如何使程序循环直到满足条件？

Question

I was trying to make it so that my program will keep asking the user to input a certain value and if the user doesn't it keeps asking until they do.

I tried to use "while" instead of "if" but I know I'm probably missing something, somewhere.

def terrain(surface):
    surface = raw_input("What surface will you be driving on? ")
    if surface == "ice":
                u = raw_input("what is the velocity of the car in meters per second? ")
                u = int(u)
                if u < 0:
                    u = raw_input("Velocity must be greater than 0")
                    return
                if u == 0:
                    u = raw_input("Velocty must be a number greater than zero")
                    return
                a = raw_input("How quickly is the vehicle decelerating? ")
                a = int(a)
                if a > 0:
                    print ("Deceleration cannot be a positive integer")
                    return
                else: 
                        s1 = u**2
                        s2 = 2*.08*9.8
                    s = s1/s2
                    print "This is how far the vehicle will travel on ice: "
                    print ("The vehicle will travel %i meters before coming to a complete stop" % (s))
terrain("ice")

Answer 1

The problem is you are using return after checking the condition which causes the function to return None you have to use break instead of return with while loop instead of if to achieve this. A better way to validate and get data is below

class ValidationError(Exception):
    pass

def validate_and_get_data_count(x):
    if int(x) < 0:
        raise ValidationError("Cannot be less than 0")
    return int(x)

def validate_and_get_data_name(x):
    if len(x) < 8:
        raise ValidationError("Length of name cannot be less than 8 chars")
    elif len(x) > 10:
        raise ValidationError("Length of name cannot be greater than 10 chars")
    return x

validators = {
    "count": validate_and_get_data_count,
    "name": validate_and_get_data_name
}

data = {}

params = [
    ("count","Please enter a count: "),
    ("name","Please enter a name: "),
]

for param in params:
    while True:
        x = input(param[1])
        try:
            data[param[0]] = validators[param[0]](x)
            break
        except ValidationError as e:
            print(e)

print(data)

What the above code does is for every param in params list it runs a while loop checking for every validation condition defined in its validator if valid it breaks the while loop and proceeds to next param and repeats the same process again