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comp.lang.python

Duplicating a variable

hnessenospam@yahoo.com

1/24/2008 5:37:00 PM

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
4 Answers

hnessenospam@yahoo.com

1/24/2008 5:43:00 PM

0

On Jan 24, 9:36 am, "hnessenos...@yahoo.com" <hnessenos...@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

Ah, found the copy module... Much better. Thanks,

-Hans

Bruno Desthuilliers

1/24/2008 5:51:00 PM

0

hnessenospam@yahoo.com a écrit :
> 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.

using the deepcopy function of the copy module.

> 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]

Python only uses object references. It never copy anything unless
explicitely asked for.

Bearophile

1/24/2008 5:56:00 PM

0

Hans:
> 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.

Python is quite high-level language, but now and then it too accepts
some compromises to increase its speed/size performance. Probably
after Python someone will invent a language that may be slower than
Python because it's higher level than Python, and avoids that problem
you talk about (and other things). (And with a strategy of smart data
sharing and copy-on-write the interpreter can avoid part of that
overhead).


> how do I go about duplicating a variable
> which I can then manipulate independently?

If your variable contains a list, then you can copy it like this:

>>> l1 = [1, 2, 3]
>>> l2 = l1[:]
>>> l2[1] = 4

As you can see now they are two distinct lists:

>>> l1
[1, 2, 3]
>>> l2
[1, 4, 3]

If you want to copy any kind of object you can use the copy function
(instead of a simpler copy method that's absent):

>>> d1 = {1:2, 3:4}
>>> from copy import copy
>>> d2 = copy(d1)
>>> d1[1] = 5
>>> d1
{1: 5, 3: 4}
>>> d2
{1: 2, 3: 4}

But as you can see copy works only one level deep:

>>> d3 = {1:[1], 3:4}
>>> d3
{1: [1], 3: 4}
>>> d4 = copy(d3)
>>> d3[1][0] = 2
>>> d3
{1: [2], 3: 4}
>>> d4
{1: [2], 3: 4}

To copy all levels you need deepcopy:

>>> from copy import deepcopy
>>> d5 = deepcopy(d3)
>>> d3[1][0] = 5
>>> d3
{1: [5], 3: 4}
>>> d4
{1: [5], 3: 4}
>>> d5
{1: [2], 3: 4}

Bye,
bearophile

hnessenospam@yahoo.com

1/24/2008 7:51:00 PM

0

On Jan 24, 9:55 am, bearophileH...@lycos.com wrote:
>
> If your variable contains a list, then you can copy it like this:
>
> >>> l1 = [1, 2, 3]
> >>> l2 = l1[:]
> >>> l2[1] = 4
>
> As you can see now they are two distinct lists:
>
> >>> l1
> [1, 2, 3]
> >>> l2
>
> [1, 4, 3]
>
> If you want to copy any kind of object you can use the copy function
> (instead of a simpler copy method that's absent):
>
> >>> d1 = {1:2, 3:4}
> >>> from copy import copy
> >>> d2 = copy(d1)
> >>> d1[1] = 5
> >>> d1
> {1: 5, 3: 4}
> >>> d2
>
> {1: 2, 3: 4}
>
> But as you can see copy works only one level deep:
>
> >>> d3 = {1:[1], 3:4}
> >>> d3
> {1: [1], 3: 4}
> >>> d4 = copy(d3)
> >>> d3[1][0] = 2
> >>> d3
> {1: [2], 3: 4}
> >>> d4
>
> {1: [2], 3: 4}
>
> To copy all levels you need deepcopy:
>
> >>> from copy import deepcopy
> >>> d5 = deepcopy(d3)
> >>> d3[1][0] = 5
> >>> d3
> {1: [5], 3: 4}
> >>> d4
> {1: [5], 3: 4}
> >>> d5
>
> {1: [2], 3: 4}
>
> Bye,
> bearophile

Works great, it is exactly what I needed thanks!

-Hans