UnboundLocalError：在使用csv-fi赋值之前引用了局部变量

Question

做一个货币兑换器，但我有一个错误，首先打印

sorry not a valid currency

之后

^{pr2}$

然后是局部变量错误

Traceback (most recent call last):
  File "<pyshell#7>", line 1, in ?
    converter()
  File "//server01/ICT2124/Task 1/Currency Converter 2.py", line 19, in converter
    exchange()
  File "//server01/ICT2124/Task 1/Currency Converter 2.py", line 65, in exchange
    newAmount = int(toPound)*float(newRt)
UnboundLocalError: local variable 'newRt' referenced before assignment
>>>

这是密码，请帮忙

def exchange():
    crntAmnt = int(input("Please enter the amount of money to convert: "))
    print(currencies)
    exRtFile = open ('exchangeRate.csv')
    exchReader = csv.reader(exRtFile)
    crntCurrency = input("Please enter the current currency: ")
    for row in exchReader:
            currency = row[0]
            if currency == crntCurrency:
                crntRt = row[1]
                print(crntRt)
                break
    else:
        print("Sorry, that is not a valid currency")


    newCurrency = input("Please enter the currency you would like to convert to: ")
    for row in exchReader:
            currency = row[0]
            if currency == newCurrency:
                newRt = row[1]
                print(newRt)
                break
    else:
        print("Sorry, that is not a valid currency")


    toPound = crntAmnt/float(crntRt)
    print(toPound)
    newAmount = int(toPound)*float(newRt)
    print("You have: " ,newAmount, newCurrency,)
    return

Answer 1

如果在exchReader中有行，newRt是设置的，而对于一行，if currency == newCurrency是{}。在

由于在没有匹配货币的情况下无法正常运行其余代码，因此只需在该点返回：

else:
    print("Sorry, that is not a valid currency")
    return

在exchReader中没有行的原因是您不能在CSV读取器上循环两次；最好将文件中的所有数据存储在字典中：

^{pr2}$

Answer 2

如果表达式：

if currency == newCurrency

将试图在中访问的变量newRt求值为false 无条件（即每次都执行）语句：

^{pr2}$

未初始化，因此 将引发错误

（编辑）正如Martin建议的那样，您应该在下面添加一个return语句：

print("Sorry, that is not a valid currency")

这意味着现在它应该是这样的：

print("Sorry, that is not a valid currency")
return