Re: [HTCondor-users] Python bindings: ExprTree vs classad.eval

[Thomas Hartmann <thomas.hartmann@xxxxxxx>]

Hi Brian,

On 05/02/2021 16.56, Bockelman, Brian wrote:
> > I had assumed, that calling the ExprTree() function on an ad would be the same as the eval() function of the ad itself - but apparently not?
>
> classad.ExprTree() is a constructor for the ExprTree object - it results in an expression object.
>
> .eval() is a method on the ClassAd object that evaluates the expression returns a "plain python object" (to the best of its ability).
>
> You'll likely will use classad.ExprTree in very few places (it's useful for manipulating expressions or handling things that cannot be represented in python - rare use cases for scripting condor); I suspect you are really wanting to use eval().

ah, good to know

I found ExprTree() in the documentation and wanted to try it out ;)

> I'm not exactly following.  Each job is represented by a single ClassAd.  Do you mean the job attributes?
>
> When you list the attributes you want, HTCondor will sometimes return additional attributes (e.g., they may be necessary to evaluate or there's a list of attributes you always need).  If you specify nothing at all, the default is "return all attributes"; be careful there as it may cause more load than you intended.

thing is, that I can query for a limited set of ads and it seems, that I 
can still successfully evaluate an ad from the list -  where the ad is a 
compound of ads, that have not been necessarily queried explicitly and 
thus are not in the query result

e.g.,

  >>> myjob = 
schedd.query('',["TotalCpus","GlobalJobId","Owner","RequestCpus"])

  >>> print(myjob.get("RequestCpus") )
  >>> ifThenElse(WantWholeNode is true, !isUndefined(TotalCpus) ? 
TotalCpus : JobCpus,OriginalCpus)

  >>> myjob.eval("RequestCpus")
  >>> 8L

  >>> if "OriginalCpus" in list(myjob.keys() ): print("OriginalCpus is 
in the ads..."
  >>> OriginalCpus is in the ads...

  >>> print(myjob.get("OriginalCpus") )
  8

where I have not been querying for 'OriginalCpus' explicitly - but it 
appears in the result ad list nevertheless
(which is good, else I would have forgotten it ;) )

Cheers,
  Thomas

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