编程新手，不明白为什么我无法打印此变量[重复]

Question

每当我运行此代码时，它都会告诉我第 19 行 distanceTravelled 没有定义，但我认为我在函数中定义了它。感谢您的帮助。

startKilometre = float(input("What is the starting kilometre?"))
finalKilometre = float(input("What is the final kilomotre"))
finalFuel = float(input("How much fuel did you put in the car?"))

def functionTravelled(startKilometre, finalKilometre):
    distanceTravelled = finalKilometre - startKilometre
    return distanceTravelled



def functionLitresKilo(distanceTravelled, finalFuel):
    litresPerKilometre = distanceTravelled / finalFuel
    return litresPerKilometre

def functionKiloLitres(finalFuel, distanceTravelled):
    kilometresPerLitre = finalFuel / distanceTravelled
    return kilometresPerLitre

print(distanceTravelled)
print(litersPerKilometre)
print(kilometresPerLitre)

Answer 1

您永远不会调用函数来获取它们返回的值，因此，您的代码运行如下：

startKilometre = float(input("What is the starting kilometre?"))
finalKilometre = float(input("What is the final kilomotre"))
finalFuel = float(input("How much fuel did you put in the car?"))

print(distanceTravelled)
print(litersPerKilometre)
print(kilometresPerLitre)

这当然不是你想要的。您需要调用每个函数并将返回结果存储在变量中：

startKilometre = float(input("What is the starting kilometre?"))
finalKilometre = float(input("What is the final kilomotre"))
finalFuel = float(input("How much fuel did you put in the car?"))

def functionTravelled(startKilometre, finalKilometre):
    distanceTravelled = finalKilometre - startKilometre
    return distanceTravelled

def functionLitresKilo(distanceTravelled, finalFuel):
    litresPerKilometre = distanceTravelled / finalFuel
    return litresPerKilometre

def functionKiloLitres(finalFuel, distanceTravelled):
    kilometresPerLitre = finalFuel / distanceTravelled
    return kilometresPerLitre

distanceTravelled = functionTravelled(startKilometre, finalKilometre)
litersPerKilometre = functionLitresKilo(distanceTravelled, finalFuel)
kilometresPerLitre = functionKiloLitres(finalFuel, distanceTravelled)

print(distanceTravelled)
print(litersPerKilometre)
print(kilometresPerLitre)

Answer 2

您正在尝试打印一个不存在的变量。该变量在您的 functionTravelled 函数中本地定义。您想打印这些函数返回的内容（并且您从未真正调用过您的函数）。

例如-

startKilometre = float(input("What is the starting kilometre?"))
finalKilometre = float(input("What is the final kilomotre"))
finalFuel = float(input("How much fuel did you put in the car?"))

def functionTravelled(startKilometre, finalKilometre):
    distanceTravelled = finalKilometre - startKilometre
    return distanceTravelled

travelled = functionTravelled(startKilometre, finalKilometre)
print(travelled)

同样适用于您的其他打印语句/函数。

Answer 3

有点像“在拉斯维加斯发生的事情，留在拉斯维加斯”，您在函数中定义了变量，并且在函数外部不可见。这是故意这样做的，这样您就不必担心在函数中为变量使用唯一名称。

您的函数是为返回变量的值而构建的，因此只需调用它们即可。这里我通过调用函数在全局范围内分配变量。

startKilometre = float(input("What is the starting kilometre?"))
finalKilometre = float(input("What is the final kilomotre"))
finalFuel = float(input("How much fuel did you put in the car?"))

def functionTravelled(startKilometre, finalKilometre):
    distanceTravelled = finalKilometre - startKilometre
    return distanceTravelled



def functionLitresKilo(distanceTravelled, finalFuel):
    litresPerKilometre = distanceTravelled / finalFuel
    return litresPerKilometre

def functionKiloLitres(finalFuel, distanceTravelled):
    kilometresPerLitre = finalFuel / distanceTravelled
    return kilometresPerLitre

distanceTravelled = functionTravelled(startKilometre, finalKilometre)
print(distanceTravelled)

etc....

Answer 4

您似乎需要对函数的定义和调用方式进行更多研究。您认为它们是为了好玩而缩进的代码块，并且没有实际使用的理由。不是这种情况。函数被定义，然后被调用，此时它们返回一个值给调用者。以下不是函数：

a = 0
print(a)

将其放入函数中的工作方式不同，正如您正在做的这个精简版本中所看到的那样：

def x():
    a = 0
    return a
print(a)

函数必须以其实际名称调用，并且您可能希望保存返回的值：

def x():
    a = 0
    return a
var = x()
print(var)

如果您在 Google 上搜索如何在 Python 中定义和使用函数，您可以从 the official tutorial's section on defining functions 以及 Internet 上的许多其他地方了解有关函数的更多信息。