^^&&^^

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 » 
 Inheritance

<h2>
 
 Inheritance
 
 </h2>
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<h3><a href="index.html">Table of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">Inheritance</a><ul>
<li><a class="reference internal" href="#nesting-blocks">Nesting Blocks</a></li>
<li><a class="reference internal" href="#rendering-a-named-block-multiple-times">Rendering a Named Block Multiple Times</a></li>
<li><a class="reference internal" href="#but-what-about-defs">But what about Defs?</a></li>
<li><a class="reference internal" href="#using-the-next-namespace-to-produce-content-wrapping">Using the <code class="docutils literal notranslate">next</code> Namespace to Produce Content Wrapping</a></li>
<li><a class="reference internal" href="#using-the-parent-namespace-to-augment-defs">Using the <code class="docutils literal notranslate">parent</code> Namespace to Augment Defs</a></li>
<li><a class="reference internal" href="#using-include-with-template-inheritance">Using <code class="docutils literal notranslate">&lt;%include&gt;</code> with Template Inheritance</a></li>
<li><a class="reference internal" href="#inheritable-attributes">Inheritable Attributes</a></li>
</ul>
</li>
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<div class="section" id="inheritance">
<h1>Inheritance<a class="headerlink" href="#inheritance" title="Permalink to this headline">¶</a></h1>
<div class="admonition note">
Note
Most of the inheritance examples here take advantage of a feature that’s
new in Mako as of version 0.4.1 called the “block”. This tag is very similar to
the “def” tag but is more streamlined for usage with inheritance. Note that
all of the examples here which use blocks can also use defs instead. Contrasting
usages will be illustrated.
</div>
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
<code class="docutils literal notranslate">A</code> inherits from template <code class="docutils literal notranslate">B</code>, then template <code class="docutils literal notranslate">A</code> agrees
to send the executional control to template <code class="docutils literal notranslate">B</code> at runtime
(<code class="docutils literal notranslate">A</code> is called the inheriting template). Template <code class="docutils literal notranslate">B</code>,
the inherited template, then makes decisions as to what
resources from <code class="docutils literal notranslate">A</code> shall be executed.
In practice, it looks like this. Here’s a hypothetical inheriting
template, <code class="docutils literal notranslate">index.html</code>:
<div class="highlight-mako notranslate"><div class="highlight"><pre>## index.html
&lt;%inherit file=&quot;base.html&quot;/&gt;

this is the body content.</pre></div>
</div>
And <code class="docutils literal notranslate">base.html</code>, the inherited template:
<div class="highlight-mako notranslate"><div class="highlight"><pre>## base.html
&lt;html&gt;
 &lt;body&gt;
 &lt;div class=&quot;header&quot;&gt;
 &lt;%block name=&quot;header&quot;/&gt;
 &lt;/div&gt;

