Convert Excel columns to index and back

alpha_columns.py

# coding: utf-8
'''Excel Column Conversion, (C) 2018 Adam C. Abernathy, Lic. MIT'''

def excel_alpha_to_index(column_name, alphabet='abcdefghijklmnopqrstuvwxyz'):
    return sum([(alphabet.find(column_name[-i - 1].lower()) + 1) * pow(len(alphabet), i)
        for i in reversed(xrange(len(column_name)))])

def excel_index_to_alpha(postion, alphabet='abcdefghijklmnopqrstuvwxyz'):
    column_name = ''
    while postion > 0:
        mod = (postion - 1) % len(alphabet)
        column_name = alphabet[mod] + column_name
        postion = (postion - mod) / len(alphabet)

    return column_name.upper()

if __name__ == '__main__':
    # -- Tests --

    # to index, base 26, normal alphabet
    print excel_alpha_to_index('CDA') == 2133
    print excel_alpha_to_index('A')   == 1
    print excel_alpha_to_index('c')   == 3
    print excel_alpha_to_index('AA')  == 27
    print excel_alpha_to_index('AB')  == 28

    # to index, base 5
    alphabet = 'abcde'
    print excel_alpha_to_index('CDA', alphabet) == 96
    print excel_alpha_to_index('A',   alphabet) == 1
    print excel_alpha_to_index('c',   alphabet) == 3
    print excel_alpha_to_index('AA',  alphabet) == 6
    print excel_alpha_to_index('AB',  alphabet) == 7

    # to alpha 
    print excel_index_to_alpha(27)   == 'AA'
    print excel_index_to_alpha(2133) == 'CDA'