[Zope] Howto test if macro evaluates to nothing

[Paul Winkler]

On Wed, Mar 10, 2004 at 09:54:07PM +0100, Dieter Maurer wrote:
> Florian Schulze wrote at 2004-3-9 11:44 +0100:
> >I would like to know wether there is a way to test if a macro evaluates to 
> >nothing.
> >
> >I tried the following:
> ><div tal:condition="here/my_macros/macros/macro_name"
> >      metal:use-macro="here/my_macros/macros/macro_name">
> ></div>
> >
> >But the macro returns a tree of lists and tuples.
> 
> Sure, this is the macro code. It is unlikely to be empty...
> 
> When you want to check, whether the result of macro rendering
> is empty, then you must render the macro and look at the result.
> In the trivial case (the macro is independent of arguments defined
> in the caller),
> you can wrap your macro use in a PageTemplate, call this template
> and check its result. When your macro needs arguments, you
> must pass them to the template.

But the macro is unlikely to produce literally nothing...
usually there will be at least a bunch of whitespace 
and/or some empty tags.  At the least you will have to 
write the macro so that all its tags are replaced with a blank
value when appropriate, call the macro in a wrapper template as 
Dieter suggests, and then in the final template that calls the wrapper,
strip() the result.

I have to wonder if there's a cleaner design that will get the
end result you want. Remember that macros are evaluated
before TAL.  I think you could set a global variable in the macro
and check it in the calling template... but I haven't tried.
Or maybe the variables needed by the macro could be retrieved
by the template that uses the macro, e.g. from a python script.

-- 

Paul Winkler
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