List-of-lists (aka array) mystery

[aardvark]

Hi everyone

A friend of mine has discovered some interesting behavior with
lists-of-lists (I suppose some call them "arrays"). When you define
(or "initialize") a list-of-lists using the range() function, then set
a value within one of the inner lists, the value of each inner list
item in that same position becomes the new value. When you explicitly
define (or "initialize") the list-of-lists the behavior is as
expected, i.e. only the value in the list position you specified,
within the list you specified, is changed.

A better way to show this is with an example:

Definition using range:

>>> x=[range(-1,0)*3]*3
>>> x
[[-1, -1, -1], [-1, -1, -1], [-1, -1, -1]]
>>> x[1][1]=0
>>> x
[[-1, 0, -1], [-1, 0, -1], [-1, 0, -1]]

I did not expect x[0][1] and x[2][1] to become 0. I only expected
x[1][1] to become 0. This does not happen when you explicitly define
the list:

>>> x=[[-1, -1, -1], [-1, -1, -1], [-1, -1, -1]]
>>> x
[[-1, -1, -1], [-1, -1, -1], [-1, -1, -1]]
>>> x[1][1]=0
>>> x
[[-1, -1, -1], [-1, 0, -1], [-1, -1, -1]]

Why is this? It would seem that multiplying the result of the range()
function twice returns pointers to a single list rather than discrete
lists. Is there a better way to initialize lists-of-lists? Should I be
going about this differently?

I'm sure it's something obvious... I'd just appreciate being pointed
in the right direction.

thanks for any help you can provide!

aardvark

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identifying string: mjunhybgt
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