Edgewall Software

Genshi Templating Basics

Genshi provides a template engine that can be used for generating either markup (such as HTML or XML) or plain text. While both share some of the syntax (and much of the underlying implementation) they are essentially separate languages.

This document describes the common parts of the template engine and will be most useful as reference to those developing Genshi templates. Templates are XML or plain text files that include processing directives that affect how the template is rendered, and template expressions that are dynamically substituted by variable data.

1   Synopsis

A Genshi markup template is a well-formed XML document with embedded Python used for control flow and variable substitution. Markup templates should be used to generate any kind of HTML or XML output, as they provide many advantages over simple text-based templates (such as automatic escaping of strings).

The following illustrates a very basic Genshi markup template:

<?python
  title = "A Genshi Template"
  fruits = ["apple", "orange", "kiwi"]
?>
<html xmlns:py="http://genshi.edgewall.org/">
  <head>
    <title py:content="title">This is replaced.</title>
  </head>
  <body>
    <p>These are some of my favorite fruits:</p>
    <ul>
      <li py:for="fruit in fruits">
        I like ${fruit}s
      </li>
    </ul>
  </body>
</html>

This example shows:

  1. a Python code block, using a processing instruction
  2. the Genshi namespace declaration
  3. usage of templates directives (py:content and py:for)
  4. an inline Python expression (${fruit}).

The template would generate output similar to this:

<html>
  <head>
    <title>A Genshi Template</title>
  </head>
  <body>
    <p>These are some of my favorite fruits:</p>
    <ul>
      <li>I like apples</li>
      <li>I like oranges</li>
      <li>I like kiwis</li>
    </ul>
  </body>
</html>

A text template is a simple plain text document that can also contain embedded Python code. Text templates can be used to generate simple non-markup text formats, such as the body of an plain text email. For example:

Dear $name,
These are some of my favorite fruits:
#for fruit in fruits
 * $fruit
#end

2   Python API

The Python code required for templating with Genshi is generally based on the following pattern:

  • Attain a MarkupTemplate or TextTemplate object from a string or file-like object containing the template source. This can either be done directly, or through a TemplateLoader instance.
  • Call the generate() method of the template, passing any data that should be made available to the template as keyword arguments.
  • Serialize the resulting stream using its render() method.

For example:

>>> from genshi.template import MarkupTemplate
>>> tmpl = MarkupTemplate('<h1>Hello, $name!</h1>')
>>> stream = tmpl.generate(name='world')
>>> print stream.render('xhtml')
<h1>Hello, world!</h1>

Note

See the Serialization section of the Markup Streams page for information on configuring template output options.

Using a text template is similar:

>>> from genshi.template import TextTemplate
>>> tmpl = TextTemplate('Hello, $name!')
>>> stream = tmpl.generate(name='world')
>>> print stream
Hello, world!

Note

If you want to use text templates, you should consider using the NewTextTemplate class instead of simply TextTemplate. See the Text Template Language page.

Using a template loader provides the advantage that “compiled” templates are automatically cached, and only parsed again when the template file changes. In addition, it enables the use of a template search path, allowing template directories to be spread across different file-system locations. Using a template loader would generally look as follows:

from genshi.template import TemplateLoader
loader = TemplateLoader([templates_dir1, templates_dir2])
tmpl = loader.load('test.html')
stream = tmpl.generate(title='Hello, world!')
print stream.render()

See the API documentation for details on using Genshi via the Python API.

3   Template Expressions and Code Blocks

Python expressions can be used in text and directive arguments. An expression is substituted with the result of its evaluation against the template data. Expressions in text (which includes the values of non-directive attributes) need to prefixed with a dollar sign ($) and usually enclosed in curly braces ({…}).

If the expression starts with a letter and contains only letters, digits, dots, and underscores, the curly braces may be omitted. In all other cases, the braces are required so that the template processor knows where the expression ends:

>>> from genshi.template import MarkupTemplate
>>> tmpl = MarkupTemplate('<em>${items[0].capitalize()} item</em>')
>>> print tmpl.generate(items=['first', 'second'])
<em>First item</em>

Expressions support the full power of Python. In addition, it is possible to access items in a dictionary using “dotted notation” (i.e. as if they were attributes), and vice-versa (i.e. access attributes as if they were items in a dictionary):

>>> from genshi.template import MarkupTemplate
>>> tmpl = MarkupTemplate('<em>${dict.foo}</em>')
>>> print tmpl.generate(dict={'foo': 'bar'})
<em>bar</em>

Because there are two ways to access either attributes or items, expressions do not raise the standard AttributeError or IndexError exceptions, but rather an exception of the type UndefinedError. The same kind of error is raised when you try to use a top-level variable that is not in the context data. See Error Handling below for details on how such errors are handled.

3.1   Escaping

If you need to include a literal dollar sign in the output where Genshi would normally detect an expression, you can simply add another dollar sign:

>>> from genshi.template import MarkupTemplate
>>> tmpl = MarkupTemplate('<em>$foo</em>') # Wanted "$foo" as literal output
>>> print tmpl.generate()
Traceback (most recent call last):
  ...
UndefinedError: "foo" not defined
>>> tmpl = MarkupTemplate('<em>$$foo</em>')
>>> print tmpl.generate()
<em>$foo</em>

But note that this is not necessary if the characters following the dollar sign do not qualify as an expression. For example, the following needs no escaping:

>>> tmpl = MarkupTemplate('<script>$(function() {})</script>')
>>> print tmpl.generate()
<script>$(function() {})</script>

On the other hand, Genshi will always replace two dollar signs in text with a single dollar sign, so you'll need to use three dollar signs to get two in the output:

>>> tmpl = MarkupTemplate('<script>$$$("div")</script>')
>>> print tmpl.generate()
<script>$$("div")</script>

3.2   Code Blocks

Templates also support full Python code blocks, using the <?python ?> processing instruction in XML templates:

<div>
  <?python
      from genshi.builder import tag
      def greeting(name):
          return tag.b('Hello, %s!' % name) ?>
  ${greeting('world')}
</div>

This will produce the following output:

<div>
  <b>Hello, world!</b>
</div>

In text templates (although only those using the new syntax introduced in Genshi 0.5), code blocks use the special {% python %} directive:

{% python
    from genshi.builder import tag
    def greeting(name):
        return 'Hello, %s!' % name
%}
${greeting('world')}

This will produce the following output:

Hello, world!

