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问题
正在编写一个接受用户输入的程序。
#note: Python 2.7 users should use `raw_input`, the equivalent of 3.X's `input` age = int(input("Please enter your age: ")) if age >= 18: print("You are able to vote in the United States!") else: print("You are not able to vote in the United States.")
只要用户输入有意义的数据,程序就会按预期工作。
C:\Python\Projects> canyouvote.py Please enter your age: 23 You are able to vote in the United States!
但如果用户输入无效数据,它就会失败:
C:\Python\Projects> canyouvote.py Please enter your age: dickety six Traceback (most recent call last): File "canyouvote.py", line 1, inage = int(input("Please enter your age: ")) ValueError: invalid literal for int() with base 10: 'dickety six'
而不是崩溃,我希望程序再次要求输入。像这样:
C:\Python\Projects> canyouvote.py Please enter your age: dickety six Sorry, I didn't understand that. Please enter your age: 26 You are able to vote in the United States!
当输入无意义的数据时,如何让程序要求有效输入而不是崩溃?
我如何拒绝像 那样的值-1,int在这种情况下这是一个有效但无意义的值?
解决方法
完成此操作的最简单方法是将input方法放入 while 循环中。使用continue时,你会得到错误的输入,并break退出循环。
当您的输入可能引发异常时
使用try和except检测用户何时输入无法解析的数据。
while True: try: # Note: Python 2.x users should use raw_input, the equivalent of 3.x's input age = int(input("Please enter your age: ")) except ValueError: print("Sorry, I didn't understand that.") #better try again... Return to the start of the loop continue else: #age was successfully parsed! #we're ready to exit the loop. break if age >= 18: print("You are able to vote in the United States!") else: print("You are not able to vote in the United States.")
实现你自己的验证规则
如果要拒绝 Python 可以成功解析的值,可以添加自己的验证逻辑。
while True: data = input("Please enter a loud message (must be all caps): ") if not data.isupper(): print("Sorry, your response was not loud enough.") continue else: #we're happy with the value given. #we're ready to exit the loop. break while True: data = input("Pick an answer from A to D:") if data.lower() not in ('a', 'b', 'c', 'd'): print("Not an appropriate choice.") else: Break
结合异常处理和自定义验证
以上两种技术都可以组合成一个循环。
while True: try: age = int(input("Please enter your age: ")) except ValueError: print("Sorry, I didn't understand that.") continue if age < 0: print("Sorry, your response must not be negative.") continue else: #age was successfully parsed, and we're happy with its value. #we're ready to exit the loop. break if age >= 18: print("You are able to vote in the United States!") else: print("You are not able to vote in the United States.")
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