Dear Robert,
Thanks a lot for your help, this is exactly what I wanted. I still
don't get why it is necessary to import a special function to perform
such a standard thing. "import copy" has the affect that suddenly I
cannot do copy(L) any more, but I have to do copy.copy(L) instead.
Since copy(L) does not do what I would expect anyway, I will avoid it.
Frankly, as intuitive as Python is in other respects, this behaviour
blows my mind.
Cheers
Stan
On Aug 17, 6:19 pm, Robert Bradshaw <rober...@math.washington.edu>
wrote:
> On Wed, Aug 17, 2011 at 9:09 AM, Stan Schymanski <schym...@gmail.com> wrote:
> > Dear all,
>
> > This has been driving me mad. According to the python documentation,
> > you can modify a copy of a list without modifying the original using
> > the following code:
>
> > sage: L = []
> > sage: M = L[:] # create a copy
> > sage: # modify L only
> > sage: L.append(1)
> > sage: M
> > []
>
> > Now, I want to do the same with a nested list, but I do not manage to
> > unlink the two. See the example below, where I also tried copy(L) to
> > no avail. I hope that someone can help. Thanks already!
>
> A "copy" of a list is a new list containing exactly the same elements
> as the original list. It sounds like what you want here is
>
> sage: import copy
> sage: L = [range(k) for k in range(5)]
> sage: M = copy.deepcopy(L)
> sage: M[0].append('something')
> sage: M
> [['something'], [0], [0, 1], [0, 1, 2], [0, 1, 2, 3]]
> sage: L
> [[], [0], [0, 1], [0, 1, 2], [0, 1, 2, 3]]
>
> - Robert
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