How to clone a list with Python 3?

Question

From list m:

 m =   [[' # ', ' # ', ' # ', ' # ', ' # ', ' # '], [' # ', ' # ', ' # ', ' # ', ' # ', ' # '], [' # ', ' # ', ' # ', ' # ', ' # ', ' # '], [' # ', ' # ', ' # ', ' # ', ' # ', ' # '], [' # ', ' # ', ' # ', ' # ', ' # ', ' # '], [' # ', ' # ', ' # ', ' # ', ' # ', ' # ']]

In doing mb = m[:] or mb = list(m) I would clone the m initial, right or not? But in doing mb[0][0] = 99999999, for example, the list m is also modified.

What I’m not seeing or don’t know about it?

Answer 1

The main problem is that you have a list of mutable objects - in this case, a list of lists. So just making a shallow copy won’t be enough.

To copy the mutable objects from your list, so they don’t keep the reference, you will need to make the deep copy.

• How to make a "deep copy" in Python?
• What "immutable" really means?

So just do:

from copy import deepcopy

mb = deepcopy(m)

Answer 2

There are several possibilities:

>>> lista = [1, 2, 3]
>>> copia = lista.copy()
>>> copia
[1, 2, 3]
>>> copia = lista[:]
>>> copia
[1, 2, 3]

In addition to many scripts you are able to see something like:

lista = [1, 2, 3]
copia = []
for item in lista:
    copia.append(item)
print(copia) # [1, 2, 3]

Something contrary to what you said to do lista[:] does not generate a reference to the original list but a copy of it, that is to say, changing the copy does not change the original.