我似乎弄不明白为什么我不能转换为int类型

Question

我试图将一个字符串转换为int类型，但由于某种原因，它似乎确实想要工作。我非常有兴趣了解在我对所写代码的理解上有什么脱节之处。谢谢!

def fourdigitnum(num):
    string = str(num)
    while (len(string) < 4):
        string = "0" + string
    return string
def ascend_desend(string,order):
    if(order == "ascend"):
        string = sorted(string,reverse = True)
    elif(order == "descend"):
        string = sorted(string,reverse = False)
    return string
def list_to_num(lst):
    string = ""
    for ele  in lst:
        string = string + ele
    return int(string)

def KaprekarsConstant(num): 
    num = int(num)
    count = 0
    while num != 6174:
        num = fourdigitnum(num)
        #ascend and descend are lists
        ascend = ascend_desend(num,"ascend")
        descend = ascend_desend(num,"descend")
        #they must be turned into nums
        ascend = list_to_num(ascend)
        descend = list_to_num(descend)
        num = descend - ascend
        count += 1

    return count

print KaprekarsConstant(raw_input())  

错误：

Traceback (most recent call last):
  File "/tmp/576159006/main.py", line 41, in <module>
    print KaprekarsConstant(2111)
  File "/tmp/576159006/main.py", line 33, in KaprekarsConstant
    ascend = list_to_num(ascend)
  File "/tmp/576159006/main.py", line 16, in list_to_num
    return int(string)
ValueError: invalid literal for int() with base 10: '999-'

Answer 1

在您的代码中，num = descend - ascend行的计算返回负(-)值，因为生成的ascend值大于descend。您可以获取绝对值并从此处继续，请参见以下几行：

num = descend - ascend
num = abs(num)
count += 1

Answer 2

您需要检查第15行的计算。

似乎在你的代码中，在这一行计算的'string‘的值是'999-’，它不能被转换成整数类型，因为它有一个'-‘。