Duplicating a variable

[hnessenospam at yahoo.com]

I have run into a bit of a subtle problem.  How do I go about
duplicating a variable (particularly a list of lists) in python.  I
was surprised when simple assignment didn't work. For example, let y =
[1,2,3]

>>> x = y
>>> x[2] = 5
>>> y
[1,2,5]

It seems that simply assigning x to y allows further modification of y
via x.  (I'm new to python and I'm sure this is obvious to experienced
users).  So my question, how do I go about duplicating a variable
which I can then manipulate independently?

Thanks,

-Hans