${self.body()}

&lt;div class=&quot;footer&quot;&gt;
 &lt;%block name=&quot;footer&quot;&gt;
 this is the footer
 &lt;/%block&gt;
 &lt;/div&gt;
 &lt;/body&gt;
&lt;/html&gt;</pre></div>
</div>
Here is a breakdown of the execution:
<ol class="arabic">
<li>When <code class="docutils literal notranslate">index.html</code> is rendered, control immediately passes to
<code class="docutils literal notranslate">base.html</code>.</li>
<li><code class="docutils literal notranslate">base.html</code> then renders the top part of an HTML document,
then invokes the <code class="docutils literal notranslate">&lt;%block name=&quot;header&quot;&gt;</code> block. It invokes the
underlying <code class="docutils literal notranslate">header()</code> function off of a built-in namespace
called <code class="docutils literal notranslate">self</code> (this namespace was first introduced in the
<a class="reference internal" href="namespaces.html">Namespaces chapter</a> in <a class="reference internal" href="namespaces.html#namespace-self">self</a>). Since
<code class="docutils literal notranslate">index.html</code> is the topmost template and also defines a block
called <code class="docutils literal notranslate">header</code>, it’s this <code class="docutils literal notranslate">header</code> block that ultimately gets
executed – instead of the one that’s present in <code class="docutils literal notranslate">base.html</code>.</li>
<li>Control comes back to <code class="docutils literal notranslate">base.html</code>. Some more HTML is
rendered.</li>
<li><code class="docutils literal notranslate">base.html</code> executes <code class="docutils literal notranslate">self.body()</code>. The <code class="docutils literal notranslate">body()</code>
function on all template-based namespaces refers to the main
body of the template, therefore the main body of
<code class="docutils literal notranslate">index.html</code> is rendered.</li>
<li>When <code class="docutils literal notranslate">&lt;%block name=&quot;header&quot;&gt;</code> is encountered in <code class="docutils literal notranslate">index.html</code>
during the <code class="docutils literal notranslate">self.body()</code> call, a conditional is checked – does the
current inherited template, i.e. <code class="docutils literal notranslate">base.html</code>, also define this block? If yes,
the <code class="docutils literal notranslate">&lt;%block&gt;</code> is not executed here – the inheritance
mechanism knows that the parent template is responsible for rendering
this block (and in fact it already has). In other words a block
only renders in its basemost scope.</li>
<li>Control comes back to <code class="docutils literal notranslate">base.html</code>. More HTML is rendered,
then the <code class="docutils literal notranslate">&lt;%block name=&quot;footer&quot;&gt;</code> expression is invoked.</li>
<li>The <code class="docutils literal notranslate">footer</code> block is only defined in <code class="docutils literal notranslate">base.html</code>, so being
the topmost definition of <code class="docutils literal notranslate">footer</code>, it’s the one that
executes. If <code class="docutils literal notranslate">index.html</code> also specified <code class="docutils literal notranslate">footer</code>, then
its version would override that of the base.</li>
<li><code class="docutils literal notranslate">base.html</code> finishes up rendering its HTML and the template
is complete, producing:
<div class="highlight-html notranslate"><div class="highlight"><pre>&lt;html&gt;
 &lt;body&gt;
 &lt;div class=&quot;header&quot;&gt;
 this is some header content
 &lt;/div&gt;

this is the body content.

&lt;div class=&quot;footer&quot;&gt;
 this is the footer
 &lt;/div&gt;
 &lt;/body&gt;
&lt;/html&gt;</pre></div>
</div>
</li>
</ol>
…and that is template inheritance in a nutshell. The main idea
is that the methods that you call upon <code class="docutils literal notranslate">self</code> always
correspond to the topmost definition of that method. Very much
the way <code class="docutils literal notranslate">self</code> 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).
<div class="section" id="nesting-blocks">
<h2>Nesting Blocks<a class="headerlink" href="#nesting-blocks" title="Permalink to this headline">¶</a></h2>
The named blocks defined in an inherited template can also be nested within
other blocks. The name given to each block is globally accessible via any inheriting
template. We can add a new block <code class="docutils literal notranslate">title</code> to our <code class="docutils literal notranslate">header</code> block:
<div class="highlight-mako notranslate"><div class="highlight"><pre>## base.html
&lt;html&gt;
 &lt;body&gt;
 &lt;div class=&quot;header&quot;&gt;
 &lt;%block name=&quot;header&quot;&gt;
 &lt;h2&gt;
 &lt;%block name=&quot;title&quot;/&gt;
 &lt;/h2&gt;
 &lt;/%block&gt;
 &lt;/div&gt;

&lt;%block name=&quot;header&quot;&gt;
 this is some header content
 ${parent.header()}
&lt;/%block&gt;

