matrix = [[1, 2, 3],
[4, 5, 6],
[7, 8, 9]]
matrix
[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
"build a new list by running an expression on each item in a sequence, one at a time, from left to right." - Learning Python
col2 = [row[1] for row in matrix]
col2
[2, 5, 8]
diag = [matrix[i][i] for i in [0, 1, 2]]
diag
[1, 5, 9]