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MAT_SIZE = 3
mat = [[0 for i in range(MAT_SIZE)] for j in range(MAT_SIZE)]

# >>> mat
# [[0, 0, 0], [0, 0, 0], [0, 0, 0]]

for i, j in [(i, j) for i in range(MAT_SIZE) for j in range(MAT_SIZE)]:
    mat[i][j] = 10*i + j

# >>> mat
# [[0, 1, 2], [10, 11, 12], [20, 21, 22]]