I have some python type code and am wondering why the python eval method return false.

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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:

Python

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

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?

Posted 31-Dec-14 6:25am

Ramza360

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Thomas Daniels · Accepted Answer · 2014-12-31T06:46:00

Solution 1

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:

Python

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

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

False == 1

False

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:

Python

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

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

Posted 31-Dec-14 6:46am

Thomas Daniels

Comments

Ramza360 31-Dec-14 12:55pm

Ok this is just an example. However the issue is how the variables are given to me to evaluate in the above layout (e.g. (string == string + string == string) == 1). Was thinking maybe the plus meant something else in python but I guess the only way around would be to do a replace on the + as and, but that would break any expression containing the + in it.. Weird situation.

Thank you for the above explanation though, just wondering why they would think that would evaluate correctly then..

Thomas Daniels 31-Dec-14 12:59pm

I don't know who thinks it would evaluate correctly, and I also don't know why they would think that. The + doesn't mean anything really special in this case, it just concatenates the two strings.

Ramza360 31-Dec-14 13:03pm

Very interesting, and that's kind of what i expected to. Only other thing i was thinking was if they thought the cases were doing the boolean check and replaceing the bool with its integer (true:1) (false:0) and seeing if they add up to 1. Ohwell.

Thanks for the quick reply.

Sergey Alexandrovich Kryukov 31-Dec-14 13:55pm

My 5. However, everything was correct only before you mentioned "how to fix it". It cannot and should not be fixed, this is the case when the main thing should be fixed: someone's brain, and you did the best by your answer and comments.
—SA