -2.  But
>> this fits in perfectly with the interpreter help, since it winds up
>> *before* index i (-1).
>
>
> Ah, but -1 isn't the "real" index. It is a convenient value for computing
> the real index if you want to figure things out from the end of the list
> instead of the start.  In your example above, the real index is 5.  As you
> would get from things.index('?') before the insert. So your insert really
> means:
>
>  things.insert(5, 'What the heck?!?')

Or, actual index = len(things) + (-1) before I do the insert.

Another subtlety for negative indexing to file away!

Thanks, Cameron. That wraps up things nicely for me.



-- 
boB