Slotting Into Place - Zope

In this concluding article, get up close and personal with METAL, the macro language that allows you to add reusability to your Zope templates.

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By: Harish Kamath, (c) Melonfire

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October 31, 2002

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METAL also introduces the concept of "slots" within a macro, which may in turn be populated by other macros. Slots are placeholders within a macro definition, which can be populated dynamically every time you use the macro.

Let's take a simple example to demonstrate how this works. Here's the macro definition, which contains a placeholder, or slot, named "link":

<p metal:define-macro="spotlight">
Today's special is <span metal:define-slot="title"></span>
</p>

And here's the template which uses it - note how the placeholder has been filled with an actual value using the "fill-slot" attribute:

<p metal:use-macro="container/macroDef/macros/spotlight">
Today's special 
is <span metal:fill-slot="title"><a
href="http://www.melonfire.com/community/columns/trog/">the 
DTML Basics
series</a></span> </p>

Here's what the output looks like:

By using a slot within a macro, I have the option of reusing the macro multiple times, and replacing the slot with different data each time I use it. This adds a tremendous amount of reusability to the macro, and can thereby substantially simplify the task of developing almost-but-not-quite-identical programming routines.

Here's another, more realistic example - a macro which sets up the basic skeleton for a Web page, and uses slots to populate each instance of that page with different content:

<span metal:define-macro="body">    
<table width="100%" cellspacing="10" 
cellpadding="10">
<tr>
<td colspan="2">Logo</td>
</tr>
<tr>
<td><span 
metal:define-slot="menu">Menu</span></td>
<td><span metal:define-slot="content">Content</span></td>
</tr>
<tr>
<td 
colspan="2"><hr>All rights reserved.</td>
</tr>
</table>
</span>

Here's a sample Web page that uses the macro, and defines a particular look for the main body:

<html>
<body>
<span metal:use-macro="container/macroDefs/macros/body">
<span 
metal:fill-slot="menu">
Select from the list below:
<br>
Item A
<br>
Item 
B
<br>
Item C
<br>
</span>
<span metal:fill-slot="content">
<center>
Content 
goes here <p>Content goes here <p>Content goes here <p>Content
goes here 
<p> </center> </span> </span>
</body>
</html>

Here's the output:

And here's another one that uses the same macro, but a different look for the body:

<html>
<body>
<span metal:use-macro="container/macroDefs/macros/body"> 
<span metal:fill-slot="menu">
&nbsp;
</span>
<span metal:fill-slot="content">
<table 
width=100% cellspacing="5" cellpadding="5">
<tr><td>Item A</td><td>Item 
B</td><td>Item C</td></tr>
<tr><td colspan="3"><hr></td></tr>
<tr><td 
colspan="3" align="center">Content goes here<p>Content goes
here</p><p>Content 
goes here</p><p>Content goes here</p><p>Content goes
here</p><p></p></td></tr> 
<table> </table></table></span> </span>
</body>
</html>

Here's the output:

As you can see, the same macro's been used in both examples; however, merely by changing the contents of the slots within the macro, I can generate a completely different look for each Web page.

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