XML Templates

# Nevow XML Templates

Stan syntax is cool, but eventually you are going to want to integrate your Python code with a template designed by an HTML monkey. Nevow accomplishes this by providing an xmlfile loader which uses the built-in Python SAX libraries to generate a tree of stan behind the scenes. The general rule is anything that is possible in stan should be possible in a pure XML template; of course, the XML syntax is generally going to be much more verbose.

## loaders.xmlfile

Wherever you have seen a loaders.stan being created in any of the example code, a `loaders.xmlfile` can be substituted instead. At the most basic, `xmlfile` merely requires the name of an xml template:

    class HelloXML(rend.Page):
        docFactory = loaders.xmlfile('hello.xml')

Placing the following xml in the `hello.xml` file will cause `HelloXML` to display a static page when it is rendered:

    <html>Hello, world!</html>

The following additional keyword arguments may be given to `xmlfile` to configure it:

- `templateDirectory`: The path to the directory which contains the template file. Defaults to ''.
- `ignoreDocType`: If True, discard any DOCTYPE declaration when building the DOM from this template. When false, preserve the DOCTYPE, causing it to show up in the final output. Useful for when you are inserting an XML fragment into a larger page and do not wish to generate invalid XML as output. Defaults to False.
- `ignoreComment`: If True, discard XML comments, causing them to disappear from the output. If False, preserve comments and render them in the final output unchanged. Defaults to False.
- `pattern`: If present, the given pattern name will be looked up and used as the root of the template. If not present, the entire document will be used as the template. Useful for embedding fragments of an XML document in a larger page. Defaults to None.

## Nevow's xmlns declaration

In order for Nevow to notice and process any XML directives in the template file, you must declare the Nevow xmlns at the top of your XML document. Nevow's xmlns is:

    http://nevow.com/ns/nevow/0.1

The syntax for declaring that your xml document uses this namespace is:

    <html xmlns:nevow="http://nevow.com/ns/nevow/0.1"></html>

You may replace the text "nevow" in the above example with any name you choose. For example, many people use "n" because it is shorter to type. If you do so, be sure to replace all occurrences of the nevow namespace in the examples with the namespace name you choose.

## Nevow's Tag Attribute Language

The markup you will add to your XHTML file in order to invoke Nevow code consists mostly of namespaced tag attributes. This approach was influenced heavily by the Zope Page Templates (ZPT) Tag Attribute Language (TAL). However, I felt that TAL did not go far enough in removing control flow and branching possibilities from the XML template. Nevow's main philosophy is that it should be as easy as possible to move from the XML document into Python code, and that the Python code should have ultimate control over manipulating the structure of the XML template.

The key is that it is easy to expose Python methods that you write to your XML template, and it is easy for the XML templates to mark nodes which it wishes the Python method to manipulate. In this way, if either the Python implementation changes or the location or content of the marked nodes change in the XML template, the other side will be isolated from these changes.

Nevow's XML templating has two attributes which invoke Python code:

- `nevow:render` -- Invokes a Python method and replaces the template node with the result
- `nevow:data` -- Invokes a Python method and sets the data special for the node to the result

It has one attribute which marks nodes as manipulatable by Python code:

- `nevow:pattern` -- Gives a node a name so that Python code may clone and mutate copies of this node

It also has two namespaced tags:

- `nevow:slot` -- Works in the same way as the slot attribute
- `nevow:attr` -- Indicates that an attribute of the parent tag should be manipulated by Python code in some way

## nevow:render

When the `nevow:render` attribute is encountered, the xmlfile loader sets the render special to a directive constructed with the attribute value. When the template is rendered, this means that the appropriate render_* method will be looked up on the `IRendererFactory` (generally the Page instance):

    <html><div nevow:render="foo" /></html>

With the `render_foo` method::

    def render_foo(self, ctx, data):
        return "Hello"

Will result in the document:

    <html>Hello</html>

Note that the return value of the render method replaces the template node in the DOM, so if you want the template node to remain, you should use `ctx.tag`.

