TALES Exists expressions


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

Note that in this case you can't use the expression, not:request/form/number, since that expression will be true if the number variable exists and is zero.