Python - 类型错误 - 类型错误 : '<' not supported between instances of 'NoneType' and 'int'

Question

TypeError: '<' not supported between instances of 'NoneType' and 'int'

我在 Stack Overflow 中寻找答案，发现我应该使用 int(input(prompt))，但这正是我正在做的

def main():      
    while True:
            vPopSize = validinput("Population Size: ")
            if vPopSize < 4:
                print("Value too small, should be > 3")
                continue
            else:
                break

def validinput(prompt):
    while True:
        try:
            vPopSize = int(input(prompt))
        except ValueError:
            print("Invalid Entry - try again")
            continue
        else:
            break

Answer 1

这个问题也出现了when migrating to Python 3 .

在 Python 2 中，将整数与 None 进行比较将“有效”，这样 None 就被认为小于任何整数，甚至是负数:

>>> None > 1
False
>>> None < 1
True

在 Python 3 中，此类比较会引发 TypeError:

>>> None > 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: '>' not supported between instances of 'NoneType' and 'int'

>>> None < 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: '<' not supported between instances of 'NoneType' and 'int'