deep copy of list in python

I tried to make a deep copy of a list l, but seems like the slicing method doesn't work somehow?I don't want the change in x to be reflected in l. So how should I make a deep copy and what is wrong in my code?

This was my code-

def processed(matrix,r,i):
    matrix[r].append(i)
    return matrix

l=[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
x=l[:]
print(processed(x,0,10))
print(l)

OUTPUT-

[[1, 2, 3, 10], [4, 5, 6], [7, 8, 9]]
[[1, 2, 3, 10], [4, 5, 6], [7, 8, 9]]

Solution

Your code does indeed succeed in creating a shallow copy. This can be seen by inspecting the IDs of the two outer lists, and noting that they differ.

>>> id(l)
140505607684808

>>> id(x)
140505607684680

Or simply comparing using is:

>>> x is l
False

However, because it is a shallow copy rather than a deep copy, the corresponding elements of the list are the same object as each other:

>>> x[0] is l[0]
True

This gives you the behaviour that you observed when the sub-lists are appended to.

If in fact what you wanted was a deep copy, then you could use copy.deepcopy. In this case the sublists are also new objects, and can be appended to without affecting the originals.

>>> from copy import deepcopy

>>> l=[[1, 2, 3], [4, 5, 6], [7, 8, 9]]

>>> xdeep = deepcopy(l)

>>> xdeep == l
True

>>> xdeep is l
False     <==== A shallow copy does the same here

>>> xdeep[0] is l[0]
False     <==== But THIS is different from with a shallow copy

>>> xdeep[0].append(10)

>>> print(l)
[[1, 2, 3], [4, 5, 6], [7, 8, 9]]

>>> print(xdeep)
[[1, 2, 3, 10], [4, 5, 6], [7, 8, 9]]

If you wanted to apply this in your function, you could do:

from copy import deepcopy

def processed(matrix,r,i):
    new_matrix = deepcopy(matrix)
    new_matrix[r].append(i)
    return new_matrix

l = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
x = processed(l,0,10)
print(x)
print(l)

If in fact you know that the matrix is always exactly 2 deep, then you could do it more efficiently than using deepcopy and without need for the import:

def processed(matrix,r,i):
    new_matrix = [sublist[:] for sublist in matrix]
    new_matrix[r].append(i)
    return new_matrix

l = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
x = processed(l,0,10)
print(x)
print(l)