Python 不从列表中获取值的函数

我想从列表中获取两个值，然后添加它们。我想用try和except块来做。皮查姆没有问题。但输出显示为“无”或“无”。代码如下：

def tryint():
    global mylist
    try:
        mylist = list[map(int, mylist)]
        ans = mylist[0] + mylist[1]
        return ans

    except:
        pass

a = input('First value: ')
b = input('Second value: ')
mylist = [a, b]
tryint()

---

我尝试将

tryint（）

更改为

print（tryint（））

，但它只是显示

“无”

。也没有错误消息。

我将使用您的代码作为改进代码的起点

首先是你的解决方案的问题

# What you are trying to do is supply two parameters 
# this is better accomplished by supplying the parameters  
def tryint():                            
    global mylist            
    try:
        # As mentioned list is a method call so you need to use ()
        mylist = list[map(int, mylist)]
        # Try to only put the code you suspect might cause an exception
        # in your try/except block.
        ans = mylist[0] + mylist[1]
        return ans

    # Always provide the exception you are expecting or at least `Exception`
    # If your code isn't going to handle the exception don't handle it.
    except:
        pass

a = input('First value: ')
b = input('Second value: ')
mylist = [a, b]
tryint()

接下来，让我们改进您的原始解决方案

# The function now excepts two parameters
def try_int(a, b):
    try:
        # Only type conversion is in the try block
        a = int(a)
        b = int(b)
    # Only ValueError and TypeError are handled
    except (ValueError, TypeError):
        return None

    # At this point we can assume we have integers and safely
    # do the addition.
    return a + b

a = input("First value: ")
b = input("Second value: ")
print(try_int(a, b))

现在，我们传入输入并仅解析处理期望值的输入

但我们可以通过立即向用户提供反馈做得更好

def input_int(msg):
    # Keep trying until a valid value is added
    while True:
        value = input(msg)

        # Allow the user an out be entering nothing
        if not value:
            return None

        try:
            # Return a valid value
            return int(value)
        except ValueError:
            # Provide feedback
            print("Expected an integer value, please try again")

a = input_int("First value: ")
if a is None:
    exit()

b = input_int("Second value: ")
if b is None:
    exit()

# At this point we know we have two integers
print(a + b)

---

有很多方法可以做你正在做的事情，但我将向你展示你犯错误的地方：-

def tryint():
    global mylist
    try:
        mylist = list(map(int, mylist))   # Here you have to use parrenthesis instead of square brackets.
        ans = mylist[0] + mylist[1]
        return ans

    except Exception as e:  # This is how you write exception handling.
        pass

a = input('First value: ')
b = input('Second value: ')
mylist = [a, b]
tryint()

输出

First value: 5
Second value: 4
9

---

不要链接到代码；在问题本身中至少举一个例子来说明你的问题。你在第4行使用大括号而不是圆括号

（

）

mylist=list[map（int，mylist）]

应该是

mylist=list（map（int，mylist））

在你的用例中使用

global

关键字是一种不好的做法，您应该这样做：

a=int（输入（'First value:'）

其次，在不指定异常的情况下，避免使用

except

。在本例中，您正在捕获一个

SyntaxError

，并默默地忽略它。Python中的默认返回值是

None

，因此返回的值就是这个值。您使用的是global，因此每次创建新列表时，它都会打印None，而不需要在此处处理异常。