&lt;%block name=&quot;title&quot;&gt;
 this is the title
&lt;/%block&gt;

this is the body content.</pre></div>
</div>
Note when we overrode <code class="docutils literal notranslate">header</code>, we added an extra call <code class="docutils literal notranslate">${parent.header()}</code> in order to invoke
the parent’s <code class="docutils literal notranslate">header</code> block in addition to our own. That’s described in more detail below,
in <a class="reference internal" href="#parent-namespace">Using the parent Namespace to Augment Defs</a>.
</div>
<div class="section" id="rendering-a-named-block-multiple-times">
<h2>Rendering a Named Block Multiple Times<a class="headerlink" href="#rendering-a-named-block-multiple-times" title="Permalink to this headline">¶</a></h2>
Recall from the section <a class="reference internal" href="defs.html#blocks">Using Blocks</a> that a named block is just like a <code class="docutils literal notranslate">&lt;%def&gt;</code>,
with some different usage rules. We can call one of our named sections distinctly, for example
a section that is used more than once, such as the title of a page:
<div class="highlight-mako notranslate"><div class="highlight"><pre>&lt;html&gt;
 &lt;head&gt;
 &lt;title&gt;${self.title()}&lt;/title&gt;
 &lt;/head&gt;
 &lt;body&gt;
 &lt;%block name=&quot;header&quot;&gt;
 &lt;h2&gt;&lt;%block name=&quot;title&quot;/&gt;&lt;/h2&gt;
 &lt;/%block&gt;
 ${self.body()}
 &lt;/body&gt;
&lt;/html&gt;</pre></div>
</div>
Where above an inheriting template can define <code class="docutils literal notranslate">&lt;%block name=&quot;title&quot;&gt;</code> just once, and it will be
used in the base template both in the <code class="docutils literal notranslate">&lt;title&gt;</code> section as well as the <code class="docutils literal notranslate">&lt;h2&gt;</code>.
</div>
<div class="section" id="but-what-about-defs">
<h2>But what about Defs?<a class="headerlink" href="#but-what-about-defs" title="Permalink to this headline">¶</a></h2>
The previous example used the <code class="docutils literal notranslate">&lt;%block&gt;</code> tag to produce areas of content
to be overridden. Before Mako 0.4.1, there wasn’t any such tag – instead
there was only the <code class="docutils literal notranslate">&lt;%def&gt;</code> tag. As it turns out, named blocks and defs are
largely interchangeable. The def simply doesn’t call itself automatically,
and has more open-ended naming and scoping rules that are more flexible and similar
to Python itself, but less suited towards layout. The first example from
this chapter using defs would look like:
<div class="highlight-mako notranslate"><div class="highlight"><pre>## index.html
&lt;%inherit file=&quot;base.html&quot;/&gt;

&lt;%def name=&quot;header()&quot;&gt;
 this is some header content
&lt;/%def&gt;

this is the body content.</pre></div>
</div>
And <code class="docutils literal notranslate">base.html</code>, the inherited template:
<div class="highlight-mako notranslate"><div class="highlight"><pre>## base.html
&lt;html&gt;
 &lt;body&gt;
 &lt;div class=&quot;header&quot;&gt;
 ${self.header()}
 &lt;/div&gt;

&lt;div class=&quot;footer&quot;&gt;
 ${self.footer()}
 &lt;/div&gt;
 &lt;/body&gt;
&lt;/html&gt;

