我有一些python类型的代码，我想知道为什么python eval方法返回false。 [英] I have some python type code and am wondering why the python eval method return false.

问题描述

我是python的新手以及eval如何工作，所以任何帮助都会受到赞赏。



我得到了一个具有以下值的变量表达式检查某些字符串。



这是表达式：

I'm kind of new to python and how eval works, so any help is appreciated.

I was given an expression of variables that have the following values to check certain strings.

Here is the expression:

eval(("PRES" == "PRES" + "PRES"=="PRES")==1)

现在，也许我误解了表达式是如何工作的，从语法看起来似乎是一个AND，看看两者是否都评估为真，

但我得到的是假的回报。



是否有一些特殊情况为+或== 1？



有人能说清楚表达/做什么吗？

Now, perhaps I am misunderstanding how the expression is working, buut from the syntax it seems like an AND to see if both evaluate to true,
but i get a false in return.

Is there some special case for the + or the == 1?

Can anyone clarify what the expression is saying/doing?

推荐答案

该代码甚至无效，因为 eval 只接受一个字符串;所以，如果你想让它发挥作用，你应该在表达式周围加上引号。除此之外，如果你的表达式是这样的硬编码，你不需要eval。



关于你的表达式，实际上它的计算结果为：

That code is not even valid, because eval only accepts a string; so if you would want to make it work, you should put quotes around your expression. Aside from that, if your expression is a hard-coded one like this, you don't need eval.

About your expression, actually it evaluates to this:

("PRES" == ("PRES" + "PRES") == "PRES") == 1

("PRES" == "PRESPRES" == "PRES") == 1

False == 1

False

这就是它的作用/>


如何解决？使用和而不是 + 。但是不要使用 + ，因为即使你使用括号来确保它是（PRES==PRES）+ （PRES==PRES）然后它将是2.



所以，为了使它真实，你的表达应该看起来像这样：

That's what it does.

How to fix it? Use use an and instead of a +. But don't use +, because even if you would use parentheses to make sure it would be ("PRES" == "PRES") + ("PRES" == "PRES") then it would be 2.

So, to make it true, your expression should look like this:

"PRES" == "PRES" and "PRES" == "PRES"

== 1 不是必需的;因为如果上面的表达式也是True，它仍然是True。

==1 is not necessary; because it would still be True if the above expression is also True.

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