How Does List.append() Work In Python - I'm Getting An Unexpected Result

I've written the following code below but the output of it is not as I expected. Does anyone know why it is behaving like this? N.B.: I know the code doesn't transpose the list c

Solution 1:

Key point is to make a copy of inner list and then append in the outer list.

copy = t_list[::]

It is because list is an object and it is a refresnce. so when you append another item in a list, it will be updated over all places where that objects exists. In your case, you are appending list into a list and then updating the inner list which causes the previous list items to update as well. You need to append a copy of a list in transposed list. Here is a solution.

transposed = []
for i in range(4):
print("i", i)
t_list = []
for row in matrix:
    print("row", row)
    t_list.append(row[i])
    print("t_list**********", t_list)
    transposed.append(t_list[::])

    print("transposed//////////////", transposed)

Solution 2:

It seems like an indentation issue in the line transposed.append(t_list):

transposed = []
for i in range(4):
    t_list = []
    for row in matrix:
        t_list.append(row[i])

    transposed.append(t_list)

This code prints:

[[1, 5, 9],
 [2, 6, 10],
 [3, 7, 11],
 [4, 8, 12]]

Transpose has been implemented in numpy and it works with python arrays:

import numpy as np
np.transpose(matrix)

Solution 3:

The problem is here:

t_list = []
for row in matrix:
    t_list.append(row[i])
    transposed.append(t_list)

Inside the loop the contents of the list object t_list changes, but the list object itself remains the same. Thus, transposed gets the same object for each row.

This is fixed by appending a copy of the current list instead of the original list object.

transposed.append(list(t_list))