doc/build/inheritance.rst - platform/external/python/mako - Git at Google

 .. _inheritance_toplevel:

 ===========
 Inheritance
 ===========

 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``:

 .. sourcecode:: mako

     ## 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:

 .. sourcecode:: mako

     ## 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
   :ref:`namespace_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:

 .. sourcecode:: html

         <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).

 Using the "next" namespace to produce content wrapping
 =======================================================

 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()``:

 .. sourcecode:: mako

     ## 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``:

 .. sourcecode:: mako

     ## 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:

 .. sourcecode:: mako

     ## 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:

 .. sourcecode:: html

     <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.

 Using the "parent" namespace to augment defs
 =============================================

 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``:

 .. sourcecode:: mako

     ## 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:

 .. sourcecode:: html

     <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 !

 Inheritable Attributes
 ======================

 The ``attr`` accessor of the :class:`.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:

 .. sourcecode:: mako

     <%!
         class_ = "grey"
     %>

     <div class="${self.attr.class_}">
         ${self.body()}
     </div>

 If a an inheriting template overrides ``class_`` to be
 ``white``, as in:

 .. sourcecode:: mako

     <%!
         class_ = "white"
     %>
     <%inherit file="parent.html"/>

     This is the body

 You'll get output like:

 .. sourcecode:: html

     <div class="white">
         This is the body
     </div>
	.. _inheritance_toplevel:

	===========
	Inheritance
	===========

	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``:

	.. sourcecode:: mako

	## 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:

	.. sourcecode:: mako

	## 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
	:ref:`namespace_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:

	.. sourcecode:: html

	<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).

	Using the "next" namespace to produce content wrapping
	=======================================================

	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()``:

	.. sourcecode:: mako

	## 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``:

	.. sourcecode:: mako

	## 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:

	.. sourcecode:: mako

	## 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:

	.. sourcecode:: html

	<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.

	Using the "parent" namespace to augment defs
	=============================================

	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``:

	.. sourcecode:: mako

	## 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:

	.. sourcecode:: html

	<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 !

	Inheritable Attributes
	======================

	The ``attr`` accessor of the :class:`.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:

	.. sourcecode:: mako

	<%!
	class_ = "grey"
	%>

	<div class="${self.attr.class_}">
	${self.body()}
	</div>

	If a an inheriting template overrides ``class_`` to be
	``white``, as in:

	.. sourcecode:: mako

	<%!
	class_ = "white"
	%>
	<%inherit file="parent.html"/>

	This is the body

	You'll get output like:

	.. sourcecode:: html

	<div class="white">
	This is the body
	</div>