Edgewall Software


Version 1 (modified by cmlenz, 4 years ago)


Internationalization and Localization

Genshi provides comprehensive supporting infrastructure for internationalizing and localizing templates. That includes functionality for extracting localizable strings from templates, as well as a template filter and special directives that can apply translations to templates as they get rendered.

This support is based on gettext message catalogs and the gettext Python module. The extraction process can be used from the API level, or through the front-ends implemented by the Babel project, for which Genshi provides a plugin.

1   Basics

The simplest way to internationalize and translate templates would be to wrap all localizable strings in a gettext() function call (which is often aliased to _() for brevity). In that case, no extra template filter is required.

<p>${_("Hello, world!")}</p>

However, this approach results in significant “character noise” in templates, making them harder to read and preview.

The genshi.filters.Translator filter allows you to get rid of the explicit gettext function calls, so you can (often) just continue to write:

<p>Hello, world!</p>

This text will still be extracted and translated as if you had wrapped it in a _() call.


For parameterized or pluralizable messages, you need to use the special template directives described below, or use the corresponding gettext function in embedded Python expressions.

You can control which tags should be ignored by this process; for example, it doesn't really make sense to translate the content of the HTML <script></script> element. Both <script> and <style> are excluded by default.

Attribute values can also be automatically translated. The default is to consider the attributes abbr, alt, label, prompt, standby, summary, and title, which is a list that makes sense for HTML documents. Of course, you can tell the translator to use a different set of attribute names, or none at all.

1.1   Language Tagging

You can control automatic translation in your templates using the xml:lang attribute. If the value of that attribute is a literal string, the contents and attributes of the element will be ignored:

<p xml:lang="en">Hello, world!</p>

On the other hand, if the value of the xml:lang attribute contains a Python expression, the element contents and attributes are still considered for automatic translation:

<html xml:lang="$locale">

2   Template Directives

Sometimes localizable strings in templates may contain dynamic parameters, or they may depend on the numeric value of some variable to choose a proper plural form. Sometimes the strings contain embedded markup, such as tags for emphasis or hyperlinks, and you don't want to rely on the people doing the translations to know the syntax and escaping rules of HTML and XML.

In those cases the simple text extraction and translation process described above is not sufficient. You could just use gettext API functions in embedded Python expressions for parameters and pluralization, but that does not help when messages contain embedded markup. Genshi provides special template directives for internationalization that attempt to provide a comprehensive solution for this problem space.

To enable these directives, you'll need to register them with the templates they are used in. You can do this by adding them manually via the Template.add_directives(namespace, factory) (where namespace would be “http://genshi.edgewall.org/i18n” and factory would be an instance of the Translator class). Or you can just call the Translator.setup(template) class method, which both registers the directives and adds the translation filter.

After the directives have been registered with the template engine on the Python side of your application, you need to declare the corresponding directive namespace in all markup templates that use them. For example:

<html xmlns:py="http://genshi.edgewall.org/"

These directives only make sense in the context of markup templates. For text templates, you can just use the corresponding gettext API calls as needed.


The internationalization directives are still somewhat experimental and have some known issues. However, the attribute language they implement should be stable and is not subject to change substantially in future versions.

2.1   Messages

2.1.1   i18n:msg

This is the basic directive for defining localizable text passages that contain parameters and/or markup.

For example, consider the following template snippet:

  Please visit <a href="${site.url}">${site.name}</a> for help.

Without further annotation, the translation filter would treat this sentence as two separate messages (“Please visit” and “for help”), and the translator would have no control over the position of the link in the sentence.

However, when you use the Genshi internationalization directives, you simply add an i18n:msg attribute to the enclosing <p> element:

<p i18n:msg="name">
  Please visit <a href="${site.url}">${site.name}</a> for help.

Genshi is then able to identify the text in the <p> element as a single message for translation purposes. You'll see the following string in your message catalog:

Please visit [1:%(name)s] for help.