&lt;%def name=&quot;header()&quot;/&gt;
&lt;%def name=&quot;footer()&quot;&gt;
 this is the footer
&lt;/%def&gt;</pre></div>
</div>
Above, we illustrate that defs differ from blocks in that their definition
and invocation are defined in two separate places, instead of at once. You can almost do exactly what a
block does if you put the two together:
<div class="highlight-mako notranslate"><div class="highlight"><pre>&lt;div class=&quot;header&quot;&gt;
 &lt;%def name=&quot;header()&quot;&gt;&lt;/%def&gt;${self.header()}
&lt;/div&gt;</pre></div>
</div>
The <code class="docutils literal notranslate">&lt;%block&gt;</code> is obviously more streamlined than the <code class="docutils literal notranslate">&lt;%def&gt;</code> for this kind
of usage. In addition,
the above “inline” approach with <code class="docutils literal notranslate">&lt;%def&gt;</code> does not work with nesting:
<div class="highlight-mako notranslate"><div class="highlight"><pre>&lt;head&gt;
 &lt;%def name=&quot;header()&quot;&gt;
 &lt;title&gt;
 ## this won&#39;t work !
 &lt;%def name=&quot;title()&quot;&gt;default title&lt;/%def&gt;${self.title()}
 &lt;/title&gt;
 &lt;/%def&gt;${self.header()}
&lt;/head&gt;</pre></div>
</div>
Where above, the <code class="docutils literal notranslate">title()</code> def, because it’s a def within a def, is not part of the
template’s exported namespace and will not be part of <code class="docutils literal notranslate">self</code>. If the inherited template
did define its own <code class="docutils literal notranslate">title</code> def at the top level, it would be called, but the “default title”
above is not present at all on <code class="docutils literal notranslate">self</code> no matter what. For this to work as expected
you’d instead need to say:
<div class="highlight-mako notranslate"><div class="highlight"><pre>&lt;head&gt;
 &lt;%def name=&quot;header()&quot;&gt;
 &lt;title&gt;
 ${self.title()}
 &lt;/title&gt;
 &lt;/%def&gt;${self.header()}

&lt;%def name=&quot;title()&quot;/&gt;
&lt;/head&gt;</pre></div>
</div>
That is, <code class="docutils literal notranslate">title</code> is defined outside of any other defs so that it is in the <code class="docutils literal notranslate">self</code> namespace.
It works, but the definition needs to be potentially far away from the point of render.
A named block is always placed in the <code class="docutils literal notranslate">self</code> namespace, regardless of nesting,
so this restriction is lifted:
<div class="highlight-mako notranslate"><div class="highlight"><pre>## base.html
&lt;head&gt;
 &lt;%block name=&quot;header&quot;&gt;
 &lt;title&gt;
 &lt;%block name=&quot;title&quot;/&gt;
 &lt;/title&gt;
 &lt;/%block&gt;
&lt;/head&gt;</pre></div>
</div>
The above template defines <code class="docutils literal notranslate">title</code> inside of <code class="docutils literal notranslate">header</code>, and an inheriting template can define
one or both in any configuration, nested inside each other or not, in order for them to be used:
<div class="highlight-mako notranslate"><div class="highlight"><pre>## index.html
&lt;%inherit file=&quot;base.html&quot;/&gt;
&lt;%block name=&quot;title&quot;&gt;
 the title
&lt;/%block&gt;
&lt;%block name=&quot;header&quot;&gt;
 the header
&lt;/%block&gt;</pre></div>
</div>
So while the <code class="docutils literal notranslate">&lt;%block&gt;</code> tag lifts the restriction of nested blocks not being available externally,
in order to achieve this it adds the restriction that all block names in a single template need
to be globally unique within the template, and additionally that a <code class="docutils literal notranslate">&lt;%block&gt;</code> can’t be defined
inside of a <code class="docutils literal notranslate">&lt;%def&gt;</code>. It’s a more restricted tag suited towards a more specific use case than <code class="docutils literal notranslate">&lt;%def&gt;</code>.
</div>
<div class="section" id="using-the-next-namespace-to-produce-content-wrapping">
<h2>Using the <code class="docutils literal notranslate">next</code> Namespace to Produce Content Wrapping<a class="headerlink" href="#using-the-next-namespace-to-produce-content-wrapping" title="Permalink to this headline">¶</a></h2>
Sometimes you have an inheritance chain that spans more than two
templates. Or maybe you don’t, but you’d 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 <code class="docutils literal notranslate">self.body()</code> 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 <code class="docutils literal notranslate">next</code>, which is the namespace of the template
immediately following the current template.
Lets change the line in <code class="docutils literal notranslate">base.html</code> which calls upon
<code class="docutils literal notranslate">self.body()</code> to instead call upon <code class="docutils literal notranslate">next.body()</code>:
<div class="highlight-mako notranslate"><div class="highlight"><pre>## base.html
&lt;html&gt;
 &lt;body&gt;
 &lt;div class=&quot;header&quot;&gt;
 &lt;%block name=&quot;header&quot;/&gt;
 &lt;/div&gt;

