templess 0.3

Templess templating system

This document is valid for Templess 0.3.

What is it?

Templess is an XML templating language for Python, that is very compact and simple, fast, and has a very strict seperation of logic and design. It is different from other templating languages because instead of ‘asking’ for data from the template, you ‘tell’ the template what content there is to render, and the template just provides placeholders.

Directives

There are only 5 directives in Templess, all of which are defined as attributes on an XML node (see the examples below):

• content

  When an XML node has a templess:content attribute, a lookup will be done in the context (see the examples below) using the value of the attribute as key. If the key can not be found, a KeyError is raised. If the value found is a string, it will be used as text content for the node. If the value found is a ‘lazyrenderer’ instance as produced by template.render() it will be used as new content for the element (iow, you can use template.render() to generate snippets of XML to be placed into other trees). If the value is a list node, the element’s contents will be repeated for each element in the list, interpolating the current item into the elements’ contents (see list interpolation below).

• replace

  Same as content, but instead of ‘filling’ the node, the node gets replaced by the result of the interpolation.

• attr

  The value of an optional templess:attr on an XML node will be split on ; to form pairs (strings), which are each split on space (’ ‘) to form key, value pairs (so ‘foo bar;baz qux’ will result in two pairs, (‘foo’, ‘bar’), (‘baz’, ‘qux’) of which each first item is the key and the second the value). For each of those pairs an attribute will be added to the XML node, with the key as name, and as value the result of a lookup in the context.

• cond

  If the value of the templess:cond condition on an XML node resolves to false, the node is removed entirely from the document.

• not

  The reverse of ‘cond’, this makes the node get rendered if the value is false only.

List interpolation

If the interpolation value found for a templess:content or templess:replace attribute is of a string or a node type, the value will just be added to a node. If the item’s value is a list type, the node will be copied and the interpolation will be executed repetetively for each item in the list, using the node as the root and the current value as the context. For each list item it will respond in the following manner:

• If the item value is a string type, the value will be used as the current element’s text attribute.

• If the item value is a node type, it will be attached to the node’s children.

• If the item value is a dict type, the content interpolation will recurse with the exact same behaviour as it has with the full template, using the current element as root and the current context value as context (note that the root context is not available anymore).

When the nodes are all copied, they are attached to the tree if the directive used was templess:content, and only their contents are attached to the tree if the directive was templess:replace.

Examples

All of the examples below assume we’re in a script with the following lines in the top somewhere:

>>> from templess import templess

Example 1: simple interpolation

As a first example we’ll interpolate the text ‘bar’ into a node ‘foo’ in a simple XML document.

>>> xml = """\
... <?xml version="1.0" ?>
... <doc xmlns:t="http://johnnydebris.net/xmlns/templess">
...   <foo t:content="foodata" />
... </doc>
... """
>>> context = {
...     'foodata': 'bar',
... }
>>> t = templess.template(xml)
>>> print t.unicode(context)
<doc>
  <foo>bar</foo>
</doc>

On calling ‘render()’ the template is searched for any Templess directives, and at some point finds the ‘t:content’ one. When this happens, it will look in the context passed in as an argument if there’s a key for the value of the attribute. In our case the value of the attribute is ‘foodata’, so it will look for the key ‘foodata’, and will find the value ‘bar’. Since this value is a string type, it will set it as the text value for the node, and remove the attribute when it’s done.

Note that if the directive used would have been templess:replace, the doc element would get the text contents, and the foo node would have been removed, so the XML would look something like <doc>bar</doc>.

Example 2: node interpolation

In this example we’ll interpolate a new XML node into the document. This functionality provides a simple way to re-use templates.

Note that (XXX currently?) the node needs to be a special Templess node to be interpolated. More convenient ways to add a node (either using an API that is less Templess specific, or perhaps by allowing XML nodes from other libraries) will be made available in the future.

>>> xml = """\
... <?xml version="1.0" ?>
... <doc xmlns:t="http://johnnydebris.net/xmlns/templess">
...   <foo t:content="foodata" />
... </doc>
... """
>>> context = {
...   'foodata': templess.elnode('bar', {}, None),
... }
>>> t = templess.template(xml)
>>> print t.unicode(context)
<doc>
  <foo><bar /></foo>
</doc>

Exactly the same happens as with a string, except that instead of setting the text value of the node to the string value of the context, the element node will be used as the new content of the node.

