要求用户提供输入，直到他们给出有效的答复 [英] Asking the user for input until they give a valid response
问题描述
我正在编写一个接受用户输入的程序.
I am writing a program that accepts an input from the user.
#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，该程序就会按预期工作.
The program works as expected as long as the the user enters meaningful data.
C:\Python\Projects> canyouvote.py
Please enter your age: 23
You are able to vote in the United States!
但是如果用户输入无效数据，它将失败:
But it fails if the user enters invalid data:
C:\Python\Projects> canyouvote.py
Please enter your age: dickety six
Traceback (most recent call last):
File "canyouvote.py", line 1, in <module>
age = int(input("Please enter your age: "))
ValueError: invalid literal for int() with base 10: 'dickety six'
我希望程序再次崩溃，而不是崩溃.像这样:
Instead of crashing, I would like the program to ask for the input again. Like this:
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!
我如何使程序要求输入有效信息，而不是在输入无意义的数据时崩溃?
How can I make the program ask for valid inputs instead of crashing when non-sensical data is entered?
如何拒绝像-1这样的值，它是有效的int，但在这种情况下是荒谬的?
How can I reject values like -1, which is a valid int, but nonsensical in this context?
推荐答案
最简单的方法是将input方法放入while循环中.使用 break(当您感到满意时)跳出循环.
The simplest way to accomplish this would be to put the input method in a while loop. Use continue when you get bad input, and break out of the loop when you're satisfied.
使用 try和except 进行检测当用户输入无法解析的数据时.
Use try and except to detect when the user enters data that can't be parsed.
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.")
实施您自己的验证规则
如果要拒绝Python可以成功解析的值，则可以添加自己的验证逻辑.
Implementing Your Own Validation Rules
If you want to reject values that Python can successfully parse, you can add your own validation logic.
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.
Combining Exception Handling and Custom Validation
Both of the above techniques can be combined into one loop.
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.")
将其全部封装在功能中
如果您需要询问用户许多不同的值，那么将此代码放入函数中可能会很有用，因此您不必每次都重新输入.
Encapsulating it All in a Function
If you need to ask your user for a lot of different values, it might be useful to put this code in a function, so you don't have to retype it every time.
def get_non_negative_int(prompt):
while True:
try:
value = int(input(prompt))
except ValueError:
print("Sorry, I didn't understand that.")
continue
if value < 0:
print("Sorry, your response must not be negative.")
continue
else:
break
return value
age = get_non_negative_int("Please enter your age: ")
kids = get_non_negative_int("Please enter the number of children you have: ")
salary = get_non_negative_int("Please enter your yearly earnings, in dollars: ")
将它们放在一起
您可以将这个想法扩展为一个非常通用的输入函数:
Putting It All Together
You can extend this idea to make a very generic input function:
def sanitised_input(prompt, type_=None, min_=None, max_=None, range_=None):
if min_ is not None and max_ is not None and max_ < min_:
raise ValueError("min_ must be less than or equal to max_.")
while True:
ui = input(prompt)
if type_ is not None:
try:
ui = type_(ui)
except ValueError:
print("Input type must be {0}.".format(type_.__name__))
continue
if max_ is not None and ui > max_:
print("Input must be less than or equal to {0}.".format(max_))
elif min_ is not None and ui < min_:
print("Input must be greater than or equal to {0}.".format(min_))
elif range_ is not None and ui not in range_:
if isinstance(range_, range):
template = "Input must be between {0.start} and {0.stop}."
print(template.format(range_))
else:
template = "Input must be {0}."
if len(range_) == 1:
print(template.format(*range_))
else:
print(template.format(" or ".join((", ".join(map(str,
range_[:-1])),
str(range_[-1])))))
else:
return ui
用法如下:
age = sanitised_input("Enter your age: ", int, 1, 101)
answer = sanitised_input("Enter your answer: ", str.lower, range_=('a', 'b', 'c', 'd'))
常见陷阱，以及为什么要避免它们
冗余使用input语句
此方法有效，但通常被认为是较差的样式:
Common Pitfalls, and Why you Should Avoid Them
The Redundant Use of Redundant input Statements
This method works but is generally considered poor style:
data = input("Please enter a loud message (must be all caps): ")
while not data.isupper():
print("Sorry, your response was not loud enough.")
data = input("Please enter a loud message (must be all caps): ")
它最初看起来很有吸引力，因为它比while True方法短，但它违反了 Don重复自己的软件开发原理.这增加了系统中错误的可能性.如果您想通过将input更改为raw_input来反向移植到2.7，而又意外地仅更改了上面的第一个input怎么办?这是一个SyntaxError，等待发生.
It might look attractive initially because it's shorter than the while True method, but it violates the Don't Repeat Yourself principle of software development. This increases the likelihood of bugs in your system. What if you want to backport to 2.7 by changing input to raw_input, but accidentally change only the first input above? It's a SyntaxError just waiting to happen.
如果您刚刚了解了递归，则可能会想在get_non_negative_int中使用它，以便可以处理while循环.
If you've just learned about recursion, you might be tempted to use it in get_non_negative_int so you can dispose of the while loop.
def get_non_negative_int(prompt):
try:
value = int(input(prompt))
except ValueError:
print("Sorry, I didn't understand that.")
return get_non_negative_int(prompt)
if value < 0:
print("Sorry, your response must not be negative.")
return get_non_negative_int(prompt)
else:
return value
在大多数情况下，这似乎可以正常工作，但是如果用户输入无效数据的次数足够多，该脚本将以RuntimeError: maximum recursion depth exceeded终止.您可能会认为没有傻瓜会连续犯1000个错误"，但是您却低估了傻瓜的聪明才智！
This appears to work fine most of the time, but if the user enters invalid data enough times, the script will terminate with a RuntimeError: maximum recursion depth exceeded. You may think "no fool would make 1000 mistakes in a row", but you're underestimating the ingenuity of fools!
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