Tim Chase wrote:
>>>> self.tasks[:] = tasks
>>>>
>>>> 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 ?
>>> It changes the list in-place. If it has been given to other objects, it
>>> might require that.
>> Nowadays it's stylistically better to write
>>
>> self.tasks = list(tasks)
>>
>> as it does just the same and makes it a little clearer what's going on
>
> Um...except it's not "just the same"?
>
> class Foo(object):
> def __init__(self, tasks):
> self.tasks1 = tasks
> self.todo1 = [self.tasks1, 42]
> self.tasks2 = tasks
> self.todo2 = [self.tasks2, 42]
> def new_tasks1(self, tasks):
> self.tasks1 = list(tasks)
> def new_tasks2(self, tasks):
> self.tasks2[:] = list(tasks)
> def __str__(self):
> return "%r\n%r" % (self.todo1, self.todo2)
>
> f = Foo([1,2,3])
>
> f.new_tasks1([4,5,6])
> print 'task1'
> print f # todo1/2 haven't been changed
>
> print 'task2'
> f.new_tasks2([4,5,6])
> print f # both todo 1 & 2 have been changed
>
> Assignment to a name just rebinds that name. Assignment to a
> slice of a list replaces the contents in-place.
>
[sigh] Right, I got the assignment the wrong way around (and clearly you
can't put list(tasks) on the left-hand side of an assignment).
Of course
self.tasks = list(tasks)
is equivalent to
self.tasks = tasks[:]
Thanks for pointing out my error.
regards
Steve
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