iter.py

# -*- coding: utf-8 -*-

row1 = ["a1", "a2", "a3", "a4", "a5", "a6", "a7", "a8", "a9"]
row2 = ["b1", "b2", "b3", "b4", "b5"]
row3 = ["c1", "c2", "c3", "c4", "c5", "c6", "c7", "c8", "c9", "c10", "c11", "c12", "c13", "c14"]

rows = [row1, row2, row3]

max_size = -1
number = 0

for row in rows:
    if len(row) > max_size:
        max_size = len(row)

for i in range(max_size-1):
    for row in rows:
        if i >= len(row):
            continue
        if i%4 != number:
            continue
        print(row[i])