Jinja2 supports extensions that can add extra filters, tests, globals or even extend the parser. The main motivation of extensions is it to move often used code into a reusable class like adding support for internationalization.
Extensions are added to the Jinja2 environment at creation time. Once the environment is created additional extensions cannot be added. To add an extension pass a list of extension classes or import paths to the environment parameter of the :class:`Environment` constructor. The following example creates a Jinja2 environment with the i18n extension loaded:
jinja_env = Environment(extensions=['jinja.ext.i18n'])
Jinja2 currently comes with one extension, the i18n extension. It can be used in combination with gettext or babel. If the i18n extension is enabled Jinja2 provides a trans statement that marks the wrapped string as translatable and calls gettext.
After enabling dummy _, gettext and ngettext functions are added to the template globals. A internationalized application has to override those methods with more useful versions.
For a web application that is available in multiple languages but gives all the users the same language (for example a multilingual forum software installed for a French community) may load the translations once and add the translation methods to the environment at environment generation time:
translations = get_gettext_translations() env = Environment(extensions=['jinja.ext.i18n']) env.globals.update( gettext=translations.ugettext, ngettext=translations.ungettext )
The get_gettext_translations function would return the translator for the current configuration. Keep in mind that Jinja2 uses unicode internally so you must pass the ugettext and ungettext functions to the template.
The default _ function injected by the extension calls gettext automatically.
If you want to pass the gettext function into the context at render time because you don't know the language/translations earlier and the optimizer is enabled (which it is per default), you have to unregister the gettext and ugettext functions first:
del env.globals['gettext'], env.globals['ugettext']
Jinja2 also provides a way to extract recognized strings. For one the jinja.ext module provides a function that can return all the occurences of gettext calls in a node (as returned by :meth:`Environment.parse`):
The usage of the i18n extension for template designers is covered as part :ref:`of the template documentation <i18n-in-templates>`.
By writing extensions you can add custom tags to Jinja2. This is a non trival task and usually not needed as the default tags and expressions cover all common use cases. The i18n extension is a good example of why extensions are useful, another one would be fragment caching.
The following example implements a cache tag for Jinja2:
In order to use the cache extension it makes sense to subclass the environment to implement the add_fragment_to_cache and load_fragment_from_cache methods. The following example shows how to use the Werkzeug caching with the extension from above:
from jinja2 import Environment from werkzeug.contrib.cache import SimpleCache cache = SimpleCache() cache_prefix = 'tempalte_fragment/' class MyEnvironment(Environment): def __init__(self): Environment.__init__(self, extensions=[CacheExtension]) def add_fragment_to_cache(self, key, value, timeout): cache.add(cache_prefix + key, value, timeout) def load_fragment_from_cache(self, key): return cache.get(cache_prefix + key)
Extensions always have to extend the :class:`jinja2.ext.Extension` class:
The parser passed to :meth:`Extension.parse` provides ways to parse expressions of different types. The following methods may be used by extensions:
The AST (Abstract Syntax Tree) is used to represent a template after parsing. It's build of nodes that the compiler then converts into executable Python code objects. Extensions that provide custom statements can return nodes to execute custom Python code.
The list below describes all nodes that are currently available. The AST may change between Jinja2 versions but will stay backwards compatible.
For more information have a look at the repr of :meth:`jinja2.Environment.parse`.