如果+或失败，即使反向条件有效

Question

我在自学python，我试图理解下面的示例失败的原因

#!/usr/bin/python3.4
vWHEN = sys.argv[1]
print (vWHEN)
if vWHEN != "before" or vWHEN != "after":
 exit("ERROR: " + vWHEN + " is invalid. Should be: 'before' or 'after'")

结果如下：

$ ptest.py before
before
ERROR: before is invalid. Should be: 'before' or 'after'

但以下方法确实有效：

#!/usr/bin/python3.4
vWHEN = sys.argv[1]
if vWHEN == "before" or vWHEN == "after":
 print (vWHEN)
else:
 exit("ERROR: " + vWHEN + " is invalid. Should be: 'before' or 'after'")

输出：

$ ptest.py before
before

Answer 1

vWHEN != "before" or vWHEN != "after"将始终为真。任何字符串都不等于“before”或“after”（例如，“before”不等于“after”）

您应该使用和布尔运算符-仅当用户输入的“before”和“after”都不需要出错时：

if vWHEN != "before" and vWHEN != "after":
    # Here      ^