Re: [HTCondor-users] Comparing evaluation result with "classad.Value.Error" throws error

["Bockelman, Brian" <BBockelman@xxxxxxxxxxxxx>]

Hi Ceyhun,

We really, really donât like backward compatibility breaks!  However, I hope you are forgiving as to why the original behavior was particularly bad and the new approach is worth the extra pain.

Glad you got to an end of your debugging!

Brian

Sent from my iPhone

On May 8, 2021, at 3:25 PM, Ceyhun Uzunoglu <ceyhun.uzunoglu@xxxxxxx> wrote:

ï

Hi Brian,

 

Now I understand why boolean comparison or isinstance() is not working, even though âclassad.ExprTree('1/0').eval()â returns âclassad.classad.Value.Errorâ

Hours of debugging finally come to an end ð  I tested and your suggestion works perfect.

 

Thanks a lot,

Ceyhun

 

From: HTCondor-users <htcondor-users-bounces@xxxxxxxxxxx> on behalf of "Bockelman, Brian" <BBockelman@xxxxxxxxxxxxx>
Reply to: HTCondor-Users Mail List <htcondor-users@xxxxxxxxxxx>
Date: Saturday, 8 May 2021 at 21:58
To: HTCondor-Users Mail List <htcondor-users@xxxxxxxxxxx>
Subject: Re: [HTCondor-users] Comparing evaluation result with "classad.Value.Error" throws error

 

Hi Ceyhun,

 

Indeed, this is a subtle one!  Excellent question.

 

Short answer: You want "value is classad.Value.Error".

 

Long answer:

 

The _expression_ "value == classad.Value.Error" results in a classad.ExprTree object which has the representation "error == error".  The "if" clause coerces the _expression_ to a boolean.  Thus, "error == error" is evaluated to the classad Value object "error" (as comparing anything to an error is still an error; "1 == error" evaluates to an error as well!)

 

Now, what's the boolean value of "error"?  Before the 8.9 series, boost converted error to the integer value "2" (sadly, this is the C++ enumerated value) and hence "error" returned "true" because bool(2) is True in python.  This was tricky because an _expression_ like "1 == classad.Value.Error" would also return true as well as "classad.Value.Undefined == classad.Value.Error".  Whoops!

 

In 8.9, converting an "error" object to a boolean raises an exception to avoid this mistake.

 

Instead, "value is classad.Value.Error" returns whether value and classad.Value.Error are the same object instead of trying to evaluate a ClassAd _expression_.  Hence, your code can be simplified to:

 

value = ad.eval(key)

if value is classad.Value.Error:

   ...

 

(It's unfortunate but these are the sort of tricky edge cases that appear when trying to model the subtly-different boolean logic of ClassAds in a pythonic manner.)

 

Brian

On May 8, 2021, at 2:27 PM, Ceyhun Uzunoglu <ceyhun.uzunoglu@xxxxxxx> wrote:

 

Dear htcondor community,

 

Iâm upgrading my codebase to htcondor v9.0.0 from v8.7.9.

As it is mentioned in this comment[1], comparison of âclassad.Value.Errorâ throws âclassad.ClassAdEvaluationError: Unable to evaluate _expression_.â exception.

What do you suggest to use instead of line-3. Iâm trying to check evaluation result is not a classad.Value.Error.

 

1. value = ad.eval(key)

2. if isinstance(value, classad.Value):
3.    if value == classad.Value.Error:

 

Kind regards,

Ceyhun Uzunoglu

 

[1] https://github.com/htcondor/htcondor/blob/db44d1948122d10b98bf101acfbd67c3f109d8f1/src/condor_tests/test_python_bindings_classad.py#L104

 

 

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