Exists expression syntax:
exists_expressions ::= 'exists:' path_expression
Exists expressions test for the existence of paths. An exists expression returns true when the path expressions following it expression returns a value. It is false when the path expression cannot locate an object.
Testing for the existence of a form variable:
<p tal:condition="not:exists:request/form/number"> Please enter a number between 0 and 5 </p>
Note that in this case you can't use the expression,
not:request/form/number, since that expression will be true if
number variable exists and is zero.