[ZPT] how do I test the existence of a variable?

[Casey Duncan]

seb bacon wrote:
> Thanks to everyone's replys, but we're still not there!
> 
> Harry's response comes closest to my requirements:

Perhaps this is indicative of a thinker than normal wall around my 
cranium, but I fail to see why this isn't what you want:

<p metal:define-macro="foo"
    tal:define="bar bar | python:<some expression>">
    ...use bar to construct wonderous things...
</p>

please enlighten...

-Casey