Using template inheritance, two or more templates can organize themselves into an inheritance chain, where content and functions from all involved templates can be intermixed. The general paradigm of template inheritance is this: if a template A inherits from template B, then template A agrees to send the executional control to template B at runtime (A is called the inheriting template). Template B, the inherited template, then makes decisions as to what resources from A shall be executed.
In practice, it looks like this. Heres a hypothetical inheriting template, index.html:
## index.html
<%inherit file="base.html"/>
<%def name="header()">
this is some header content
</%def>
this is the body content.
And base.html, the inherited template:
## base.html
<html>
<body>
<div class="header">
${self.header()}
</div>
${self.body()}
<div class="footer">
${self.footer()}
</div>
</body>
</html>
<%def name="footer()">
this is the footer
</%def>
Here is a breakdown of the execution:
When index.html is rendered, control immediately passes to base.html.
base.html then renders the top part of an HTML document, then calls the method header() off of a built in namespace called self (this namespace was first introduced in the Namespaces chapter in self). Since index.html is the topmost template and also defines a def called header(), its this header() def that gets executed.
Control comes back to base.html. Some more HTML is rendered.
base.html executes self.body(). The body() function on all template-based namespaces refers to the main body of the template, therefore the main body of index.html is rendered.
Control comes back to base.html. More HTML is rendered, then the self.footer() expression is invoked.
The footer def is only defined in base.html, so being the topmost definition of footer, its the one that executes. If index.html also specified footer, then its version would override that of the base.
base.html finishes up rendering its HTML and the template is complete, producing:
<html>
<body>
<div class="header">
this is some header content
</div>
this is the body content.
<div class="footer">
this is the footer
</div>
</body>
</html>
...and that is template inheritance in a nutshell. The main idea is that the methods that you call upon self always correspond to the topmost definition of that method. Very much the way self works in a Python class, even though Mako is not actually using Python class inheritance to implement this functionality. (Mako doesn't take the "inheritance" metaphor too seriously; while useful to setup some commonly recognized semantics, a textual template is not very much like an object-oriented class construct in practice).
Sometimes you have an inheritance chain that spans more than two templates. Or maybe you don't, but youd like to build your system such that extra inherited templates can be inserted in the middle of a chain where they would be smoothly integrated. If each template wants to define its layout just within its main body, you can't just call self.body() to get at the inheriting template's body, since that is only the topmost body. To get at the body of the next template, you call upon the namespace next, which is the namespace of the template immediately following the current template.
Lets change the line in base.html which calls upon self.body() to instead call upon next.body():
## base.html
<html>
<body>
<div class="header">
${self.header()}
</div>
${next.body()}
<div class="footer">
${self.footer()}
</div>
</body>
</html>
<%def name="footer()">
this is the footer
</%def>
Lets also add an intermediate template called layout.html, which inherits from base.html:
## layout.html
<%inherit file="base.html"/>
<ul>
${self.toolbar()}
</ul>
<div class="mainlayout">
${next.body()}
</div>
<%def name="toolbar()">
<li>selection 1</li>
<li>selection 2</li>
<li>selection 3</li>
</%def>
And finally change index.html to inherit from layout.html instead:
## index.html
<%inherit file="layout.html"/>
## .. rest of template
In this setup, each call to next.body() will render the body of the next template in the inheritance chain (which can be written as base.html -> layout.html -> index.html). Control is still first passed to the bottommost template base.html, and self still references the topmost definition of any particular def.
The output we get would be:
<html>
<body>
<div class="header">
this is some header content
</div>
<ul>
<li>selection 1</li>
<li>selection 2</li>
<li>selection 3</li>
</ul>
<div class="mainlayout">
this is the body content.
</div>
<div class="footer">
this is the footer
</div>
</body>
</html>
So above, we have the <html>, <body> and header/footer layout of base.html, we have the <ul> and mainlayout section of layout.html, and the main body of index.html as well as its overridden header def. The layout.html template is inserted into the middle of the chain without base.html having to change anything. Without the next namespace, only the main body of index.html could be used; there would be no way to call layout.html's body content.
Lets now look at the other inheritance-specific namespace, the opposite of next called parent. parent is the namespace of the template immediately preceding the current template. What is most useful about this namespace is the methods within it which can be accessed within overridden versions of those methods. This is not as hard as it sounds and is very much like using the super keyword in Python. Lets modify index.html to augment the list of selections provided by the toolbar function in layout.html:
## index.html
<%inherit file="layout.html"/>
<%def name="header()">
this is some header content
</%def>
<%def name="toolbar()">
## call the parent's toolbar first
${parent.toolbar()}
<li>selection 4</li>
<li>selection 5</li>
</%def>
this is the body content.
Above, we implemented a toolbar() function, which is meant to override the definition of toolbar within the inherited template layout.html. However, since we want the content from that of layout.html as well, we call it via the parent namespace whenever we want it's content, in this case before we add our own selections. So the output for the whole thing is now:
<html>
<body>
<div class="header">
this is some header content
</div>
<ul>
<li>selection 1</li>
<li>selection 2</li>
<li>selection 3</li>
<li>selection 4</li>
<li>selection 5</li>
</ul>
<div class="mainlayout">
this is the body content.
</div>
<div class="footer">
this is the footer
</div>
</body>
</html>
and you're now a template inheritance ninja !
back to section topThe attr accessor of the Namespace object allows access to module level variables declared in a template. By accessing self.attr, you can access regular attributes from the inheritance chain as declared in <%! %> sections. Such as:
<%!
class_ = "grey"
%>
<div class="${self.attr.class_}">
${self.body()}
</div>
If a an inheriting template overrides class_ to be white, as in:
<%!
class_ = "white"
%>
<%inherit file="parent.html"/>
This is the body
You'll get output like:
<div class="white">
This is the body
</div>