## Built-in renderers

Nevow comes with various built in renderers on the Page class.

- `data`: Renders the current data as-is inside the current node.
- `string`: Renders the current data as a string inside the current node.
- `sequence`: Iterates the current data, copying the "item" pattern for each item. Sets the the data special of the new node to the item, and inserts the result in the current node. See the nevow.rend.sequence docstring for information about other used patterns, including "header", "divider", "footer" and "empty".
- `mapping`: Calls .items() on the current data, and calls ctx.fillSlots(key, value) for every key, value pair in the result. Returns the template tag.
- `xml`: Inserts the current data into the template after wrapping it in an xml instance. Not very useful in practice.

## nevow:data

When the `nevow:data` attribute is encountered, the xmlfile loader sets the data special of the current node to a directive constructed with the attribute value. When the template is rendered, this means that the appropriate data_* method will be looked up on the current `IContainer` (generally the Page instance). The data_* method will be called, and the result will be set as the data special of the current Tag:

    <html><div nevow:data="name" nevow:render="data" /></html>

With the `data_name` method:

    def data_name(self, ctx, data):
        return "Hello!"

Will result in the document:

    <html><div>Hello!</div></html>

Note that with a data attribute on a node but no renderer, the result of the data method will be set as the data special for that tag, and child render methods will be passed this data.

## nevow:pattern

When the `nevow:pattern` attribute is encountered, the xmlfile loader sets the pattern special of the current node to the attribute value as a string. Renderers which are above this node may then make copies of it using the `nevow.inevow.IQ` of the current context. With the template:

    <html nevow:render="stuff"><div nevow:pattern="somePattern" nevow:render="data" /></html>

And the renderer:

    def render_stuff(self, ctx, data):
        pat = inevow.IQ(ctx).patternGenerator('somePattern')
        return [pat(data=1), pat(data=2)]

Will result in the document:

    <html><div>1</div><div>2</div></html>

## nevow:slot

When the `nevow:slot` tag is encountered, the xmlfile loader constructs a `nevow.stan.slot` instance, passing the name attribute value as the slot name. The children of the slot node are added as children of the new slot instance. This is useful if you wish to put patterns inside the slot. With the template:

    <html nevow:render="stuff"><nevow:slot name="slotName" /></html>

And the render method:

    def render_stuff(self, ctx, data):
        ctx.fillSlots('slotName', "Hello.")
        return ctx.tag

This document will be produced:

    <html>Hello.</html>

## nevow:attr

When the `nevow:attr` tag is encountered, the contents of the nevow:attr node will be assigned to the attribute of the parent tag with the name of the value of the name attribute. Perhaps an example will be a little clearer:

    <html><a><nevow:attr name="href">HELLO!</nevow:attr>Goodbye</a></html>

This document will be produced:

    <html><a href="HELLO!">Goodbye</a></html>

While this syntax is somewhat awkward, every other type of nevow tag and attribute may be used inside the `nevow:attr` node. This makes setting attributes of tags uniform with every other method of manipulating the XML template.

## nevow:invisible

Sometimes you need to group some elements, because you need to use a renderer for a group of children.

However, it may not be desirable to give these elements a parent/child relationship in your XML structure. For these cases, use `nevow:invisible`.

As suggested by the name, a `nevow:invisible` tag is removed in the rendered XML. Here is an example:

    <html><nevow:invisible nevow:data="name" nevow:render="data" /></html>

With the `data_name` method:

    def data_name(self, ctx, data):
        return "Hello!"

Will result in the document:

    <html>Hello!</html>

## xmlstr, htmlfile, and htmlstr

xmlstr is a loader which is identical to xmlfile except it takes a string of XML directly.

htmlfile and htmlstr should generally be avoided. They are similar to xmlfile and xmlstr, except they use twisted.web.microdom in beExtremelyLenient mode to attempt to parse badly-formed HTML (non-XHTML) templates. See the nevow.loaders docstrings for more information.

## Conclusions

Nevow's xmlfile tag attribute language allows you to integrate externally-designed XHTML templates into the Nevow rendering process.