英文:
Python: real order of execution for equalities/inequalities in expressions?
问题
请看下面这段“狡猾”的Python代码：
>>> 1 == 2 < 3
False
根据Python文档，所有运算符in, not in, is, is not, <, <=, >, >=, !=, ==的优先级相同，但这里发生的情况似乎矛盾。
在实验后，我得到了更奇怪的结果：
>>> (1 == 2) < 3
True
>>> 1 == (2 < 3)
True
发生了什么？
（注意）
>>> True == 1
True
>>> True == 2
False
>>> False == 0
True
>>> False == -1
False
布尔类型是int的子类，True代表1，False代表0。
这可能是一个实现细节，不同版本可能会有所不同，所以我主要关心的是Python 3.10版本。
英文:
Imagine this "sneaky" python code:
>>> 1 == 2 < 3
False
According to Python documentation all of the operators in, not in, is, is not, <, <=, >, >=, !=, == have the same priority, but what happens here seems contradictory.
I get even weirder results after experimenting:
>>> (1 == 2) < 3
True
>>> 1 == (2 < 3)
True
What is going on?
(Note)
>>> True == 1
True
>>> True == 2
False
>>> False == 0
True
>>> False == -1
False
Boolean type is a subclass of int and True represents 1 and False represents 0.
This is likely an implementation detail and may differ from version to version, so I'm mostly interested in python 3.10.
答案1
得分: 1
Python允许您链接条件，它使用and来组合它们。所以
1 == 2 and 2 < 3
等同于
1 == 2 and 2 < 3
这在不等式链中非常有用，例如 1 < x < 10 将在 x 在 1 和 10 之间时为真。
当您添加括号时，它不再是一个链。所以
(1 == 2) < 3
等同于
False < 3
True 和 False 等同于 0 和 1，因此 False < 3 与 0 < 3 是相同的，也就是 True。
类似地，
1 == (2 < 3)
等同于
1 == True
这等同于
1 == 1
显然是 True。
英文:
Python allows you to chain conditions, it combines them with and. So
1 == 2 < 3
is equivalent to
1 == 2 and 2 < 3
This is most useful for chains of inequalities, e.g. 1 < x < 10 will be true if x is between 1 and 10.
WHen you add parentheses, it's not a chain any more. So
(1 == 2) < 3
is equivalent to
False < 3
True and False are equivalent to 0 and 1, so False < 3 is the same as 0 < 3, which is True.
Similarly,
1 == (2 < 3)
is equivalent to
1 == True
which is equivalent to
1 == 1
which is obviously True.
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