initialized list: strange behavior

Steve Holden steve at holdenweb.com
Tue Nov 25 15:32:10 CET 2008


Arnaud Delobelle wrote:
> Jason Scheirer <jason.scheirer at gmail.com> writes:
> 
>> On Nov 24, 10:34 pm, alexander.gen... at gmail.com wrote:
>>> Hi Python experts! Please explain this behavior:
>>>
>>>>>> nn=3*[[]]
>>>>>> nn
>>> [[], [], []]
>>>>>> mm=[[],[],[]]
>>>>>> mm
>>> [[], [], []]
>>>
>>> Up till now, 'mm' and 'nn' look the same, right? Nope!
>>>
>>>>>> mm[1].append(17)
>>>>>> mm
>>> [[], [17], []]
>>>>>> nn[1].append(17)
>>>>>> nn
>>> [[17], [17], [17]]
>>>
>>> ???
>>>
>>> Python 2.5 Win XP
>>>
>>> Thanks!
>> You're creating three references to the same list with the
>> multiplication operator.
> 
> There's no need to introduce references: you're creating a list with the
> same object at each position.
> 
> [...]
>> Python is pass-by-reference, not pass-by-value.
> 
> It's certainly not pass-by-reference, nor is it pass-by-value IMHO.
> 
Since no lists are being passed as arguments in these examples it's not
pass-by-anything. Jump off that horse right now!

regards
 Steve
-- 
Steve Holden        +1 571 484 6266   +1 800 494 3119
Holden Web LLC              http://www.holdenweb.com/




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