Wrong code for converting number to Excel row ref

gistfile1.txt

import string

def base_(x, acc):
    if x == 0:
        return acc
    string.uppercase
    q, r = divmod(x, 26)
    l = string.uppercase[r]
    return base_(q, l + acc)

def base(x):
    return base_(x, "")

print(base(0))
print(base(27))
print(base(2**20-1))