The tal:condition attribute

The tal:condition attribute is used to conditionally render an element and its descendents. The attribute value is a TALES expression and if the expression result is 'truthy' then the element and its content/descendents are rendered. If the expression result is not 'truthy' then the element, its content & descendents are removed from the DOM and not rendered or further processed by ZPT.

Aborting a condition attribute

If the result of an expression in a tal:condition attribute value is an instance of AbortZptActionToken, such as via the root context default, then a tal:condition attribute will behave as if it were not present. In other words, the element and content/descendents will be rendered.

Further processing is halted if the expression is not 'truthy'

If the condition expression is not 'truthy' and the element & contents/descendents is to be removed then additionally, further processing of TAL attributes upon that element is also halted. Usually this is not an issue, because the element will be removed anyway, so further manipulation is irrelevant.

Be aware that global variable definitions present upon the same element will also not be processed in this case though; this could feasibly have an impact upon other parts of the document. It is probably best not to place global definitions upon elements that have condition attributes, so as to avoid this potentially-confusing scenario.

Example

In this example, the first <p> element will be rendered but the second will not. This is because a non-empty-string is truthy but a null reference is not.

<div tal:define="isTruthy string:I am truthy;isFalsey nothing">
    <p tal:condition="isTruthy">This text will be displayed.</p>
    <p tal:condition="isFalsey">This text <em>will not</em> be displayed.</p>
</div>