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# Genshi How-To

Genshi was chosen as the default for two reasons.
  - Genshi’s syntax is a mere augmentation of the kid templating language which was the default for TurboGears from 0.8-1.0. Genshi is the new default in TurboGears 1.1 Genshi improves on Kid’s error reporting by providing better debug messages.
  - By default, Genshi is forces (x)html compliance, and error if your template does not provide valid (x)html.

Genshi is an XML template language based on kid, which in turn was inspired by Zope’s TAL.

Genshi Templates look like XHTML. Here’s a sample Genshi template:

## A Simple Genshi Template

    <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
                          "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
    <html xmlns="http://www.w3.org/1999/xhtml"
          xmlns:py="http://genshi.edgewall.org/"
          xmlns:xi="http://www.w3.org/2001/XInclude">

    <xi:include href="master.html" />

    <head>
      <meta content="text/html; charset=UTF-8" http-equiv="content-type" py:replace="''"/>
      <title>Sample Template, for looking at template locals</title>
    </head>

    <body>
        <h1>All objects from locals():</h1>

        <div py:for="item in sorted(locals()['data'].keys())">
          ${item}: ${repr(locals()['data'][item])}</div>
    </body>
    </html>

This particular template does a couple of things, but its main function is to take each variable that’s been made available in the template namespace and display its name and its value.

Every template language in the world needs to provide a variable substitution syntax, and the standard way in python seems to be `${item}`, and that’s exactly what you do in Genshi. Take a variable wrap it in curly braces and throw a $ in front, and the value of the variable will be substituted in when the template is rendered. Genshi is nice in that it allows you to use full python expressions in the substitution.

So, in our above template when Genshi sees:

    ${repr(locals()['data'][item])}

it evaluates the python expression `repr(locals()['data'][item])` and provides the string representation of the proper item.

And if you look a line up, you’ll see where item is defined as one of the list of keys in the dictionary representing the local variables. The way this works is that py:for acts just like a standard python for loop, repeating the <div> that it’s in (and its children if there were any) once for each item in the dictionary.

In TurboGears 2 the template namespace is going to be populated with the items you return in your controller’s return dictionary, along with a few extras. This particular template can be very helpful when debugging a controller’s return values, and is included in default quickstarted projects for you as project_name/templates/debug.html.

The general way that Genshi works it that it allows you to add special attributes to your xml elements, called Template Directives. Each of these template directives should be given a value, which can be ANY python expression. So, learning Genshi is pretty much about leaning how those directives work, since the rest is just python. And like py:for, most of the directives are very “python like”.

## Available Processing Directives:

Here’s a list of all the Template Directives in Genshi, along with a brief description.

Genshi Directive | Definition
--- | ---
`py:if` | Displays the element and its children if the condition is true.
`py:choose` | Used with py:when and py:otherwise to select one of several options to be rendered.
`py:when` | Used with py:choose – displays an element and its children when the condition is true.
`py:otherwise` | Used with py:when and py:choose, displays if non of the when clauses are true.
`py:for` | Repeats the element (and its children) for each item in some iterable
`py:with` | Lets you assign expressions to variables
`py:replace` | Replaces the element with the contents of the expression, stripping out the element itself.
`py:def` | Creates a re-usable “template function” that can be used to render template snippets based on the arguments passed in.
`py:match` | given an XPath expression, it finds and replaces every element in the template that matches the expression – with the content of the element containing the py:match.
`py:strip` | Removes just the containing element (not its children) if the condition is true.

There are examples of how each of these template directives works on the Genshi web site.

## Genshi gotchas

Note

DO NOT USE ‘data’ as a key in the return dictionary of your controller. This can provide a somewhat confusing AttributeError on the Context object. Currently the error message provides no mention of ‘data’ being a reserved word.

## Dotted Lookup Support

Since TurboGears relies on dotted template support for it’s standard, this standard also applies to Genshi. Therefore, all templates are referenced using a dotted name, instead of slashes, and this applies to xincluded templates within your template as well.

## Local Support

Genshi support also includes support for `local:` in your template name. What this allows you to do is to tell TurboGears to look for the referenced template in the locally executing namespace, as apposed to a fully-dotted name. This allows you to write extensions that can “plug in” to an existing TurboGears project by providing direct access to a project’s master template. tgext.admin takes advantage of this; most templates have the following code at the beginning of their files:

    <%inherit file="local:templates.master"/>

## Exposing a mako template

If you have your project’s default set to mako, don’t fret, you may still use genshi within your app. Simply preface your template name with mako, producing an expose decorator that might look like this:

    @expose('genshi:mytgapp.templates.my_awesome_genshi_template')
    def my_awesome_controller_method(self, **kw):
        ...

## Further Reading

Genshi web site