函數break和pass似乎在正確的輸入下不會起作用，程序會停止，而在沒有正確輸入的情況下，程序決定不中斷而是打印字符串，告訴用戶有問題并繼續運行。currency = str(raw_input ("""what currency would you like to covert: GBP, EURO, USD OR YEN?"""))exchange = str(raw_input("""what currency would you like in exchange? : GBP, EURO, USD OR YEN?                              """))amount = int(input("""how much would you like to convert?                      """))valid_input = ('EUR','eur','GBP','gbp' ,'USD','usd','JPY','jpy')while True:    if currency in valid_input and exchange in valid_input:        pass    else:        print("incorrect input, please restart program")        breakdecision = str(raw_input("""Please enter u for user input exchange rate or s for the preset exchange rate    """))if decision == "u" :    user_rate = raw_input("Please enter the current exchange rate")    exchange_value = int(amount) *  int(user_rate)    print ("At the user found exchange rate you will receive",exchange_value,exchange)    elif decision == "s" :    if currency  == "GBP" and exchange == "USD":        exchange_value= int(amount) * 1.6048        print ("At the preset exchange rate you will receive",exchange_value,exchange)    if currency  == "GBP" and exchange == "EUR":        exchange_value= int(amount) * 1.2399        print ("At the preset exchange rate you will receive",exchange_value,exchange)