${next.body()}

## .. rest of template</pre></div>
</div>
In this setup, each call to <code class="docutils literal notranslate">next.body()</code> will render the body
of the next template in the inheritance chain (which can be
written as <code class="docutils literal notranslate">base.html -&gt; layout.html -&gt; index.html</code>). Control
is still first passed to the bottommost template <code class="docutils literal notranslate">base.html</code>,
and <code class="docutils literal notranslate">self</code> still references the topmost definition of any
particular def.
The output we get would be:
<div class="highlight-html notranslate"><div class="highlight"><pre>&lt;html&gt;
 &lt;body&gt;
 &lt;div class=&quot;header&quot;&gt;
 this is some header content
 &lt;/div&gt;

&lt;div class=&quot;mainlayout&quot;&gt;
 this is the body content.
 &lt;/div&gt;

&lt;div class=&quot;footer&quot;&gt;
 this is the footer
 &lt;/div&gt;
 &lt;/body&gt;
&lt;/html&gt;</pre></div>
</div>
So above, we have the <code class="docutils literal notranslate">&lt;html&gt;</code>, <code class="docutils literal notranslate">&lt;body&gt;</code> and
<code class="docutils literal notranslate">header</code>/<code class="docutils literal notranslate">footer</code> layout of <code class="docutils literal notranslate">base.html</code>, we have the
<code class="docutils literal notranslate">&lt;ul&gt;</code> and <code class="docutils literal notranslate">mainlayout</code> section of <code class="docutils literal notranslate">layout.html</code>, and the
main body of <code class="docutils literal notranslate">index.html</code> as well as its overridden <code class="docutils literal notranslate">header</code>
def. The <code class="docutils literal notranslate">layout.html</code> template is inserted into the middle of
the chain without <code class="docutils literal notranslate">base.html</code> having to change anything.
Without the <code class="docutils literal notranslate">next</code> namespace, only the main body of
<code class="docutils literal notranslate">index.html</code> could be used; there would be no way to call
<code class="docutils literal notranslate">layout.html</code>’s body content.
</div>
<div class="section" id="using-the-parent-namespace-to-augment-defs">
<h2>Using the <code class="docutils literal notranslate">parent</code> Namespace to Augment Defs<a class="headerlink" href="#using-the-parent-namespace-to-augment-defs" title="Permalink to this headline">¶</a></h2>
Lets now look at the other inheritance-specific namespace, the
opposite of <code class="docutils literal notranslate">next</code> called <code class="docutils literal notranslate">parent</code>. <code class="docutils literal notranslate">parent</code> is the
namespace of the template immediately preceding the current
template. What’s useful about this namespace is that
defs or blocks can call upon their overridden versions.
This is not as hard as it sounds and
is very much like using the <code class="docutils literal notranslate">super</code> keyword in Python. Lets
modify <code class="docutils literal notranslate">index.html</code> to augment the list of selections provided
by the <code class="docutils literal notranslate">toolbar</code> function in <code class="docutils literal notranslate">layout.html</code>:
<div class="highlight-mako notranslate"><div class="highlight"><pre>## index.html
&lt;%inherit file=&quot;layout.html&quot;/&gt;

