TALES Exists expressions

  Syntax

    Exists expression syntax::

      exists_expressions ::= 'exists:' path_expression

  Description

    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.

  Examples

    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
    the 'number' variable exists and is zero.