[Tutor] Manipulate list in place or append to a new list

[spir]

Sander Sweers a écrit :
> On Sun, Nov 2, 2008 at 13:32, Kent Johnson <kent37 at tds.net> wrote:
>> Use a list comprehension:
>> somelist = [ x+1 for x in somelist ]
> 
> Got it.
> 
>> Note that this creates a new list, replacing the one that was in
>> somelist. If you need to actually modify somelist in place (rare) then
>> use somelist[:] =  [ x+1 for x in somelist ]
>>
>> which creates a new list, then replaces the *contents* of somelist
>> with the contents of the new list.
> 
> In what (rare) situation would you use this?

[post sent again -- seems not to have reached the list -- ??]
When you need to cleanup or normalize the objects held in a list. For instance,
in text processing:
lines = text.splitlines()
lines = [line.strip() for line in lines]
or e.g.
lines = [line.lower() for line in lines]
Both "pre-processing" actions that will make further processing more
straitforward. Note that you can chain actions:
lines = [line.strip().lower() for line in lines]

Denis