[Python-Dev] List insert at index that is well out of range - behaves like append

[Mark Lawrence]

On 15/09/2014 23:29, Mark Shannon wrote:
>
>
> On 15/09/14 12:31, Tal Einat wrote:
>> On Mon, Sep 15, 2014 at 6:18 AM, Harish Tech <technews.full at gmail.com>
>> wrote:
>>> I had a list
>>>
>>>   a = [1, 2, 3]
>>>
>>> when I did
>>>
>>> a.insert(100, 100)
>>>
>>> [1, 2, 3, 100]
>>>
>>> as list was originally of size 4 and I was trying to insert value at
>>> index
>>> 100 , it behaved like append instead of throwing any errors as I was
>>> trying
>>> to insert in an index that did not even existed .
>>>
>>>
>>> Should it not throw
>>>
>>>
>>> IndexError: list assignment index out of range
>>>
>>>
>>> exception as it throws when I attempt doing
>>>
>>>
>>> a[100] = 100
>>>
>>> Question : 1. Any idea Why has it been designed to silently handle this
>>> instead of informing the user with an exception ?
>>>
>>>
>>> Personal Opinion : Lets see how other dynamic languages behave in such a
>>> situation : Ruby :
>>>
>>>
>>>      > a = [1, 2]
>>>
>>>      > a[100] = 100
>>>
>>>      > a
>>>
>>>   => [1, 2, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil,
>>> nil, nil,
>>> nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil,
>>> nil,
>>> nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil,
>>> nil,
>>> nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil,
>>> nil,
>>> nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil,
>>> nil,
>>> nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil,
>>> nil,
>>> nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, 100]
>>>
>>> The way ruby handles this is pretty clear and sounds meaningful (and
>>> this is
>>> how I expected to behave and it behaved as per my expectation) at
>>> least to
>>> me . So what I felt was either it should throw exception or do the
>>> way ruby
>>> handles it .
>>>
>>>
>>> Is ruby way of handling not the obvious way ?
>>>
>>> I even raised it in stackoverflow
>>> http://stackoverflow.com/questions/25840177/list-insert-at-index-that-is-well-out-of-range-behaves-like-append
>>>
>>>
>>> and got some responses .
>>
>> Hello Harish,
>>
>> The appropriate place to ask questions like this is python-list [1],
>> or perhaps Stack Overflow.
>
> I think this is an OK forum for this question.
> If someone isn't sure if something is a bug or not, then why not ask
> here before reporting it on the bug tracker?
>
> This does seem strange behaviour, and the documentation for list.insert
> gives no clue as to why this behaviour was chosen.
>
> Cheers,
> Mark.

I assume it's based on the concepts of slicing.  From the docs 
"s.insert(i, x) - inserts x into s at the index given by i (same as 
s[i:i] = [x])".  Although shouldn't that read s[i:i+1] = [x] ?

-- 
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.

Mark Lawrence