a trick with lists ?

Tim Chase python.list at tim.thechases.com
Thu Feb 7 22:42:58 CET 2008

>>>     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])

   print 'task1'
   print f # todo1/2 haven't been changed

   print 'task2'
   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.


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