UnboundLocalError：在 Python3 中赋值之前引用了局部变量“sumOfOdd”

Question

I'm trying to run this code:

number = input("Number: ")
valid = False
sumOfOdd = 0

def validation(credit_num):
    for i in range(len(credit_num)):
        if i % 2 != 0:
            sumOfOdd += i


def main():
    print(f"Your credit card number is {number}, it's third digit is {number[2]}")
    print(f'sum of odds: {sumOfOdd}')

validation(number)
main()

But I get this error:

Traceback (most recent call last):
  File "credit.py", line 15, in <module>
    validation(number)
  File "credit.py", line 8, in validation
    sumOfOdd += i
UnboundLocalError: local variable 'sumOfOdd' referenced before assignment

I'm able to run, but when I input any number it gives me this error

Answer 1

This error occurs because the variable sumOfOdd is not accessible from within the function. You could declare the variable global in the function , but you should be careful using the global statement.

In my opinion, a better way to do this is to supply sumOfOdd as an argument to the function and return the updated variable:

def validation(credit_num, sumOfOdd):
    for i in range(len(credit_num)):
        if i % 2 != 0:
            sumOfOdd += i
    return sumOfOdd

validation(number, 0)
# Returns the correct sum.

Or, if you know that the sumOfOdd should always be initialized by zero, you could define the variable locally:

def validation(credit_num):
    sumOfOdd = 0
    for i in range(len(credit_num)):
        if i % 2 != 0:
            sumOfOdd += i
    return sumOfOdd

Answer 2

Here's a working version of your code.

Note that it now iterates through credit_num as for num in credit_num . If you use for i in range(len(credit_num)) you are iterating through a list of indexes and would need to use if int(credit_num[i]) % 2 != 0 , reason being that range(N) returns a list [0, 1, 2... N-1] where for num in credit_num iterates through [1,1,1,1,2,2,2,2,3,3,3,3,4,4,4,4] if your input string was 1111222233334444

number = input("Number: ")
valid = False
sumOfOdd = 0

def validation(credit_num):
    global sumOfOdd
    for num in credit_num:
        if int(num) % 2 != 0:
            sumOfOdd += 1


def main():
    print(f"Your credit card number is {number}, it's third digit is {number[2]}")
    print(f'sum of odds: {sumOfOdd}')

validation(number)
main()

Note that validation(number) is called in global scope along with main() , hence the global sumOfOdd declaration inside to allow def validation to gain access to that variable.

A different way to write this code to make it more readable would be:

if __name__ == "__main__":
    number = input("Number: ")
    valid = False
    sumOfOdd = 0
    
    def validation(credit_num):
        sumOfOddToReturn = 0
        for num in credit_num:
            if int(num) % 2 != 0:
                sumOfOddToReturn += 1
        return sumOfOddToReturn
    
    sumOfOdd = validation(number)
    
    print(f"Your credit card number is `{number}`, its third digit is `{number[2]}`.")
    print(f'sum of odds: {sumOfOdd}')