python - "a is a is a"比较结果

Question

这个问题已经有答案了:

Chaining "is" operators (4 个回答)

Why does `False is False is False` evaluate to `True`? (5 个回答)

已关闭 5 年前。

A = 314

if A == A == A:
    print('True #1')

if A == A == 271:
    print('True #2')

lie = 0
if lie is lie is lie:
    print('no matter how white, how small,')
    print('how incorporating of a smidgeon')
    print('of truth there be in it.')

结果:

True #1
no matter how white, how small,
how incorporating of a smidgeon
of truth there be in it.

我知道在 if 语句中使用两个“=”和“is”是不正常的。但我想知道Python解释器如何解释if声明。

表达式是 lie is lie is lie同时解释，还是短路方式？

Answer 1

您遇到的情况称为“运算符链”。

来自 Comparisons 的文档:

Comparisons can be chained arbitrarily, e.g., x < y <= z is equivalent to x < y and y <= z, except that y is evaluated only once (but in both cases z is not evaluated at all when x < y is found to be false).

强调我的。

所以，这意味着 lie is lie is lie被解释为(lie is lie) and (lie is lie) ，仅此而已。

更一般地说，a op b op c op d ...评估结果与 a op b and b op c and c op d ... 相同等等。根据python的grammar rules解析表达式。特别是；

comparison    ::=  or_expr ( comp_operator or_expr )*
comp_operator ::=  "<" | ">" | "==" | ">=" | "<=" | "!="
                   | "is" ["not"] | ["not"] "in"