pythonで変数のコピーがうまくいかない

python

1mymatlix = [2 [1,2,3],3 [4,5,6],4 [7,8,9]]5# pre_mymatlix = [6# [1,2,3],7# [4,5,6],8# [7,8,9]]9pre_mymatlix = mymatlix 10N = 311for i in range(N):12 for j in range(N):13 mymatlix[N-i-1][j] = pre_mymatlix[j][i]14 print(pre_mymatlix)15 16''' 17出力結果 18[[1, 2, 3], [4, 5, 6], [7, 8, 9]] 19[[1, 2, 3], [4, 5, 6], [7, 8, 9]] 20[[1, 2, 3], [4, 5, 6], [1, 8, 9]] 21[[1, 2, 3], [4, 5, 6], [1, 4, 9]] 22[[1, 2, 3], [4, 5, 6], [1, 4, 1]] 23[[1, 2, 3], [2, 5, 6], [1, 4, 1]] 24[[1, 2, 3], [2, 5, 6], [1, 4, 1]] 25[[1, 2, 3], [2, 5, 4], [1, 4, 1]] 26[[3, 2, 3], [2, 5, 4], [1, 4, 1]] 27[[3, 4, 3], [2, 5, 4], [1, 4, 1]] 28'''