XML template XPATH conditionals always cast to strings?

[allenc]

I have a template-based service which includes this YANG snippet:

leaf receive-traps {
  type boolean;
  default false;
}

The corresponding template includes this:

<?if {/receive-traps}?>
  <some stuff here that enables trap processing />
<?end?>

When I create an instance of this service and set receive-traps to false, the above if condition apparently evaluates as true because the <some stuff here...> shows up.

Looking at the xpath.trace, I see it being evaluated as follows:

10-Jan-2019::15:54:54.574 template snmp-template: evaluating: string(boolean(/receive-traps))
10-Jan-2019::15:54:54.574 template snmp-template: result: true

/receive-traps is false, so this evaluates as string(false), which I assume is the string "false", 
which is boolean true since it is a non-empty string. Am I interpreting this correctly? Why is the condition being cast into a string? How do I make it just treat the expression as the boolean it is?

Thanks,

-Allen

[Abdel el Anazi · Accepted solution]

I guess you are only checking the existence of the receive-traps, perhaps you can change it to:

<?if {/receive-traps = 'false'}?>

[allenc]

Thanks, that works.  I think you must be right about it checking for existence.  I would point out that is not how it's supposed to work according to the NSO Developer's Guide, which says a "conditional expression will evaluate to false when...the argument evaluates to a boolean false..."

However, this might explain why the devtools xpath command has different options for `eval`, `must`, and `when`...  I'm guessing that 'if' expressions are evaluated in the same way as `xpath when` .

Thanks

[vleijon]

To answer this, we have to look in the XPath specification, https://www.w3.org/TR/1999/REC-xpath-19991116/#function-boolean note the wording *a node-set is true if and only if it is non-empty".

Now, the expression /receive-traps is a path and evaluating it returns a node-set, not a single node, so boolean on a path actually asks if the path exists or not, and doesn't care what it contains.

The reason the comparison works is in the definition of equality:

If one object to be compared is a node-set and the other is a string, then the comparison will be true if and only if there is a node in the node-set such that the result of performing the comparison on the string-value of the node and the other string is true. 

It is worth remembering that XPath often gives surprising results, precisely because things are often node-sets instead of nodes.

I hope you find my explanation helpful and not nit-picky, to me XPath evaluation in NSO is one of the trickier things to wrap your head around.

[allenc]

Thank you very much for the clarification.  It's not nit-picky at all.  In fact, that really helps to explain why it works the way it does.  XPATH has definitely been the most difficult aspect of service development to understand.

[vleijon]

I agree, I often avoid complex XPath and do things in python or java instead - though simple things like an if-statement I would probably leave in the template.

Speaking of python/java - when applying a template through the API any variables passed in as template variables from the programming language vill always be a scalar, never a node-set.