Как после создания новой скопированной переменной, не изменить старую?

Имеется такой генератор

import time
import itertools
import random

def main(size):
    global global_board
    global global_board_clear
    size = int(size)

    L = []
    for i in range(1, size + 1):
        L.append(i)
    per = list(itertools.permutations(L))
    number = random.randint(0, len(per))

    boardd = [[0] * (size + 2) for j in range(size + 2)]
    lst_4 = [[2, 3, 2, 4, 1, 2], [1, 4, 1, 3, 2, 3], [3, 1, 3, 2, 4, 1], [2, 2, 4, 1, 3, 2]]

    lst_5 = [[2, 3, 5, 1, 4, 2, 3], [3, 2, 1, 3, 5, 4, 2], [2, 4, 2, 5, 3, 1, 3],
             [4, 1, 3, 4, 2, 5, 1], [1, 5, 4, 2, 1, 3, 3]]
    lst_6 = [[4, 3, 4, 1, 2, 5, 6, 1], [4, 2, 3, 4, 1, 6, 5, 2], [2, 5, 6, 3, 4, 1, 2, 3],
             [1, 6, 1, 5, 3, 2, 4, 3], [3, 4, 5, 2, 6, 3, 1, 3], [3, 1, 2, 6, 5, 4, 3, 4]]
    lst_7 = [[3, 5, 1, 6, 7, 2, 4, 3, 3, ], [3, 4, 2, 1, 5, 7, 3, 6, 2],
             [2, 1, 7, 3, 6, 4, 5, 2, 4], [2, 6, 5, 4, 3, 1, 2, 7, 1], [3, 3, 6, 7, 2, 5, 1, 4, 3],
             [1, 7, 4, 2, 1, 3, 6, 5, 3], [5, 2, 3, 5, 4, 6, 7, 1, 2]]
    lst_8 = [[1, 8, 7, 6, 4, 1, 5, 3, 2, 6], [2, 5, 8, 7, 3, 2, 6, 1, 4, 4],
             [2, 6, 3, 2, 8, 4, 7, 5, 1, 4], [4, 1, 4, 3, 2, 7, 8, 6, 5, 3],
             [2, 7, 5, 1, 6, 3, 4, 2, 8, 1], [2, 2, 1, 8, 7, 5, 3, 4, 6, 3],
             [3, 4, 6, 5, 1, 8, 2, 7, 3, 3], [5, 3, 2, 4, 5, 6, 1, 8, 7, 2]]
    lst_9 = [[4, 2, 3, 8, 1, 5, 9, 6, 4, 7, 2], [3, 3, 6, 9, 7, 4, 8, 5, 1, 2, 4],
             [3, 7, 8, 4, 5, 3, 1, 9, 2, 6, 2], [2, 6, 5, 3, 9, 1, 2, 4, 7, 8, 2],
             [2, 8, 4, 1, 3, 6, 7, 2, 9, 5, 2], [3, 4, 7, 5, 2, 9, 6, 8, 3, 1, 4],
             [2, 5, 9, 2, 6, 7, 4, 1, 8, 3, 3], [1, 9, 1, 6, 8, 2, 3, 7, 5, 4, 5],
             [5, 1, 2, 7, 4, 8, 5, 3, 6, 9, 1]]
    if size == 4:
        lst = lst_4
    elif size == 5:
        lst = lst_5
    elif size == 6:
        lst = lst_6
    elif size == 7:
        lst = lst_7
    elif size == 8:
        lst = lst_8
    elif size == 9:
        lst = lst_9

    for i in range(1, size + 1):
        boardd[i] = lst[per[number][i - 1] - 1]
    boardd[0][0] = -1
    boardd[0][-1] = -1
    boardd[-1][0] = -1
    boardd[-1][-1] = -1

    for j in range(1, size + 1):
        curr = boardd[1][j]
        heig = 1
        for i in range(1, size + 1):
            if boardd[i][j] > curr:
                curr = boardd[i][j]
                heig += 1
        boardd[0][j] = heig
    for j in range(1, size + 1):
        curr = boardd[-2][j]
        heig = 1
        for i in range(1, size + 1):
            if boardd[-i - 1][j] > curr:
                curr = boardd[-i - 1][j]
                heig += 1
        boardd[-1][j] = heig
    global_board = boardd
    print(boardd)



    board_clear = boardd[:]
    for v in range(1, size + 1):
        #print(boardd)
        for f in range(1, size + 1):
            #print(boardd)
            board_clear[v][f] = 0
    boardd_clear = board_clear
    global_board_clear = boardd_clear
    print(board_clear)
    print(boardd)

main(4)

После трех print, вывод в консоль:

[[-1, 2, 3, 1, 2, -1], [2, 3, 2, 4, 1, 2], [3, 1, 3, 2, 4, 1], [2, 2, 4, 1, 3, 2], [1, 4, 1, 3, 2, 3], [-1, 1, 2, 2, 3, -1]]
[[-1, 2, 3, 1, 2, -1], [2, 0, 0, 0, 0, 2], [3, 0, 0, 0, 0, 1], [2, 0, 0, 0, 0, 2], [1, 0, 0, 0, 0, 3], [-1, 1, 2, 2, 3, -1]]
[[-1, 2, 3, 1, 2, -1], [2, 0, 0, 0, 0, 2], [3, 0, 0, 0, 0, 1], [2, 0, 0, 0, 0, 2], [1, 0, 0, 0, 0, 3], [-1, 1, 2, 2, 3, -1]]

Вопрос: Почему boardd меняется на board_clear и как его можно оставить прежним, чтобы вывод в консоль был такой:

[[-1, 2, 3, 1, 2, -1], [2, 3, 2, 4, 1, 2], [3, 1, 3, 2, 4, 1], [2, 2, 4, 1, 3, 2], [1, 4, 1, 3, 2, 3], [-1, 1, 2, 2, 3, -1]]
[[-1, 2, 3, 1, 2, -1], [2, 0, 0, 0, 0, 2], [3, 0, 0, 0, 0, 1], [2, 0, 0, 0, 0, 2], [1, 0, 0, 0, 0, 3], [-1, 1, 2, 2, 3, -1]]
[[-1, 2, 3, 1, 2, -1], [2, 3, 2, 4, 1, 2], [3, 1, 3, 2, 4, 1], [2, 2, 4, 1, 3, 2], [1, 4, 1, 3, 2, 3], [-1, 1, 2, 2, 3, -1]]