[Tutor] Copying lists of lists

[Norman Charnley]

As the subject line says, I want to make a copy of a list of lists, but
I've encountered a problem which I can't understand.

First the obvious:

>>> a=[1,2,3]
>>> b=a[:]
>>> b[0]=99
>>> print a,b
>>> [1, 2, 3] [99, 2, 3]

OK, so far, so good. But:

>>> a=[[1,2,3],[4,5,6],[7,8,9]]
>>> b=a[:]
>>> b[0][0]=99
>>> print a
[[99, 2, 3], [4, 5, 6], [7, 8, 9]]
>>> print b
[[99, 2, 3], [4, 5, 6], [7, 8, 9]]

i.e. a is reflecting the change made in b.

However, the following does work as I expected:
>>> a=[[1,2,3],[4,5,6],[7,8,9]]
>>> b=a[:]
>>> b[0]=99
>>> print a
[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
>>> print b
[99, [4, 5, 6], [7, 8, 9]]

Help, I'm a little confused, and I'd be grateful for an explanation. If
this were C I'd think it was a pointer problem...!

Many thanks,

Norman

 =================================================
 Dr. Norman Charnley
 Department of Earth Sciences
 University of Oxford
 Oxford OX1 3PR, UK.
==================================================