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

我目前正在上一门编程课，这位教授希望我们“逻辑地”结束所有程序，因此我们无法使用系统退出或中断。 我以前曾经成功完成过此任务，但是由于我不知道如何终止程序以致其他功能都无法运行，因此增加的功能因素使我停顿了下来。 我的程序只是吐出错误消息，但继续进行下一个计算，我需要它完全终止。

这是我的代码：

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()

先感谢您！！

Answer 1

一种解决方案是添加一个全局布尔值，以跟踪用户是否已经失败3次

def main():

    notFailedYet = True

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

在每个函数中放置一个if语句，以检查它们是否失败

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

虽然通常用break语句进行无限循环是我所看到的方式，但肯定可以使用循环来完成：

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

现在，当此函数返回时，它将以正确转换的浮点数返回，或者返回None（在超过max_retries的情况下）。

因此，在您的主程序逻辑中：

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`.

您最终会看到一个非常难看的嵌套if结构，但是它将起作用。

另一种方法可能是定义“阶段”：

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.

这将循环并进入if / elif结构的另一个分支，直到（a）您完成了所有阶段，或者（b）其中一个阶段设置err=True （因为prompt_float的返回值为None 。是支持switch语句的语言中常见的编程习惯，但Python不支持，因此我们使用if/elif

Answer 3

万一其他人以与我相同的限制来查看它，这是我编写的有效代码的一部分。 （对不起，这很难看，评论也不美观）

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)

...等等等等

我将函数嵌套在主函数的ifs中，并将转换函数内部确定的任何值返回给主函数，以评估返回的值是否大于0。如果是，则程序继续。 如果不是，则程序由于逻辑退出。 这也可以用做try和except但我走，我们只允许使用我们所学之类的目的。 所以我知道有更好或更简单的方法。