a trick with lists ?

[James Turk]

On Feb 7, 12:20 pm, "Sébastien Vincent" <sebastien_nimp73<@>free.fr>
wrote:
> I've found some class on the Net which takes basically this form :
>
> ######
> class Foo:
>     def __init__(self):
>         self.tasks = []
>    ...
>
>     def method1(self):
>         tasks = []
>         while True:
>   ...
>   append/pop elements into/from tasks
>   ...
>   if condition : break
>
>     self.tasks[:] = tasks
>         return
> ######
>
> What I do not fully understand is the line "self.tasks[:] = tasks". Why does
> the guy who coded this did not write it as "self.tasks = tasks"? What is the
> use of the "[:]" trick ?

if you do
a = [1,2,3]
b = []
b = a

then assign: b[1] = 9
now a[1] == 9 as well

with a[:] = b you are actually getting a copy of the list rather than
an alias

it's hard to say if this is  needed in the case you described without
context, but that's what the a[:] = b idiom does

-James