[Zope] dtml-var weirdness

[Joseph A Knapka]

"Thomas B. Passin" wrote:
> 
> [Joseph A Knapka]
> >
> > Since I'm checking to see whether REQUEST has a
> > key 'xyz', and setting it if not, then by the time we get
> > to the <dtml-var xyz> line, REQUEST definitely *does* have
> > an 'xyz' key; and since REQUEST is undoubtedly in the
> > namespace (right?), how can <dtml-var xyz> possibly result
> > in a KeyError?
> >
> 
> Sounds mysterious, all right.  Let's get clear on a few things, then maybe
> Dieter or someone else will know the answer.
> 
> 1) Is it the case that nothing else every happens on your page between the
> setting of the xyz property in the REQUEST and the <dtml-var xyz> rendering?

Nothing else happens within the document - I've put the
<dtml-var xyz> immediately after the if/then/else block
to be sure. However, the document containing the problematic
code is being invoked by another. I suspect that may be an
important fact, which I apologize for omitting from my
original post.

> 2) When you say that you "sometimes" get an error, can you tell what has
> preceded  it when you do and do not get an error?

That was a bit disingenuous of me :-) By "sometimes" I mean that
in a particular bit of production code the problem occurs, but
I haven't been able to reproduce the problem in a simple,
postable-to-the-list case. I constructed a test case that
is identical in form AFAICT: a document D1 is invoked as a
request handler, and that document invokes a second document
D2 that contains the code in question. That test case works
fine.

> 3)  Add a line after you assign the variable,  like this:
> 
> <dtml-if expr="REQUEST.has_key('xyz')">
>     The property already exists, and its value is ==<dtml-var
> "REQUEST.xyz">==<br>
> <dtml-else>
>    <dtml-call expr="REQUEST.set('xyz','Hello world')">
>    We just set the property<br>
>  </dtml-if>
> 
> This will let you know if there is a property with that name whose value is
> empty, or whether it is in fact getting set.  The "== ==" characters are
> ther to let you see if there is an empty string.  It may seem obvious that
> it has to get set, but best to cover all possiblitites when strange things
> happen.
> 
> You could also add <dtml-var REQUEST> before and after to see whether the
> xyz property did get into the REQUEST after all.
> 
> Let us know the results.

Will do.

Of course, as you've pointed out, it's not hard to work
around the problem, and I've already done so, but I just
want to understand what's really going on here. Software
behaving in apparently mysterious ways does not inspire
confidence :-)

Thanks very much,

-- Joe
"I should like to close this book by sticking out any part of my neck
 which is not yet exposed, and making a few predictions about how the
 problem of quantum gravity will in the end be solved."
 --- Physicist Lee Smolin, "Three Roads to Quantum Gravity"