[Tutor] append vs list addition

[Denis Heidtmann]

On Sun, May 4, 2014 at 6:44 PM, Dave Angel <davea at davea.name> wrote:
> C Smith <illusiontechniques at gmail.com> Wrote in message:
>> Sorry.
>>
>> I meant for example:
>> list1 = [1,2,3]
>> list2 = [3,4,5]
>>
>> newList = list1 + list2
>>
>> versus
>>
>> for x in list2:
>>     list1.append(x)
>>
>> Which is the preferred way to add elements from one list to another?
>
> Thank you for switching to text mail.
>
> These examples still aren't equivalent.  But in any similar
>  example,  if list2 is type list, then avoid the list. Use either
>  extend, or the equivalent += . And if you aren't permitted to
>  change list1, you should use +=
....
> DaveA
>

Lurking here.  I am confused by "avoid the list".  What does that mean?
 Also, you say extend and += are equivalent, yet say " if you aren't
permitted to change list1, you should use +="  If they are equivalent,
why choose one over the other?  Doesn't += always change the left-hand
side?

Not being critical--just confused.

Thanks,
-Denis