devoff2.py

# coding: utf-8
import math


def transpose(matrix):
    row_length = len(matrix[0])
    transpose_matrix = [[] for _ in range(row_length)]

    for i, m1 in enumerate(matrix):
        for j, m2 in enumerate(m1):
            transpose_matrix[j].append(matrix[i][j])

    return transpose_matrix


foo = "devoff se puso atr mono"
length = 4
submatrices_length = (len(foo) // length) + 1
matrix = [[] for _ in range(submatrices_length)]

for i, char in enumerate(foo):
    matrix_idx = math.ceil(i // length)
    matrix[matrix_idx].append(char)
padding = length - len(matrix[-1])

if padding > 0:
    for _ in range(padding):
        matrix[-1].append('')

transpose_matrix = transpose(matrix)

real_matrix = transpose(transpose_matrix)

matrix_to_vector = []
for row in real_matrix:
    for c in row:
        matrix_to_vector.append(c)

print(''.join(matrix_to_vector))