a trick with lists ?

[Matthew_WARREN at bnpparibas.com]

> 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

Be wary of using a[:]=b when the list holds objects;

>>> a=[[1],[2],[3]]
>>> b[:]=a
>>> b
[[1], [2], [3]]
>>> b[0].append(4)
>>> b
[[1, 4], [2], [3]]
>>> a
[[1, 4], [2], [3]]
>>>

see reply to 'Brain stuck whats occurring here' by Gabriel for more info.

Matt
(do I really need to put an apology for the disclaimers. Like, a disclaimer
for the disclaimers???. if people stopped reposting them and complainin'
there would be a lot less of them.

However, you will be glad to hear today is my last day with Paribas, so
after today, there will be no more Paribas discalimers on my emails :)
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