Code blocks can import modules, define classes and functions, and basically do anything you can do in normal Python code. What code blocks can not do is to produce content that is emitted directly tp the generated output.

Note

Using the print statement will print to the standard output stream, just as it does for other Python code in your application.

Unlike expressions, Python code in <?python ?> processing instructions can not use item and attribute access in an interchangeable manner. That means that “dotted notation” is always attribute access, and vice-versa.

The support for Python code blocks in templates is not supposed to encourage mixing application code into templates, which is generally considered bad design. If you're using many code blocks, that may be a sign that you should move such code into separate Python modules.

If you'd rather not allow the use of Python code blocks in templates, you can simply set the allow_exec parameter (available on the Template and the TemplateLoader initializers) to False. In that case Genshi will raise a syntax error when a <?python ?> processing instruction is encountered. But please note that disallowing code blocks in templates does not turn Genshi into a sandboxable template engine; there are sufficient ways to do harm even using plain expressions.

Warning

Unfortunately, code blocks are severely limited when running under Python 2.3: For example, it is not possible to access variables defined in outer scopes. If you plan to use code blocks extensively, it is strongly recommended that you run Python 2.4 or later.

3.3   Error Handling

By default, Genshi raises an UndefinedError if a template expression attempts to access a variable that is not defined:

>>> from genshi.template import MarkupTemplate
>>> tmpl = MarkupTemplate('<p>${doh}</p>')
>>> tmpl.generate().render('xhtml')
Traceback (most recent call last):
  ...
UndefinedError: "doh" not defined

You can change this behavior by setting the variable lookup mode to "lenient". In that case, accessing undefined variables returns an Undefined object, meaning that the expression does not fail immediately. See below for details.

If you need to check whether a variable exists in the template context, use the defined or the value_of function described below. To check for existence of attributes on an object, or keys in a dictionary, use the hasattr(), getattr() or get() functions, or the in operator, just as you would in regular Python code:

>>> from genshi.template import MarkupTemplate
>>> tmpl = MarkupTemplate('<p>${defined("doh")}</p>')
>>> print tmpl.generate().render('xhtml')
<p>False</p>

Note

Lenient error handling was the default in Genshi prior to version 0.5. Strict mode was introduced in version 0.4, and became the default in 0.5. The reason for this change was that the lenient error handling was masking actual errors in templates, thereby also making it harder to debug some problems.

3.3.1   Lenient Mode

If you instruct Genshi to use the lenient variable lookup mode, it allows you to access variables that are not defined, without raising an UndefinedError.

This mode can be chosen by passing the lookup='lenient' keyword argument to the template initializer, or by passing the variable_lookup='lenient' keyword argument to the TemplateLoader initializer:

>>> from genshi.template import MarkupTemplate
>>> tmpl = MarkupTemplate('<p>${doh}</p>', lookup='lenient')
>>> print tmpl.generate().render('xhtml')
<p></p>

You will however get an exception if you try to call an undefined variable, or do anything else with it, such as accessing its attributes:

>>> from genshi.template import MarkupTemplate
>>> tmpl = MarkupTemplate('<p>${doh.oops}</p>', lookup='lenient')
>>> print tmpl.generate().render('xhtml')
Traceback (most recent call last):
  ...
UndefinedError: "doh" not defined

If you need to know whether a variable is defined, you can check its type against the Undefined class, for example in a conditional directive:

>>> from genshi.template import MarkupTemplate
>>> tmpl = MarkupTemplate('<p>${type(doh) is not Undefined}</p>',
...                       lookup='lenient')
>>> print tmpl.generate().render('xhtml')
<p>False</p>

Alternatively, the built-in functions defined or value_of can be used in this case.

3.3.2   Custom Modes

In addition to the built-in "lenient" and "strict" modes, it is also possible to use a custom error handling mode. For example, you could use lenient error handling in a production environment, while also logging a warning when an undefined variable is referenced.

See the API documentation of the genshi.template.eval module for details.

3.4   Built-in Functions & Types

The following functions and types are available by default in template code, in addition to the standard built-ins that are available to all Python code.

3.4.1   defined(name)

This function determines whether a variable of the specified name exists in the context data, and returns True if it does.

3.4.2   value_of(name, default=None)

This function returns the value of the variable with the specified name if such a variable is defined, and returns the value of the default parameter if no such variable is defined.

3.4.3   Markup(text)

The Markup type marks a given string as being safe for inclusion in markup, meaning it will not be escaped in the serialization stage. Use this with care, as not escaping a user-provided string may allow malicious users to open your web site to cross-site scripting attacks.

3.4.4   Undefined

The Undefined type can be used to check whether a reference variable is defined, as explained in error handling.

4   Template Directives

Directives provide control flow functionality for templates, such as conditions or iteration. As the syntax for directives depends on whether you're using markup or text templates, refer to the XML Template Language or Text Template Language pages for information.

See also: genshi.template, Genshi XML Template Language, Genshi Text Template Language, Documentation, GenshiRecipes

Last modified 9 years ago Last modified on Dec 14, 2015, 6:22:32 AM