[Tutor] List indexing problem

[Steve Willoughby]

Mike Meisner wrote:
> I need to do some statistical analysis by binning values into an array.
>  
> Being new to Python, I tried to use a list of lists.  I've extracted 
> just the minimum code that I'm having trouble with:

What you need to remember is that Python works with *objects*, and 
variables are simply *references* to those objects.  So if I say
a = [1,2,3]
b = a

b and a both refer to the *SAME* list object (not two lists which happen 
to have the same elements).

>     temp = [[0, 0, 0],[0, 0, 0],[0, 0, 0]]

temp now refers to a list object containing 3 lists, each containing 3 
integers.

>     # initialize to zero
>     for i in range(20):
>         IP.append(temp)

Now IP contains 20 copies of references to the *same* list object.
So you were modifying the underlying list object which happens to be 
referenced many times in IP.