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>