如果用户输入无效答案，如何重做输入

Question

I'm new to programming, and I was wondering how I can repeat an input section, if the user types in invalid data.

I want the application to just repeat the input section, instead of having to run the function all over again and making the user type everything all over again.

My guess is that I would have to change the "return main()" into something else.

condition = input("What is the condition of the phone(New or Used)?")
if condition not in ["New", "new", "Used", "used"]:
    print("Invalid input")
    return main()

gps = input("Does the phone have gps(Yes or No)?")
if gps not in ["Yes", "yes", "No", "no"]:
    print("Invalid input")
    return main()

Answer 1

You can make a method to check it in a loop:

def check_input(values,  message):
    while True:
        x = input(message) 
        if x in values:
            return x
        print "invalid values, options are "   + str(values) 

Answer 2

Here is a good reading about Control Flows .

Also in your case, you can use strip() and lower() for user inputs.

>>> 'HeLLo'.lower()
'hello'
>>> ' hello   '.strip()
'hello'

Here is the solution for Python 3:

while True:
    condition=input("What is the condition of the phone(New or Used)?")
    if condition.strip().lower() in ['new', 'used']:
        break
    print("Invalid input")

while True:
    gps=input("Does the phone have gps(Yes or No)?")
    if gps.strip().lower() in ['yes','no']:
        break
    print("Invalid input")

Answer 3

You can generalise the code to use a message prompt and a validating function:

def validated_input(prompt, validate):
    valid_input = False
    while not valid_input:
        value = input(prompt)
        valid_input = validate(value)
    return value

eg:

>>> def new_or_used(value):
...     return value.lower() in {"new", "used"}

>>> validate_input("New, or used?", new_or_used)

Or, simpler, but less flexible, pass in the valid values:

def validated_input(prompt, valid_values):
    valid_input = False
    while not valid_input:
        value = input(prompt)
        valid_input = value.lower() in valid_values
    return value

And use:

>>> validate_input("New, or used?", {"new", "used"})

You could even use the valid values to create the input prompt:

def validated_input(prompt, valid_values):
    valid_input = False
    while not valid_input:
        value = input(prompt + ': ' + '/'.join(valid_values))
        valid_input = value.lower() in valid_values
    return value

Which gives a prompt:

>>> validate_input("What is the condition of the phone?", {"new", "used"})
What is the condition of the phone?: new/used