A somewhat more useful example interpolates the results of another template (somewhat similar to macros in other systems):

>>> xml_bar = """\
... <?xml version="1.0" ?>
... <bar xmlns:t="http://johnnydebris.net/xmlns/templess"
...      t:content="bardata" />
... """
>>> context_bar = {
...   'bardata': 'baz',
... }
>>> t_bar = templess.template(xml_bar)
>>> xml = """\
... <?xml version="1.0" ?>
... <doc xmlns:t="http://johnnydebris.net/xmlns/templess">
...   <foo t:content="foodata" />
... </doc>
... """
>>> context = {
...   'foodata': t_bar.render(context_bar)
... }
>>> t = templess.template(xml)
>>> print t.unicode(context)
<doc>
  <foo><bar>baz</bar></foo>
</doc>

Example 3: list interpolation with string values

In this example we’ll repeat a certain element inside the document, and interpolate the contents of an array into each instance.

>>> xml = """\
... <?xml version="1.0" ?>
... <doc xmlns:t="http://johnnydebris.net/xmlns/templess">
...   <foo t:content="foodata" />
... </doc>
... """
>>> context = {
...   'foodata': ['foo', 'bar'],
... }
>>> t = templess.template(xml)
>>> print t.unicode(context)
<doc>
  <foo>foo</foo><foo>bar</foo>
</doc>

When the Templess engine notices the value is a list type, it will iterate through it, creating a clone of the node for each iteration. From there on the interpolation works exactly the same as normal, but then on the clone instead of on the original node. When it’s done iterating the list, the original will be removed.

Note: the interpolation of nodes (templess.node subclasses) works exactly the same, also it’s possible to use templess:replace the same way as before.

Note: when the values of the list are not of a string type, interpolation will not behave the same on the node, see the next example.

Example 4: list interpolation with dict values

In this example we’ll use dict values for the list, effectively recursing into the interpolation process. The context for the recursion is the current context value (the current dict), and the root for the recursion is the current element.

>>> xml = """\
... <?xml version="1.0" ?>
... <doc xmlns:t="http://johnnydebris.net/xmlns/templess">
...   <foo t:content="foodata"><bar t:content="bardata" /></foo>
... </doc>
... """
>>> context = {
...   'foodata': [
...     {'bardata': 'baz'},
...     {'bardata': 'qux'},
...   ],
... }
>>> t = templess.template(xml)
>>> print t.unicode(context)
<doc>
  <foo><bar>baz</bar></foo><foo><bar>qux</bar></foo>
</doc>

Of course this also works the same with templess.node instances instead of strings, and recurses infinitely, so you could use lists as the values, too. Also behaves as you’d expect when the directive is t:replace.

Example 5: conditional rendering

Very simple example for conditional rendering, because it’s a very simple directive. The end result would be something like <doc><bar /></doc>.

>>> xml = """\
... <?xml version="1.0" ?>
... <doc xmlns:t="http://johnnydebris.net/xmlns/templess">
...   <foo t:cond="foodata" /><bar t:cond="bardata" />
... </doc>
... """
>>> context = {
...   'foodata': False,
...   'bardata': True,
... }
>>> t = templess.template(xml)
>>> print t.unicode(context)
<doc>
  <bar />
</doc>

When the t:cond resolves to True, the element is left in tact, when it resolves to False, it’s removed entirely from the document. There is a counter directive called ‘not’, which has exactly the opposite effect from ‘cond’, so instead of adding the node on a True value, it removes it.

Example 6: setting attributes

Setting attributes is also not too hard. The ‘attr’ directive generates attributes for each contained item, using the value in the context as value for the attribute. If the context value resolves to False, the attribute is skipped (unless the value is ‘’, then it will result in an empty attribute).

>>> xml = """\
... <?xml version="1.0" ?>
... <doc xmlns:t="http://johnnydebris.net/xmlns/templess">
...   <foo t:attr="bar bardata; baz bazdata; qux quxdata" />
... </doc>
... """
>>> context = {
...   'bardata': 'quux',
...   'bazdata': 'quuux',
...   'quxdata': False,
... }
>>> t = templess.template(xml)
>>> print t.unicode(context)
<doc>
  <foo bar="quux" baz="quuux" />
</doc>