TALES Not expressions

Syntax

Not expression syntax:

      not_expression ::= 'not:' expression

Description

Not expression evaluate the expression string (recursively) as a full expression, and returns the boolean negation of its value. If the expression supplied does not evaluate to a boolean value, not will issue a warning and coerce the expression's value into a boolean type based on the following rules:

1. the number 0 is false
2. numbers > 0 are true
3. an empty string or other sequence is false
4. a non-empty string or other sequence is true
5. a non-value (e.g. void, None, Nil, NULL, etc) is false
6. all other values are implementation-dependent.

If no expression string is supplied, an error should be generated.

Zope considers all objects not specifically listed above as false (including negative numbers) to be true.

Examples

Testing a sequence:

      <p tal:condition="not:here/objectIds">
        There are no contained objects.
      </p>