&lt;%block name=&quot;toolbar&quot;&gt;
 ## call the parent&#39;s toolbar first
 ${parent.toolbar()}
 &lt;li&gt;selection 4&lt;/li&gt;
 &lt;li&gt;selection 5&lt;/li&gt;
&lt;/%block&gt;

this is the body content.</pre></div>
</div>
Above, we implemented a <code class="docutils literal notranslate">toolbar()</code> function, which is meant
to override the definition of <code class="docutils literal notranslate">toolbar</code> within the inherited
template <code class="docutils literal notranslate">layout.html</code>. However, since we want the content
from that of <code class="docutils literal notranslate">layout.html</code> as well, we call it via the
<code class="docutils literal notranslate">parent</code> 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:
<div class="highlight-html notranslate"><div class="highlight"><pre>&lt;html&gt;
 &lt;body&gt;
 &lt;div class=&quot;header&quot;&gt;
 this is some header content
 &lt;/div&gt;

&lt;ul&gt;
 &lt;li&gt;selection 1&lt;/li&gt;
 &lt;li&gt;selection 2&lt;/li&gt;
 &lt;li&gt;selection 3&lt;/li&gt;
 &lt;li&gt;selection 4&lt;/li&gt;
 &lt;li&gt;selection 5&lt;/li&gt;
 &lt;/ul&gt;

&lt;div class=&quot;footer&quot;&gt;
 this is the footer
 &lt;/div&gt;
 &lt;/body&gt;
&lt;/html&gt;</pre></div>
</div>
and you’re now a template inheritance ninja!
</div>
<div class="section" id="using-include-with-template-inheritance">
<h2>Using <code class="docutils literal notranslate">&lt;%include&gt;</code> with Template Inheritance<a class="headerlink" href="#using-include-with-template-inheritance" title="Permalink to this headline">¶</a></h2>
A common source of confusion is the behavior of the <code class="docutils literal notranslate">&lt;%include&gt;</code> tag,
often in conjunction with its interaction within template inheritance.
Key to understanding the <code class="docutils literal notranslate">&lt;%include&gt;</code> tag is that it is a dynamic, e.g.
runtime, include, and not a static include. The <code class="docutils literal notranslate">&lt;%include&gt;</code> is only processed
as the template renders, and not at inheritance setup time. When encountered,
the referenced template is run fully as an entirely separate template with no
linkage to any current inheritance structure.
If the tag were on the other hand a static include, this would allow source
within the included template to interact within the same inheritance context
as the calling template, but currently Mako has no static include facility.
In practice, this means that <code class="docutils literal notranslate">&lt;%block&gt;</code> elements defined in an <code class="docutils literal notranslate">&lt;%include&gt;</code>
file will not interact with corresponding <code class="docutils literal notranslate">&lt;%block&gt;</code> elements in the calling
template.
A common mistake is along these lines:
<div class="highlight-mako notranslate"><div class="highlight"><pre>## partials.mako
&lt;%block name=&quot;header&quot;&gt;
 Global Header
&lt;/%block&gt;

## parent.mako
&lt;%include file=&quot;partials.mako&quot;&gt;

## child.mako
&lt;%inherit file=&quot;parent.mako&quot;&gt;
&lt;%block name=&quot;header&quot;&gt;
 Custom Header
&lt;/%block&gt;</pre></div>
</div>
Above, one might expect that the <code class="docutils literal notranslate">&quot;header&quot;</code> block declared in <code class="docutils literal notranslate">child.mako</code>
might be invoked, as a result of it overriding the same block present in
<code class="docutils literal notranslate">parent.mako</code> via the include for <code class="docutils literal notranslate">partials.mako</code>. But this is not the case.
Instead, <code class="docutils literal notranslate">parent.mako</code> will invoke <code class="docutils literal notranslate">partials.mako</code>, which then invokes
<code class="docutils literal notranslate">&quot;header&quot;</code> in <code class="docutils literal notranslate">partials.mako</code>, and then is finished rendering. Nothing
from <code class="docutils literal notranslate">child.mako</code> will render; there is no interaction between the <code class="docutils literal notranslate">&quot;header&quot;</code>
block in <code class="docutils literal notranslate">child.mako</code> and the <code class="docutils literal notranslate">&quot;header&quot;</code> block in <code class="docutils literal notranslate">partials.mako</code>.
Instead, <code class="docutils literal notranslate">parent.mako</code> must explicitly state the inheritance structure.
In order to call upon specific elements of <code class="docutils literal notranslate">partials.mako</code>, we will call upon
it as a namespace:
<div class="highlight-mako notranslate"><div class="highlight"><pre>## partials.mako
&lt;%block name=&quot;header&quot;&gt;
 Global Header
&lt;/%block&gt;

