Having trouble in creating 2D array/list

Question

I'm having difficulty creating a 2D list of permutations. Here is a minimal code to reproduce the problem

class Solution:
def permute(self, A):
    A = sorted(A)
    print A
    A_out = []
    A_out.append(A)
    for iter0 in range(4):
        A[0] = A[0] + 1
        print A
        A_out.append(A)
    return A_out

sol = Solution()
A = [1, 2, 3]
print sol.permute(A)

For this particular input(1, 2, 3) the output is

[1, 2, 3]
[2, 2, 3]
[3, 2, 3]
[4, 2, 3]
[5, 2, 3]
[[5, 2, 3], [5, 2, 3], [5, 2, 3], [5, 2, 3], [5, 2, 3]]

but the desired output is

[1, 2, 3]
[2, 2, 3]
[3, 2, 3]
[4, 2, 3]
[5, 2, 3]
[[1, 2, 3], [2, 2, 3], [3, 2, 3], [4, 2, 3], [5, 2, 3]]

I think it has something to do deep copy/ shallow copy thing but I am not sure how to correct this as I am not very familiar with Python. How can I fix it?

Answer 1

It is indeed about shallow copies. The list A you keep appending always refers to the same value. This is related to the mutability of lists in Python.

You need to append a new copy of the list each time to make them independent of each other. You can do that using the slice operator [:] because it creates a new copy of the list. So you can just use it when calling append

def permute(self, A):
    A = sorted(A)
    print A
    A_out = []
    A_out.append(A[:])
    while (self.checkVal(A) != -1) :
        A = self.nextState(A,self.checkVal(A))
        print A
        A_out.append(A[:])
    return A_out