The Macro Expansion Template Attribute Language (METAL) standard is a facility for HTML/XML macro preprocessing. It can be used in conjunction with or independently of TAL and TALES.
Macros provide a way to define a chunk of presentation in one template, and share it in others, so that changes to the macro are immediately reflected in all of the places that share it. Additionally, macros are always fully expanded, even in a template’s source text, so that the template appears very similar to its final rendering.
A single Page Template can accomodate multiple macros.
The METAL namespace URI and recommended alias are currently defined as:
xmlns:metal="http://xml.zope.org/namespaces/metal"
Just like the TAL namespace URI, this URI is not attached to a web page; it’s just a unique identifier. This identifier must be used in all templates which use METAL.
METAL defines a number of statements:
Although METAL does not define the syntax of expression non-terminals, leaving that up to the implementation, a canonical expression syntax for use in METAL arguments is described in TALES Specification.
The metal:define-macro statement defines a macro. The macro is named by the statement expression, and is defined as the element and its sub-tree.
Simple macro definition:
<p metal:define-macro="copyright">
Copyright 2004, <em>Foobar</em> Inc.
</p>
The metal:define-slot statement defines a macro customization point or slot. When a macro is used, its slots can be replaced, in order to customize the macro. Slot definitions provide default content for the slot. You will get the default slot contents if you decide not to customize the macro when using it.
The metal:define-slot statement must be used inside a metal:define-macro statement.
Slot names must be unique within a macro.
Simple macro with slot:
<p metal:define-macro="hello">
Hello <b metal:define-slot="name">World</b>
</p>
This example defines a macro with one slot named name. When you use this macro you can customize the b element by filling the name slot.
The metal:fill-slot statement customizes a macro by replacing a slot in the macro with the statement element (and its content).
The metal:fill-slot statement must be used inside a metal:use-macro statement.
Slot names must be unique within a macro.
If the named slot does not exist within the macro, the slot contents will be silently dropped.
Given this macro:
<p metal:define-macro="hello">
Hello <b metal:define-slot="name">World</b>
</p>
You can fill the name slot like so:
<p metal:use-macro="container['master.html'].macros.hello">
Hello <b metal:fill-slot="name">Kevin Bacon</b>
</p>
The metal:use-macro statement replaces the statement element with a macro. The statement expression describes a macro definition.
Note
In Chameleon the expression may point to a template instance; in this case it will be rendered in its entirety.
In zc.pt the expression will generally be a expression referring to a macro defined in another template which is passed in to the rendering template. See metal:define-macro for more information.
The effect of expanding a macro is to graft a subtree from another document (or from elsewhere in the current document) in place of the statement element, replacing the existing sub-tree. Parts of the original subtree may remain, grafted onto the new subtree, if the macro has slots. See metal:define-slot for more information. If the macro body uses any macros, they are expanded first.
When a macro is expanded, its metal:define-macro attribute is replaced with the metal:use-macro attribute from the statement element. This makes the root of the expanded macro a valid use-macro statement element.
Basic macro usage:
<p metal:use-macro="other.macros.header"> header macro from
defined in other.html template </p>
This example refers to the header macro defined in the other template which has been passed as a keyword argument to zc.pt‘s render method. When the macro is expanded, the p element and its contents will be replaced by the macro.
Note
there will still be a metal:use-macro attribute on the replacement element.