.. -*- mode: rst; encoding: utf-8 -*-

=====================================
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.

.. _`gettext`: http://www.gnu.org/software/gettext/
.. _`gettext python module`: http://docs.python.org/lib/module-gettext.html
.. _`babel`: http://babel.edgewall.org/


.. contents:: Contents
   :depth: 2
.. sectnum::


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.

.. code-block:: genshi

${_("Hello, world!")}

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:

.. code-block:: genshi

Hello, world!

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

.. note:: 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.

----------------
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:

.. code-block:: genshi

Hello, world!

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:

.. code-block:: genshi

    ...

.. _`template directives`:

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:

.. code-block:: genshi

    …

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.

.. note:: 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.

.. _`markup templates`: xml-templates.html
.. _`text templates`: text-templates.html

--------
Messages
--------

``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:

.. code-block:: genshi

    Please visit ${site.name} 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:

.. code-block:: genshi

    Please visit ${site.name} 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:

.. code-block:: xml

    Um Hilfe zu erhalten, besuchen Sie bitte
    Example (http://example.com/)

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

.. code-block:: genshi

    Please visit the site ${site.name}
    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:

.. code-block:: xml

    Um Hilfe zu erhalten besuchen Sie bitte
    Example (http://example.com/), das ist eine Web-Site

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

.. warning:: 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.

``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:

.. code-block:: genshi

    <i18n:singular>You have ${len(messages)} new message.</i18n:singular>
    <i18n:plural>You have ${len(messages)} 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:

.. code-block:: genshi

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

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).

--------------------
Comments and Domains
--------------------

``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:

.. code-block:: genshi

    Please visit ${site.name} 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.

``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:

.. code-block:: genshi

Hello, world!

``i18n.ctxt``
-------------

Sometimes a source string can have two different meanings. Without resorting to
splitting these two occurrences into different domains, gettext provides a
means to specify a *context* for each translatable string. For instance, the
word "volunteer" can either mean the noun, one who volunteers, or the verb,
to volunteer.

The ``i18n:ctxt`` directive allows you to mark a scope with a particular
context. Here is a rather contrived example:

.. code-block:: genshi

A volunteer can really help their community.
    Why don't you volunteer some time today?

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.

-----------------
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`_.

.. _`setuptools`: http://peak.telecommunity.com/DevCenter/setuptools
.. _`distutils`: http://docs.python.org/dist/dist.html

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:

.. code-block:: ini

  # Python souce
  [python:**.py]

  # Genshi templates
  [genshi:**/templates/**.html]
  include_attrs = title

  [genshi:**/templates/**.txt]
  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.


---------------------
Configuration Options
---------------------

The Genshi extraction plugin for Babel supports the following options:

``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`_.

.. _`text templates`: text-templates.html

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

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

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

``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.

.. note:: 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.


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:

.. code-block:: python

  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:

.. code-block:: python

  from genshi.filters import Translator
  from genshi.template import MarkupTemplate

  template = MarkupTemplate("...")
  translator = Translator(translations.ugettext)
  translator.setup(template)

.. warning:: 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.

.. _`callback function`: loader.html#callback-interface


Related Considerations
======================

If you intend to produce an application that is fully prepared for an
international audience, there are a couple of other things to keep in mind:

-------
Unicode
-------

Use ``unicode`` internally, not encoded bytestrings. Only encode/decode where
data enters or exits the system. This means that your code works with characters
and not just with bytes, which is an important distinction for example when
calculating the length of a piece of text. When you need to decode/encode, it's
probably a good idea to use UTF-8.

-------------
Date and Time
-------------

If your application uses datetime information that should be displayed to users
in different timezones, you should try to work with UTC (universal time)
internally. Do the conversion from and to "local time" when the data enters or
exits the system. Make use the Python `datetime`_ module and the third-party
`pytz`_ package.

--------------------------
Formatting and Locale Data
--------------------------

Make sure you check out the functionality provided by the `Babel`_ project for
things like number and date formatting, locale display strings, etc.

.. _`datetime`: http://docs.python.org/lib/module-datetime.html
.. _`pytz`: http://pytz.sourceforge.net/
