Convert spreadsheet number to column letter

Question

I\'m looking for the opposite to this Q&A: Convert an excel or spreadsheet column letter to its number in Pythonic fashion.

or this one but in python How to conv

Answer 1

Just to complicate everything a little bit I added caching, so the name of the same column will be calculated only once. The solution is based on a recipe by @Alex Benfica

import string


class ColumnName(dict):
    def __init__(self):
        super(ColumnName, self).__init__()
        self.alphabet = string.uppercase
        self.alphabet_size = len(self.alphabet)

    def __missing__(self, column_number):
        ret = self[column_number] = self.get_column_name(column_number)
        return ret

    def get_column_name(self, column_number):
        if column_number <= self.alphabet_size:
            return self.alphabet[column_number - 1]
        else:
            return self.alphabet[((column_number - 1) / self.alphabet_size) - 1] + self.alphabet[((column_number - 1) % self.alphabet_size)]

Usage example:

column = ColumnName()

for cn in range(1, 40):
    print column[cn]

for cn in range(1, 50):
    print column[cn]