Duplicating a variable

[Guest]

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

 

[Guest]

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]

(e-mail address removed) 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.

 

[bearophileHUGS]

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

 

[Guest]

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