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Python replacing all list elements instead of the indexed list element in for loop

May 24, 2022

I want to replace the kth element of the kth element of the list.

E.g.,

[[0, 0, 0],
 [0, 0, 0],
 [0, 0, 0]]

[[1, 0, 0],
 [0, 1, 0],
 [0, 0, 1]]

BUT python does not seem to want to do that and instead is replacing each value for each element.

I run this:

    # Triangle Vertices
    V = [[0, 1], [-1, 0], [1, 0]]
    
    # Triangles (indices of V in clockwise direction)
    T = [[1, 0, 2]]
    
    # Creating sub-triangles
    bary_point = [0, 0.5]
    
    v_list = []
    
    for t in T:
        print(t)
        for k in range(3):
            v_list.append(V)
            v_list[k][k] = bary_point # <-- This line
    
    print('v_list: ')
    print(v_list)

and it produces this:

v_list:
[[[0, 0.5], [0, 0.5], [0, 0.5]],
 [[0, 0.5], [0, 0.5], [0, 0.5]],
 [[0, 0.5], [0, 0.5], [0, 0.5]]]

but I want this:

v_list:
[[[0, 0.5], [-1, 0],  [1, 0]],
 [[0, 1],   [0, 0.5], [1, 0]],
 [[0, 1],   [-1, 0],  [0, 0.5]]]

Am I doing something wrong? I am on Python 3.10.0

Thank you.

EDIT Solution:
Change

v_list.append(V)

v_list.append(V.copy())

Thank you!

>Solution :

You are appending the same reference to the list V, and when you change v_list[k][k] you are changing V, one solution would be to append a copy of the list, by using V.copy() as argument to append.