How to clone a list with Python 3?

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 will not be enough.

To copy the mutable objects from your list, so that they do not keep the reference, you need to do the deep copy.

• How to do a deep copy in Python?
• What does "immutable" really mean?

So, just do:

from copy import deepcopy

mb = deepcopy(m)

There are several possibilities:

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

In addition to what in 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 rather a copy of it, ie change the copy does not change the original.