excel_columns.py

import sys


def columns(end):
    char_a = ord('A')
    alpha_iter = lambda: (chr(c) for c in range(char_a, char_a + 26))

    p = [alpha_iter()]
    v = [None]

    for n in range(0, end):
        # at the end of the 0 place iterator
        if n % 26 == 0 and n != 0:
            # setup 0 place carry
            p[0] = alpha_iter()
            l = len(v)

            for i in range(1, l + 1):
                if i == l:
                    # new digit
                    p.append(alpha_iter())
                    v.append(None)

                # increment digit in `i` place
                u = next(p[i], None)

                if u is None:
                    # setup `i` place carry
                    p[i] = alpha_iter()
                    v[i] = next(p[i])
                else:
                    # increment resolved
                    v[i] = u
                    break

        # normal increment
        v[0] = next(p[0])

        yield ''.join(v[::-1])


if __name__ == "__main__":
    count = int(sys.argv[1])

    for name in columns(count):
        print(name)