Python：如何在用户输入3个无效值后结束程序，并防止以下功能在不使用系统退出/中断的情况下运行？

Question

i'm currently taking a programming class and the professor would like us to end all of our programs "logically" so we are not able to use system exits or breaks. I did this successfully before but the added in factor of functions has halted me as I do not know how I could terminate the program so that the other functions do not run either. My program just spits out error message but continues to the next calculation, I need it to terminate completely.

here is my code:

def main():

    #miles to kilometers input
    miles_km = float(input('Please enter how many miles you would like to convert.'))

    #function call 
    milestoKm(miles_km)

        #Fahrenheit to Celsius
    fah_cel = float(input('Please enter how many degrees Farenheit you would like to convert.'))
        #Function call
    FahToCel(fah_cel)

    gal_liters = float(input('Please enter how many gallons you would like to convert.'))
    #Function call
    GaltoLit(gal_liters)


    #Pounds to Kilograms
    pounds_kg = float(input('Please enter how many pounds you would like to convert.'))
    #Function call
    PoundsToKg(pounds_kg)


    #Inches to Centimeters
    inches_cm = float(input('Please enter how many inches you would like to convert.'))
    #function call
    InchesToCm(inches_cm)
def milestoKm(miles):

    tries = 0
    while miles <0 and tries <3:
        print('Error: enter a valid value')
        miles = float(input('Please enter how many miles you would like to convert.'))
        tries+=1
        if tries ==3 and miles <0:
            print('Error: valid value not entered.')
            return miles

    if tries <=2:
        km_miles = (miles *1.6)
        return print(miles, 'miles is equal to', format(km_miles ,',.2f'), 'kilometers.')

def FahToCel(faren_deg):
    tries = 0
    while faren_deg <1000 and tries <3:
            print('Error: enter a valid value')
            faren_deg = float(input('Please enter how many degrees Farenheit you would like to convert.'))
            tries+=1
            if tries ==3:
                 print('Error: valid value not entered.')
            return faren_deg
    if tries <=2:
        cels_deg = ((faren_deg -32) *(5/9))
        return print(faren_deg, 'degrees Farenheit is equal to', format(cels_deg, ',.2f'), 'degrees Celsius.')

def GaltoLit(galtoLiters):

    tries = 0
    while galtoLiters <0 and tries <3:
        print('Error: enter a valid value')
        galtoLiters = float(input('Please enter how many gallons you would like to convert.'))
        tries+=1
        if tries ==3:
            print('Error: valid value not entered.')
    if tries <=2:
        liters_gal = (galtoLiters * 3.9)
        return print(galtoLiters, 'gallons is equal to', format(liters_gal, ',.2f'), 'liters,')

def PoundsToKg(poundstoKg):

    tries = 0
    while poundstoKg <0 and tries <3:
        print('Error: enter a valid value')
        poundstoKg = float(input('Please enter how many pounds you would like to convert..'))
        tries+=1
        if tries ==3:
            print('Error: valid value not entered.')
    if tries <=2:
        kg_Pounds = (poundstoKg * .45)
        return print(poundstoKg, 'pounds is equal to', kg_Pounds, 'kilograms.')

def InchesToCm(inchestoCm):

    tries = 0
    while inchestoCm <0 and tries <3:
        print('Error: enter a valid value')
        inchestoCm = float(input('Please enter how many inches you would like to convert.'))
        tries+=1
        if tries ==3:
            print('Error: valid value not entered.')
    if tries <=2:
        cm_inch = (inchestoCm * 2.54)
        return print(inchestoCm, 'inches is equal to', cm_inch, 'centimeters.')




main()

thank you in advance!!

Answer 1

One solution is to add a global boolean to track whether the user has failed 3 times already or not

def main():

    notFailedYet = True

    #miles to kilometers input
    miles_km = float(input('Please enter how many miles you would like to convert.'))

Within each function place an if statement to check if they've failed or not

if notFailedYet:
    tries = 0
    while miles <0 and tries <3:
        print('Error: enter a valid value')
        miles = float(input('Please enter how many miles you would like to convert.'))
        tries+=1
        if tries ==3 and miles <0:
            notFailedYet = False #Set this to false so that the other functions don't execute
            print('Error: valid value not entered.')
            return miles

    if tries <=2:
        km_miles = (miles *1.6)
        return print(miles, 'miles is equal to', format(km_miles ,',.2f'), 'kilometers.')

Answer 2

While an infinite loop with a break statement is usually the way I've seen this done, it could certainly be done with a loop:

def prompt_float(msg, max_retries=3):
    for _ in range(max_retries):
        resp = input(msg)
        try:
            resp = float(resp)
            return resp         # Only return if float() was successful
        except ValueError:
            pass                # Do nothing

Now, when this function returns it'll either return with a properly converted float, or None (in the case max_retries was exceeded).

So in your main program logic:

miles_km = prompt_float('Please enter how many miles you would like to convert.'))

if miles_km is not None:
    milestoKm(miles_km)

    fah_cel = prompt_float('Please enter how many degrees Farenheit you would like to convert.'))
    if fah_cel is not None:
        FahToCel(fah_cel)

        # ... more code here, each time indented inside the previous `if`.

You will end up with a pretty ugly nested if structure, but it would work.

The other way might be to define "stages":

stage = 1
while (stage <= 5) and (not err):
    if stage == 1:
        miles_km = prompt_float('Please enter how many miles you would like to convert.'))
        if miles_km is not None:
            milestoKm(miles_km)
        else:
            err = True

    elif stage == 2:
        fah_cel = prompt_float('Please enter how many degrees Farenheit you would like to convert.'))
        if fah_cel is not None:
            FahToCel(fah_cel)
        else:
            err = True

    elif <...>:
        # The rest of your question/processing each in its own `elif stage ==` block.

    stage += 1  # Increment `stage` so that next iteration of the while loop
                #   will enter the next branch of the if/elif ladder.

Which would loop and enter a different branch of the if/elif structure until either (a) you've finished all the stages, or (b) one of the stages set err=True (because the return value of prompt_float was None . This is a common programming idiom in languages that support the switch statement, but Python does not so we use if/elif

Answer 3

In case anyone else is looking at this with the same restrictions I had, here's a part of the updated code that I wrote that works. (sorry it's kind of ugly and the comments are unsightly)

def main():

#miles to kilometers input
miles_km = float(input('Please enter how many miles you would like to convert.'))

#function call 
milestoKm(miles_km)
if miles_km >0:
    #Fahrenheit to Celsius
    fah_cel = float(input('Please enter how many degrees Farenheit you would like to convert.'))
    #Function call
    FahToCel(fah_cel)
    if fah_cel>0:
        gal_liters = float(input('Please enter how many gallons you would like to convert.'))
#Function call
        GaltoLit(gal_liters)

        if gal_liters>0:
#Pounds to Kilograms
            pounds_kg = float(input('Please enter how many pounds you would like to convert.'))
#Function call
            PoundsToKg(pounds_kg)

...and so on and so forth

I nested the functions in ifs in the main function and returned whatever value was determined inside of the conversion functions back to the main function to evaluate if the returned value was greater than 0. if so, the program goes on. if not, the program exits because of logic. This can also be done with try and except but for the purpose of the class i'm taking we were only allowed to use what we learned. so I know there are better or easier ways.