SĂ©bastien Vincent <sebastien_nimp73 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 ?
>
I've just run into this difference myself.
As several others have pointed out, assignment to
self.task[:] modifies this list in place.
Here my example showing a striking difference
class MyClass(object) :
def shorten_list(self,outer_list) :
ll=len(outer_list)
if ll > 0 :
outer_list[:]= outer_list[:ll-1]
mylist=[1,2,3]
MyClass().shorten_list(mylist)
print mylist
// this prints [1, 2] (as expected)
class MyClass2(object) :
def shorten_list(self,outer_list) :
ll=len(outer_list)
if ll > 0 :
outer_list= outer_list[:ll-1]
mylist=[1,2,3]
MyClass2().shorten_list(mylist)
print mylist
# this prints [1, 2, 3]
The shortened list outer_list[:ll-1] has been assigned (bound in Python terms)
to the LOCAL reference (to a list) 'outer_list'
--
Helmut Jarausch
Lehrstuhl fuer Numerische Mathematik
RWTH - Aachen University
D 52056 Aachen, Germany
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