Python - UnboundLocalError：在循环中赋值之前引用的局部变量

Question

我是 Python 的新手，我正在尝试从 Zed Shaw 的书中做一个练习。我试图为我的问题找到答案并自行调试代码，但没有成功。

我收到“分配前引用的局部变量”错误，但仅在之后 循环其中一个分支。例如。当我在输入中选择非整数字符 (chosen_car = int(input("> "))) 时，该函数应该从头开始并允许我选择正确的值。但是当我之后选择正确的数字时，我得到一个错误。在 shell 中它看起来像这样：

You're about to start a race.
You should buy a car before you start.
Let's see how much you have in your pocket.
> Check your poocket (hit enter to proceed)
Oh, you have 1 thousands. Let's see what you can buy with it
> Ok
Choose your car:
    1. Race Car 4k
    2. Sedan 2k
    3. Pickup 1k
> w
Don't waste my time, choose a correct number.
Choose your car:
    1. Race Car 4k
    2. Sedan 2k
    3. Pickup 1k
> 3
Congratulations, you bought a Pickup
Traceback (most recent call last):
  File "ex36.py", line 35, in <module>
    choose_a_car()
  File "ex36.py", line 17, in choose_a_car
    if chosen_car >= 0 and chosen_car <= 3:
UnboundLocalError: local variable 'chosen_car' referenced before assignment

这是代码。我会很乐意为您提供帮助。

from random import randint
from sys import exit

def choose_a_car():

    # the list of cars [cost, name]
    cars = [[4, "Hidden Super Car"], [4, "Race Car"], [2, "Sedan"], [1, 
    "Pickup"]]

    print(f"Choose your car:\n\t1. {cars[1][1]} {cars[1][0]}k \n\t2. 
    {cars[2][1]} {cars[2][0]}k \n\t3. {cars[3][1]} {cars[3][0]}k")

    try:
        chosen_car = int(input("> "))
    except ValueError:
        print("Don't waste my time, choose a correct number.")
        choose_a_car()

    if chosen_car >= 0 and chosen_car <= 3:
        if cars[chosen_car][0] <= money:
            print(f"Congratulations, you bought a {cars[chosen_car][1]}")
        else:
            print("Unfortunately, you don't have enough money to buy this 
            car")
            choose_a_car()
    else:
        print("There is no such a car")
        choose_a_car()


print("You're about to start a race.")
print("You should buy a car before you start.")
print("Let's see how much you have in your pocket.")
input("> Check your poocket (hit enter to proceed)")
money = int(randint(1,4))
print(f"Oh, you have {money} thousands. Let's see what you can buy with it")
input("> Ok")
choose_a_car()

print("Let's start a race!")
print("1! \n2! \n3! \nSTART!")

Answer 1

try:
    chosen_car = int(input("> "))
except ValueError:
    print("Don't waste my time, choose a correct number.")
    choose_a_car()

此代码 sn-p 使用递归。因此，如果用户输入了无效的输入，程序将进入except 块并再次调用该函数。如果用户在第二次调用中输入了正确的号码，第二次调用将成功终止，您将返回到第一次函数调用。但是，在第一次调用中，chosen_car 未定义，因为输入无效。因此程序因错误而崩溃。您可以尝试使用如下标志来代替递归：

myFlag = True
while( myFlag):
    try:
        chosen_car = int(input("> "))
        myFlag = False
    except ValueError:
        print("Don't waste my time, choose a correct number.")

通过这样做，如果程序在chosen_car = int(input("> ")) 行崩溃，myFlag 不会设置为False，程序会继续从用户那里获取输入。

我还没有检查代码，但我想应该可以。

Answer 2

通过在 try 语句之前将 selected_car 设置为 None 来初始化它。您得到了异常，因为当您到达第 17 行时，该变量尚未初始化。

Answer 3

这是你的问题：

try:
    chosen_car = int(input("> "))
except ValueError:
    print("Don't waste my time, choose a correct number.")
    choose_a_car()

if chosen_car >= 0 and chosen_car <= 3:

如果这进入了例外，那么选择的汽车将不会被定义。我建议您在 try 之外定义它，或者将整个内容放在 while 循环中，而不是使用 choose_a_car() 递归。

Answer 4

你可以这样做：

from random import randint


def get_number(msg, minimum, maximum):
    while True:
        try:
            num = int(input(msg))
        except ValueError:
            print("Don't waste my time, choose a correct number.")
        else:
            if num >= minimum and num <= maximum:
                return num


def choose_a_car():

    # the list of cars [cost, name]
    cars = [[4, "Hidden Super Car"], [4, "Race Car"], [2, "Sedan"], [1, "Pickup"]]

    print(f"Choose your car:\n\t1. {cars[1][1]} {cars[1][0]}k \n\t2. {cars[2][1]} {cars[2][0]}k \n\t3. {cars[3][1]} {cars[3][0]}k")

    chosen_car = get_number('> ', minimum=1, maximum=3)

    if cars[chosen_car][0] <= money:
        print(f"Congratulations, you bought a {cars[chosen_car][1]}")
    else:
        print("Unfortunately, you don't have enough money to buy this car")


print("You're about to start a race.")
print("You should buy a car before you start.")
print("Let's see how much you have in your pocket.")
input("> Check your pocket (hit enter to proceed)")
money = int(randint(1,4))
print(f"Oh, you have {money} thousands. Let's see what you can buy with it")
input("> Ok")
choose_a_car()