Question

How one does attributes which may or may not exist? Exampls are checkbox checked=”checked” or disabled=”disabled” HTML attributes.

What kind of tal:attributes expression is involved?

Answer 1

When an attribute listed in tal:attributes is set to None, the attribute is omitted:

<span tal:attributes="title python:len(item['title']) < 10 and item['title'] or None">
   Only a title if shorter than 10 characters.
</span>

The same applies to a path expression to a non-existing object:

<span tal:attributes="title item/title" />

Now the title attribute will only be set if there is a title key or attribute on item. Note that if item/title resolves to None, the attribute is omitted as well.

From the TALES 1.4 specification, attributes section:

If the expression associated with an attribute assignment evaluates to nothing, then that attribute is deleted from the statement element.

Where python None is interpreted as “nothing”.