The <a> element with its attribute has been replaced by a part in square brackets, which does not include the tag name or the attributes of the element.

The value of the i18n:msg attribute is a comma-separated list of parameter names, which serve as simplified aliases for the actual Python expressions the message contains. The order of the paramer names in the list must correspond to the order of the expressions in the text. In this example, there is only one parameter: its alias for translation is “name”, while the corresponding expression is ${site.name}.

The translator now has complete control over the structure of the sentence. He or she certainly does need to make sure that any bracketed parts are not removed, and that the name parameter is preserved correctly. But those are things that can be easily checked by validating the message catalogs. The important thing is that the translator can change the sentence structure, and has no way to break the application by forgetting to close a tag, for example.

So if the German translator of this snippet decided to translate it to:

Um Hilfe zu erhalten, besuchen Sie bitte [1:%(name)s]

The resulting output might be:

  Um Hilfe zu erhalten, besuchen Sie bitte
  <a href="http://example.com/">Example</a>

Messages may contain multiple tags, and they may also be nested. For example:

<p i18n:msg="name">
  <i>Please</i> visit <b>the site <a href="${site.url}">${site.name}</a></b>
  for help.

This would result in the following message ID:

[1:Please] visit [2:the site [3:%(name)s]] for help.

Again, the translator has full control over the structure of the sentence. So the German translation could actually look like this:

Um Hilfe zu erhalten besuchen Sie [1:bitte]
[3:%(name)s], [2:das ist eine Web-Site]

Which Genshi would recompose into the following outout:

  Um Hilfe zu erhalten besuchen Sie <i>bitte</i>
  <a href="http://example.com/">Example</a>, <b>das ist eine Web-Site</b>

Note how the translation has changed the order and even the nesting of the tags.


Please note that i18n:msg directives do not support other nested directives. Directives commonly change the structure of the generated markup dynamically, which often would result in the structure of the text changing, thus making translation as a single message ineffective.

2.1.2   i18n:choose, i18n:singular, i18n:plural

Translatable strings that vary based on some number of objects, such as “You have 1 new message” or “You have 3 new messages”, present their own challenge, in particular when you consider that different languages have different rules for pluralization. For example, while English and most western languages have two plural forms (one for n=1 and an other for n<>1), Welsh has five different plural forms, while Hungarian only has one.

The gettext framework has long supported this via the ngettext() family of functions. You specify two default messages, one singular and one plural, and the number of items. The translations however may contain any number of plural forms for the message, depending on how many are commonly used in the language. ngettext will choose the correct plural form of the translated message based on the specified number of items.

Genshi provides a variant of the i18n:msg directive described above that allows choosing the proper plural form based on the numeric value of a given variable. The pluralization support is implemented in a set of three directives that must be used together: i18n:choose, i18n:singular, and i18n:plural.

The i18n:choose directive is used to set up the context of the message: it simply wraps the singular and plural variants.

The value of this directive is split into two parts: the first is the numeral, a Python expression that evaluates to a number to determine which plural form should be chosen. The second part, separated by a semicolon, lists the parameter names. This part is equivalent to the value of the i18n:msg directive.

For example:

<p i18n:choose="len(messages); num">
  <i18n:singular>You have <b>${len(messages)}</b> new message.</i18n:singular>
  <i18n:plural>You have <b>${len(messages)}</b> new messages.</i18n:plural>

All three directives can be used either as elements or attribute. So the above example could also be written as follows:

<i18n:choose numeral="len(messages)" params="num">
  <p i18n:singular="">You have <b>${len(messages)}</b> new message.</p>
  <p i18n:plural="">You have <b>${len(messages)}</b> new messages.</p>

When used as an element, the two parts of the i18n:choose value are split into two different attributes: numeral and params. The i18n:singular and i18n:plural directives do not require or support any value (or any extra attributes).

