Python - what is the difference between those statements?

Question

What's the difference between A=B and A = B[:]?

Answer 1

When you make a copy of a list:
A = B.copy()

It makes sure that A and B are not the same.
It does not make sure, that their items are copied too.

Thus A[1] and B[1] are still the same, even though A and B are not the same. It can be quite complicated, but I think there is a solution in module "copy".

Answer 2

Let's assume B is an ordered iterable.

This:
A = B
will give A the original B.
changes on A will also change B and
changes on B will also change A.

This:
A = B[:]
will give A only a copy of B.
changes on A will not change B and
changes on B will not change A.
BUT
changes on A's items can change B's items and
changes on B's items can change A's items.

This:
from copy import deepcopy

A = deepcopy(B)
will also give A a copy of B, but it will also make copies of all the mutable items in the list.
changes on A will not change B and
changes on B will not change A.
AND
changes on A's items will not change B's items and
changes on B's items will not change A's items.

Answer 3

https://code.sololearn.com/cx3U3Kv0f2ln/?ref=app

Answer 4

I get it now, b= a[:] is the same as b= copy(a). And if I don't want to keep references to list inside lists, then I use a=deepcopy (b).

Answer 5

Gabriel Yes, b = a[:] and b = a.copy() don't have much difference, except for which datatypes they can be used, but I would recommend a.copy() because it's more readable.

Here are some speed comparisons for different methods with same result:

https://code.sololearn.com/cvlGwbcjQXOO/?ref=app

Answer 6

Solution:
from copy import deepcopy

a = [5, [6, 7]]
b = deepcopy(a)

Answer 7

Ok, but I still have a question about it, something is still not clear. Here is a complete code that shows where I don't get it...
https://code.sololearn.com/cP3lHeM48mR2/?ref=app

Answer 8

Yeah, I saw that as a[1] is a list, a[1] IS the same as b[1], so if a change a[1], it also changes b[1]. But if I change other indexes of a that are a list themselves, the changes does not apply to b. I Just don't know exactly why that works that way, this could cause some trouble if this is not clear for me...I'm gonna check the module "copy", I hope the detail I am searching for is there, thanks for the help.

Answer 9

Sorry, you posted more stuff before I give you a feed back, I will check that out