python报表自动化系列 - 译码：将纯数字译码为Excel列坐标的字母索引表示形式

def num_to_Letter(num):
    bit = 1
    i = 1
    a = 0
    x = int(num) - 26**(bit-i)
    Letter = ''
    if x < 0:
        raise ValueError('error:列坐标索引的数值表示必须为一个正整数')
    if  num - 26**(bit-i) == 0:
        return 'A'
    x = num
    while x >= 0:
        if x == 0:
            a = 1            #标志位置1，表示由全A构成
        x = x - 26**(bit - i)
        i = i-1
    if a == 1:
        for k in range(abs(i)):
            Letter = Letter + 'A'
        return Letter
    Num_of_letters = abs(i)
    D = {1:'A',2:'B',3:'C',4:'D',5:'E',6:'F',7:'G',8:'H',
         9:'I',10:'J',11:'K',12:'L',13:'M',14:'N',15:'O',16:'P',
         17:'Q',18:'R',19:'S',20:'T',21:'U',22:'V',23:'W',24:'X',
         25:'Y',26:'Z',}
    alist = []
    #从最高位开始逐个确定字母
    for k in range(Num_of_letters+1):
        if k != 0:
            alist = [k] + alist
    #print('alist:',alist)
    n = num
    for k in alist:        # k 为从高位向低位的第k个字母
        i = k
        j = 1              # 用于得到 j X (26)的i次方的数字k，再将k由数字映射为字母
        while (n - j*(26**(i-1))) > 0:
            j = j + 1
        j = j - 1
        n = n - j*(26**(i-1))
        u = j
        if k == 1:
            u = u+1
        Letter = Letter + D.get(u)
    return Letter