UnboundLocalError：在使用csv文件时局部变量被引用前未赋值

Question

我在做一个货币转换器，但遇到了错误，首先打印出

sorry not a valid currency

然后是

Pound Sterling


Please enter the amount of money to convert: 100
['Pound Sterling', 'Euro', 'US Dollar', 'Japanese Yen']
Please enter the current currency: 'Euro'
1.22
Please enter the currency you would like to convert to: 'Pound Sterling'
Sorry, that is not a valid currency
81.9672131148

接着出现了一个本地变量错误

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
>>>

这是我的代码，请帮帮我

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

在这种情况下，如果表达式：

if currency == newCurrency

的结果是假的，那么在下面这条不带条件的语句中：

newAmount = int(toPound)*float(newRt)

试图访问的变量 newRt 还没有被初始化，因此会出现错误。

（补充）正如马丁所建议的，你应该在下面加一个返回语句：

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

这意味着现在代码应该像这样：

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

Answer 2

newRt 只有在 exchReader 中有数据行，并且某一行的 currency == newCurrency 为 True 时才会被设置。

因为后面的代码在没有匹配的货币时是无法合理运行的，所以在这个时候就直接返回吧：

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

你在 exchReader 中没有数据行的原因是，你不能对 CSV 读取器进行 两次 循环；你最好把文件中的所有数据存储到一个字典里：

with open('exchangeRate.csv') as exRtFile:
    exchReader = csv.reader(exRtFile)
    currencies = {row[0]: float(row[1]) for row in extchReader}

crntCurrency = input("Please enter the current currency: ")
if crntCurrency not in currencies:
    print("Sorry, that is not a valid currency")
    return

newRt = currencies[newCurrency]

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