使用Try和Except块，但在Except部分中，它表示未定义Python

Question

我的代码将华氏温度转换为摄氏温度。就是这个。

def again(): 
    calc = raw_input("Press y to convert again. Press n to quit. ").lower() 
    if calc == "y": 
     main() 
    elif calc == "n": 
     quit() 
    else: 
     again() 

def main(): 
    fahrenheit = input("What do you want to convert from Fahrenheit to Celsius? ") 
    try: 
     celsius = (fahrenheit - 32) *5/float(9) 
     print('%0.2F degree Celsius is equal to about %0.1f degree Fahrenheit' %(celsius,fahrenheit)) 
     again() 
    except ValueError: 
     print("That is not a number") 
     check() 

如果您运行它，并键入一个字母而不是数字进行转换。它不执行Except位，但表示该字母未被定义。如果我做了 fahrenheit = input("What do you want to convert from Fahrenheit to Celsius? ")，我该如何让Try位将引号中的数字转换为不带引号的正常数字？我认为这是因为我选择了Value Error，但我不知道。任何帮助，将不胜感激！

Answer 1

您的代码时，尝试执行fahrenheit - 32，这导致该异常从字符串减去整数：

TypeError: unsupported operand type(s) for -: 'str' and 'int' 

这是不是ValueError，所以它不会被你的except条款捕获。

如果你想赶上ValueError，你需要把fahrenheit = int(fahrenheit)放在你的try:块内。

Answer 2

您使用的是Python 2.x，而不是3.x.在python 2.x中，你需要使用raw_input而不是input。因此，改变这种：

fahrenheit = input(
    "What do you want to convert from Fahrenheit to Celsius? ") 

这样：

fahrenheit = raw_input(
    "What do you want to convert from Fahrenheit to Celsius? ") 

预演input()在Python 2.7失败：

C:\Python27\python.exe test.py 
What do you want to convert from Fahrenheit to Celsius? d 
Traceback (most recent call last): 
    ... 
    "What do you want to convert from Fahrenheit to Celsius? ") 
    File "<string>", line 1, in <module> 
NameError: name 'd' is not defined 

预演input()使用Python 3.5的工作：

"C:\Program Files (x86)\Python35-32\python.exe" test.py 
What do you want to convert from Fahrenheit to Celsius? d 
Traceback (most recent call last): 
    ... 
    File "C:/Users/stephen/Documents/src/testcode/test.py", line 31, in main 
    celsius = (fahrenheit - 32) * 5/float(9) 
TypeError: unsupported operand type(s) for -: 'str' and 'int' 

预演raw_input()使用Python 2.7的工作：

C:\Python27\python.exe test.py awk_data.txt 
What do you want to convert from Fahrenheit to Celsius? d 
Traceback (most recent call last): 
... 
    File "C:/Users/stephen/Documents/src/testcode/test.py", line 31, in main 
    celsius = (fahrenheit - 32) * 5/float(9) 
TypeError: unsupported operand type(s) for -: 'str' and 'int' 

Answer 3

转换从用户输入到一个整数尝试如下：

fahrenheit = int(fahrenheit) 

如果你使用python 3.x中，这应该是你的最终代码

def again(): 
calc = input("Press y to convert again. Press n to quit. ").lower() 
if calc == "y": 
    main() 
elif calc == "n": 
    quit() 
else: 
    again() 

def main(): 
fahrenheit = input("What do you want to convert from Fahrenheit to Celsius? ") 
try: 
    fahrenheit = int(fahrenheit) 
    celsius = (fahrenheit - 32) *5/float(9) 
    print('%0.2F degree Celsius is equal to about %0.1f degree Fahrenheit' %(celsius,fahrenheit)) 
    again() 
except ValueError: 
    print("That is not a number") 
    #check() 
main() 

希望它有助于