Question

为什么在此try /上没有引发ValueError，除非isalpha失败。

我知道如果给出一个数字，isalpha会返回false

In [9]: ans = input("Enter a Letter")
Enter a Letter4

In [10]: ans.isalpha()
Out[10]: False

如果提供数字而不是y或n，我如何得到值错误？因为如果尝试是假的，它不应该停止是真的而不是打印我的轨迹吗？

import sys

v0 = float(input("What velocity would you like? "))
g = float(input("What gravity would you like? "))
t = float(input("What time decimal would you like? "))

print("""
    We have the following inputs.
       v0 is %d
       g is  %d
       t is  %d
    Is this correct? [Y/n]
""" % (v0, g, t))
while True:
    try:
        answer = input("\t >> ").isalpha()
        print(v0 * t - 0.5 * g * t ** 2)
    except ValueError as err:
        print("Not a valid entry", err.answer)
        sys.exit()
    finally:
            print("would you like another?")
            break

例如，如果用户键入5而不是y或n仍然得到答案

$ python3 ball.py 
What velocity would you like? 2
What gravity would you like? 3
What time decimal would you like? 4

    We have the following inputs.
       v0 is 2
       g is  3
       t is  4
    Is this correct? [Y/n]

     >> 5
-16.0
would you like another?

Answer 1

只有在抛出ValueError时才会发生

except ValueError as err:。 answer的值为False，但这只是一个任意的布尔值，而不是错误。

有关错误的示例，请参阅{{3}}。

在您的情况下，只需测试：

answer = input("\t >> ")
if answer.isalpha():
    print(v0 * t - 0.5 * g * t ** 2)
    break

Answer 2

通常，您应该更喜欢使用普通的控制流逻辑来处理一系列用户输入，而不是提出/捕获异常。

Answer 3

您需要自己提出错误。键入您不喜欢的内容不会引发异常：

try:
    answer = input("\t >> ").isalpha()
    if not answer:
        raise ValueError
    print(v0 * t - 0.5 * g * t ** 2)
except ValueError as err:
    print("Not a valid entry", err.answer)
    sys.exit()

Answer 4

通过对字符串和 int的的处理，提供一种更加一致和明确的方法，以便清晰地发布答案。通过使用 isinstance ，我向明确阅读我的代码的人宣告我的价值观有望提高可读性。

answer = input("\t >> ")
if isinstance(int(answer), int) is True:
    raise ValueError("Ints aren't valid input")
    sys.exit()
elif isinstance(answer, str) is True:
        print(v0 * t - 0.5 * g * t ** 2)
else:
    print("Ok please ammend your entries")

如果我以后有不同的要求，可以很容易地将其抽象为一个函数，因为isinstance允许检查多种类型增加灵活性。

参考 How to properly use python's isinstance() to check if a variable is a number?

def testSomething(arg1, **types):
    if isintance(arg1, [types]):
        do_something

Python - isalpha对int没有任何评估错误

4 个答案: