为什么“a==x或y或z”的计算结果总是为真？

Question

我正在编写一个安全系统，拒绝未经授权的用户访问

name = input("Hello. Please enter your name: ")
if name == "Kevin" or "Jon" or "Inbar":
    print("Access granted.")
else:
    print("Access denied.")

它按预期授予授权用户访问权限，但也允许未经授权的用户进入

Hello. Please enter your name: Bob
Access granted.

为什么会发生这种情况？我已经明确声明，只有当name等于Kevin、Jon或Inbar时才授予访问权限。我也尝试过相反的逻辑，if "Kevin" or "Jon" or "Inbar" == name，但结果是一样的

Note: this question is intended as the canonical duplicate target of this very common problem. There is another popular question How to test multiple variables against a single value? that has the same fundamental problem, but the comparison targets are reversed. This question should not be closed as a duplicate of that one as this problem is encountered by newcomers to Python who might have difficulties applying the knowledge from the reversed question to their problem.

Answer 1

在if name == "Kevin" or "Jon" or "Inbar":中有3个条件检查

• 姓名==“凯文”
• “乔恩”
• “Inbar”

这个if语句等价于

if name == "Kevin":
    print("Access granted.")
elif "Jon":
    print("Access granted.")
elif "Inbar":
    print("Access granted.")
else:
    print("Access denied.")

由于elif "Jon"始终为真，因此授予任何用户访问权限

解决方案

---

您可以使用下面的任何一种方法

快速

if name in ["Kevin", "Jon", "Inbar"]:
    print("Access granted.")
else:
    print("Access denied.")

慢

if name == "Kevin" or name == "Jon" or name == "Inbar":
    print("Access granted.")
else:
    print("Access denied.")

缓慢+不必要的代码

if name == "Kevin":
    print("Access granted.")
elif name == "Jon":
    print("Access granted.")
elif name == "Inbar":
    print("Access granted.")
else:
    print("Access denied.")