Duplicating list of lists [newbie]

[yatsek]

Hi there.
I felt like I already know my toys around but it looks that I'm still
newbie on some points.

So here goes problem:

Lets say that we have list to copy:

a = [1,2,3]
b = a[:]
a[0] = 5

a

[5,2,3] b
[1,2,3]

So far so good

But... let's say that "a" also contains lists:

a = [1,2,[5,6,7]]
b = a[:]

a[0] = 55
a

[55,2,[5,6,7]] b
[1,2,[5,6,7]]

Looks OK but...

a[2][0] = 99
a

[55,2,[99,6,7]] b
[1,2,[99,6,7]]

So - it looks that in list "b" there copy of all objects from list "a"
including not copy of list [5,6,7] but reference to it.

Is there simple way to copy a into b (like a[:]) with all copies of
all objects going as deep as possible? Or it can be done only
manually?

Regards
Yatsek

 

[George Sakkis]

Hi there.
I felt like I already know my toys around but it looks that I'm still
newbie on some points.

So here goes problem:

Lets say that we have list to copy:

a = [1,2,3]
b = a[:]
a[0] = 5

a

[5,2,3] b
[1,2,3]

So far so good

But... let's say that "a" also contains lists:

a = [1,2,[5,6,7]]
b = a[:]

a[0] = 55
a

[55,2,[5,6,7]] b
[1,2,[5,6,7]]

Looks OK but...

a[2][0] = 99
a

[55,2,[99,6,7]] b
[1,2,[99,6,7]]

So - it looks that in list "b" there copy of all objects from list "a"
including not copy of list [5,6,7] but reference to it.

Right; that's what it's called a "shallow copy".

Is there simple way to copy a into b (like a[:]) with all copies of
all objects going as deep as possible?

Yes, there is:

from copy import deepcopy
b = deepcopy(a)


HTH,
George

 

[Ryan Ginstrom]

On Behalf Of (e-mail address removed)

So - it looks that in list "b" there copy of all objects from list "a"
including not copy of list [5,6,7] but reference to it.

Is there simple way to copy a into b (like a[:]) with all
copies of all objects going as deep as possible? Or it can be
done only manually?

I'd suggest checking out copy.deepcopy.

a = [1, [1, 2, 3], 2]
b = a[:]
a[1][2] = 'spam'
b [1, [1, 2, 'spam'], 2]
from copy import deepcopy
b = deepcopy(a)
a[1][2] = 'deepcopy is your friend'
b

[1, [1, 2, 'spam'], 2]

Regards,
Ryan Ginstrom

 

[bearophileHUGS]

Yatsek:

Is there simple way to copy a into b (like a[:]) with all copies of
all objects going as deep as possible?

If you only want to copy up to level-2, then you can do something
like:
cloned = [subl[:] for subl in somelist]
Or sometimes safer:
cloned = [list(subl) for subl in somelist]

If you want to go all the way down, you can use the deepcopy function
of the copy module. In many situations the level-2 copy may be enough,
and it can be faster.

Bye,
bearophile