Python循环无法正确遍历字典

我正在为我的个人理财项目工作，我试图让用户能够选择他们希望交易的货币。我试图进行错误处理，如果他们输入的键在字典中不存在，它会显示一条消息并关闭程序。但现在，即使我输入了正确的键，它仍然会关闭

print("Currency's supported: USD, EUR, CAN, YEN, GBP")
currencyCheck = input("Input the currency you would like to use. Ex: 'USD' or 'EUR'...etc ").upper()


#currencySYM is a dictionary of currency ticker symbols and currency symbols
currencySYM = {'USD':'$', 'EUR':'€', 'CAN':'C$','YEN':'¥','GBP':'£'}
#the for loop takes the input from Currencycheck and applies the correct symbol to the letters
for key in currencySYM:
    if currencyCheck == key:
        currencyCheck = currencySYM[key]

    elif currencyCheck != key:
        print("Make sure you type the correct three letter symbol.")
        exit()

---

如果我取出else语句，它可以工作，但不符合预期，我可以键入任何单词，它不必是键，它会将它分配给变量，甚至不检查它是否作为键存在于字典中

在检查第一个键后，您不想立即中断。一个解决办法可能是

found = False

for key in currencySYM:
    if currencyCheck == key:
        currencyCheck = currencySYM[key]
        found = True

if !found:
    print("Make sure you type the correct three letter symbol.")
    exit()

当然，更好的方法是检查键是否在字典中：

if currencyCheck in currencySYM:
    currencyCheck = currencySYM[key]
else:
    print("Make sure you type the correct three letter symbol.")
    exit()

---

您正在检查字典中的每个键是否都已输入，这是不可能的。您可以通过在

if

子句末尾添加

break

语句轻松解决此问题：

 if currencyCheck == key:
    currencyCheck = currencySYM[key]
    # found the key, so stop checking
    break

在任何情况下，此代码都不是很pythonic，您可以用整个循环替换此代码，如果dict中没有键，则将返回None：

currencyCheck = currencySYM.get(key,None)
if not currencyCheck:
    print("Make sure you type the correct three letter symbol.")
    exit()

---

如果要检查给定词典中是否存在

键

，可以在dict中执行此操作

键

currencyCheck = input("Input the currency you would like to use. Ex: 'USD' or 'EUR'...etc ").upper()
currencySYM = {'USD':'$', 'EUR':'€', 'CAN':'C$','YEN':'¥','GBP':'£'}
if currenyCheck in currencySYM:
    currencyCheck = currencySYM[key]
else:
    exit()

或者您可以使用

.get（）

字典是无序的-由于字典中的值由键索引，因此它们不按任何特定顺序保存，这与列表不同，列表中的每个项都可以根据其在列表中的位置进行定位

这应该是可行的：

currencyCheck = input("Input the currency you would like to use. Ex: 'USD' or 'EUR'...etc ").upper()

#currencySYM is a dictionary of currency ticker symbols and currency symbols
currencySYM = {'USD':'$', 'EUR':'€', 'CAN':'C$','YEN':'¥','GBP':'£'}
#the for loop takes the input from Currencycheck and applies the correct symbol to the letters
key_list = currencySYM.keys()
if currencyCheck in key_list:
    currencyCheck = currencySYM[currencyCheck]

elif currencyCheck not in key_list:
    print("Make sure you type the correct three letter symbol.")
    exit()

---

为什么会存在这个循环<代码>如果输入currencySYM:…您在第一项不匹配后退出。如果没有匹配的项，您希望退出，但这里也不需要循环。变量和函数名称应遵循带有下划线的

小写形式。另外，您使用了
exit（）
，但我认为您可能一直在寻找
break
。谢谢！，我想我应该读一下pythoniccode的指南，我想我只是假设我需要一个字典循环，谢谢@WillArmentrout您想要使用循环的解决方案吗？
currencyCheck = input("Input the currency you would like to use. Ex: 'USD' or 'EUR'...etc ").upper()

#currencySYM is a dictionary of currency ticker symbols and currency symbols
currencySYM = {'USD':'$', 'EUR':'€', 'CAN':'C$','YEN':'¥','GBP':'£'}
#the for loop takes the input from Currencycheck and applies the correct symbol to the letters
key_list = currencySYM.keys()
if currencyCheck in key_list:
    currencyCheck = currencySYM[currencyCheck]

elif currencyCheck not in key_list:
    print("Make sure you type the correct three letter symbol.")
    exit()