错误处理后 while 循环中的 Python 回溯错误

Question

我用 Python 3.4.1 创建了一个程序:要求输入一个整数，验证它是一个整数，如果不是整数，则抛出错误消息并要求重新输入数字，验证号码后将其添加到列表中，如果输入-1则结束。

mylist = []

def checkint(number):
    try:
        number = int(number)
    except:
        print ("Input not accepted, please only enter whole numbers.")
        number = input("Enter a whole number. Input -1 to end: ")
        checkint(number)


number = input("Enter a whole number. Input -1 to end: ")
checkint(number)

while int(number) != -1:

    mylist.append(number)
    number = input("Enter a whole number. Input -1 to end: ")
    checkint(number)   

除一种情况外，一切正常。如果输入非整数，例如p(给出错误消息)后跟 -1 来结束程序，我收到此消息:

Traceback (most recent call last):
  File "C:/Users/************/Documents/python/ws3q4.py", line 15, in <module>
    while int(number) != -1:
ValueError: invalid literal for int() with base 10: 'p'

我不明白为什么会发生这种情况，因为 p 的输入永远不应该达到

while int(number) != -1:

Answer 1

这是一个说明问题的最小示例:

>>> def change(x):
        x = 2
        print x

>>> x = 1
>>> change(x)
2 # x inside change
>>> x
1 # is not the same as x outside

您需要修复该函数以返回某些内容，并将其分配给外部作用域中的number:

def checkint(number):
    try:
        return int(number) # return in base case
    except:
        print ("Input not accepted, please only enter whole numbers.")
        number = input("Enter a whole number. Input -1 to end: ")
        return checkint(number) # and recursive case

number = input("Enter a whole number. Input -1 to end: ")
number = checkint(number) # assign result back to number

此外，最好迭代地执行此操作而不是递归地执行此操作 - 请参阅例如Asking the user for input until they give a valid response 。