## parent.mako
&lt;%namespace name=&quot;partials&quot; file=&quot;partials.mako&quot;/&gt;
&lt;%block name=&quot;header&quot;&gt;
 ${partials.header()}
&lt;/%block&gt;

## parent.mako
&lt;%inherit file=&quot;base.mako&quot;&gt;
&lt;%include file=&quot;child.mako&quot;&gt;

## child.mako
&lt;%block name=&quot;SectionA&quot;&gt;
 child.mako
&lt;/%block&gt;</pre></div>
</div>
The resolution is similar; instead of using <code class="docutils literal notranslate">&lt;%include&gt;</code>, we call upon the blocks
of <code class="docutils literal notranslate">child.mako</code> using a namespace:
<div class="highlight-mako notranslate"><div class="highlight"><pre>## parent.mako
&lt;%inherit file=&quot;base.mako&quot;&gt;
&lt;%namespace name=&quot;child&quot; file=&quot;child.mako&quot;&gt;

&lt;%block name=&quot;SectionA&quot;&gt;
 ${child.SectionA()}
&lt;/%block&gt;</pre></div>
</div>
</div>
<div class="section" id="inheritable-attributes">
<h2>Inheritable Attributes<a class="headerlink" href="#inheritable-attributes" title="Permalink to this headline">¶</a></h2>
The <a class="reference internal" href="namespaces.html#mako.runtime.Namespace.attr" title="mako.runtime.Namespace.attr"><code class="xref py py-attr docutils literal notranslate">attr</code></a> accessor of the <a class="reference internal" href="namespaces.html#mako.runtime.Namespace" title="mako.runtime.Namespace"><code class="xref py py-class docutils literal notranslate">Namespace</code></a> object
allows access to module level variables declared in a template. By accessing
<code class="docutils literal notranslate">self.attr</code>, you can access regular attributes from the
inheritance chain as declared in <code class="docutils literal notranslate">&lt;%! %&gt;</code> sections. Such as:
<div class="highlight-mako notranslate"><div class="highlight"><pre>&lt;%!
 class_ = &quot;grey&quot;
%&gt;

&lt;div class=&quot;${self.attr.class_}&quot;&gt;
 ${self.body()}
&lt;/div&gt;</pre></div>
</div>
If an inheriting template overrides <code class="docutils literal notranslate">class_</code> to be
<code class="docutils literal notranslate">&quot;white&quot;</code>, as in:
<div class="highlight-mako notranslate"><div class="highlight"><pre>&lt;%!
 class_ = &quot;white&quot;
%&gt;
&lt;%inherit file=&quot;parent.html&quot;/&gt;

This is the body</pre></div>
</div>
you’ll get output like:
<div class="highlight-html notranslate"><div class="highlight"><pre>&lt;div class=&quot;white&quot;&gt;
 This is the body
&lt;/div&gt;</pre></div>
</div>
<div class="admonition seealso">
See also
<a class="reference internal" href="namespaces.html#namespace-attr-for-includes">Version One - Use Namespace.attr</a> - a more sophisticated example using
<a class="reference internal" href="namespaces.html#mako.runtime.Namespace.attr" title="mako.runtime.Namespace.attr"><code class="xref py py-attr docutils literal notranslate">Namespace.attr</code></a>.
</div>
</div>
</div>

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