initialized list: strange behavior

[Gary Herron]

alexander.genkin at gmail.com wrote:
> Hi Python experts! Please explain this behavior:
>   

[] make an empty list.
[ [],[],[] ] makes a list of three different empty lists.
3*[[]]  makes a list of three references to the same list.

Realy, that should explain it all, but perhaps there are enough empty
lists here to obscure things.  Try this:

Here b is a list that contains three references to a.   Modify a, and
all three references to a show the modification:

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

Gary Herron



>   
>>>> 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!
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>