# Duplicating a variable

Jeff McNeil jeff at jmcneil.net
Thu Jan 24 18:49:19 CET 2008

```Have a look at the copy module if you have a somewhat "complex" object graph
to duplicate.  Remember, you're essentially just creating another reference
to a singular list object with simple assignment (a = b).

>>> a = [1,2,3,4, ['5a', '5b', '5c', ['6a', '6b','6c']], 7,8]
>>> b = a
>>> a
[1, 2, 3, 4, ['5a', '5b', '5c', ['6a', '6b', '6c']], 7, 8]
>>> a[4][1] = None
>>> a
[1, 2, 3, 4, ['5a', None, '5c', ['6a', '6b', '6c']], 7, 8]
>>> b
[1, 2, 3, 4, ['5a', None, '5c', ['6a', '6b', '6c']], 7, 8]
>>> import copy
>>> d = copy.deepcopy(a)
>>> d
[1, 2, 3, 4, ['5a', None, '5c', ['6a', '6b', '6c']], 7, 8]
>>> a
[1, 2, 3, 4, ['5a', None, '5c', ['6a', '6b', '6c']], 7, 8]
>>> a[4][1] = "Something"
>>> a
[1, 2, 3, 4, ['5a', 'Something', '5c', ['6a', '6b', '6c']], 7, 8]
>>> b
[1, 2, 3, 4, ['5a', 'Something', '5c', ['6a', '6b', '6c']], 7, 8]
>>> d
[1, 2, 3, 4, ['5a', None, '5c', ['6a', '6b', '6c']], 7, 8]
>>>

Thanks,

Jeff

On 1/24/08, hnessenospam at yahoo.com <hnessenospam at yahoo.com> wrote:
>
> 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
> --
> http://mail.python.org/mailman/listinfo/python-list
>
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