2.2   Comments and Domains

2.2.1   i18n:comment

The i18n:comment directive can be used to supply a comment for the translator. For example, if a template snippet is not easily understood outside of its context, you can add a translator comment to help the translator understand in what context the message will be used:

<p i18n:msg="name" i18n:comment="Link to the relevant support site">
  Please visit <a href="${site.url}">${site.name}</a> for help.

This comment will be extracted together with the message itself, and will commonly be placed along the message in the message catalog, so that it is easily visible to the person doing the translation.

This directive has no impact on how the template is rendered, and is ignored outside of the extraction process.

2.2.2   i18n:domain

In larger projects, message catalogs are commonly split up into different domains. For example, you might have a core application domain, and then separate domains for extensions or libraries.

Genshi provides a directive called i18n:domain that lets you choose the translation domain for a particular scope. For example:

<div i18n:domain="examples">
  <p>Hello, world!</p>

3   Extraction

The Translator class provides a class method called extract, which is a generator yielding all localizable strings found in a template or markup stream. This includes both literal strings in text nodes and attribute values, as well as strings in gettext() calls in embedded Python code. See the API documentation for details on how to use this method directly.

3.1   Babel Integration

This functionality is integrated with the message extraction framework provided by the Babel project. Babel provides a command-line interface as well as commands that can be used from setup.py scripts using Setuptools or Distutils.

The first thing you need to do to make Babel extract messages from Genshi templates is to let Babel know which files are Genshi templates. This is done using a “mapping configuration”, which can be stored in a configuration file, or specified directly in your setup.py.

In a configuration file, the mapping may look like this:

# Python souce
# Genshi templates
include_attrs = title
template_class = genshi.template.TextTemplate
encoding = latin-1

Please consult the Babel documentation for details on configuration.

If all goes well, running the extraction with Babel should create a POT file containing the strings from your Genshi templates and your Python source files.

3.2   Configuration Options

The Genshi extraction plugin for Babel supports the following options:

3.2.1   template_class

The concrete Template class that the file should be loaded with. Specify the package/module name and the class name, separated by a colon.

The default is to use genshi.template:MarkupTemplate, and you'll want to set it to genshi.template:TextTemplate for text templates.

3.2.2   encoding

The encoding of the template file. This is only used for text templates. The default is to assume “utf-8”.

3.2.3   include_attrs

Comma-separated list of attribute names that should be considered to have localizable values. Only used for markup templates.

3.2.4   ignore_tags

Comma-separated list of tag names that should be ignored. Only used for markup templates.

3.2.5   extract_text

Whether text outside explicit gettext function calls should be extracted. By default, any text nodes not inside ignored tags, and values of attribute in the include_attrs list are extracted. If this option is disabled, only strings in gettext function calls are extracted.


If you disable this option, and do not make use of the internationalization directives, it's not necessary to add the translation filter as described above. You only need to make sure that the template has access to the gettext functions it uses.

4   Translation

If you have prepared MO files for use with Genshi using the appropriate tools, you can access the message catalogs with the gettext Python module. You'll probably want to create a gettext.GNUTranslations instance, and make the translation functions it provides available to your templates by putting them in the template context.

The Translator filter needs to be added to the filters of the template (applying it as a stream filter will likely not have the desired effect). Furthermore it needs to be the first filter in the list, including the internal filters that Genshi adds itself:

from genshi.filters import Translator
from genshi.template import MarkupTemplate
template = MarkupTemplate("...")
template.filters.insert(0, Translator(translations.ugettext))

The Translator class also provides the convenience method setup(), which will both add the filter and register the i18n directives:

from genshi.filters import Translator
from genshi.template import MarkupTemplate
template = MarkupTemplate("...")
translator = Translator(translations.ugettext)


If you're using TemplateLoader, you should specify a callback function in which you add the filter. That ensures that the filter is not added everytime the template is rendered, thereby being applied multiple times.

See also: genshi.filters.i18n, Documentation