django/docs/topics/i18n/internationalization.txt

.. _topics-i18n-internationalization:

====================
Internationalization
====================

Overview
========

The goal of internationalization is to allow a single Web application to offer
its content and functionality in multiple languages and locales.

For text translations, you, the Django developer, can accomplish this goal by
adding a minimal amount of hooks to your Python and templates. These hooks
are called **translation strings**. They tell Django: "This text should be
translated into the end user's language, if a translation for this text is
available in that language." It's your responsibility to mark translatable
strings; the system can only translate strings it knows about.

Django takes care of using these hooks to translate Web apps, on the fly,
according to users' language preferences.

Specifying translation strings: In Python code
==============================================

Standard translation
--------------------

Specify a translation string by using the function ``ugettext()``. It's
convention to import this as a shorter alias, ``_``, to save typing.

.. note::
    Python's standard library ``gettext`` module installs ``_()`` into the
    global namespace, as an alias for ``gettext()``. In Django, we have chosen
    not to follow this practice, for a couple of reasons:

      1. For international character set (Unicode) support, ``ugettext()`` is
         more useful than ``gettext()``. Sometimes, you should be using
         ``ugettext_lazy()`` as the default translation method for a particular
         file. Without ``_()`` in the global namespace, the developer has to
         think about which is the most appropriate translation function.

      2. The underscore character (``_``) is used to represent "the previous
         result" in Python's interactive shell and doctest tests. Installing a
         global ``_()`` function causes interference. Explicitly importing
         ``ugettext()`` as ``_()`` avoids this problem.

.. highlightlang:: python

In this example, the text ``"Welcome to my site."`` is marked as a translation
string::

    from django.utils.translation import ugettext as _

    def my_view(request):
        output = _("Welcome to my site.")
        return HttpResponse(output)

Obviously, you could code this without using the alias. This example is
identical to the previous one::

    from django.utils.translation import ugettext

    def my_view(request):
        output = ugettext("Welcome to my site.")
        return HttpResponse(output)

Translation works on computed values. This example is identical to the previous
two::

    def my_view(request):
        words = ['Welcome', 'to', 'my', 'site.']
        output = _(' '.join(words))
        return HttpResponse(output)

Translation works on variables. Again, here's an identical example::

    def my_view(request):
        sentence = 'Welcome to my site.'
        output = _(sentence)
        return HttpResponse(output)

(The caveat with using variables or computed values, as in the previous two
examples, is that Django's translation-string-detecting utility,
``django-admin.py makemessages``, won't be able to find these strings. More on
``makemessages`` later.)

The strings you pass to ``_()`` or ``ugettext()`` can take placeholders,
specified with Python's standard named-string interpolation syntax. Example::

    def my_view(request, m, d):
        output = _('Today is %(month)s, %(day)s.') % {'month': m, 'day': d}
        return HttpResponse(output)

This technique lets language-specific translations reorder the placeholder
text. For example, an English translation may be ``"Today is November, 26."``,
while a Spanish translation may be ``"Hoy es 26 de Noviembre."`` -- with the
placeholders (the month and the day) with their positions swapped.

For this reason, you should use named-string interpolation (e.g., ``%(day)s``)
instead of positional interpolation (e.g., ``%s`` or ``%d``) whenever you
have more than a single parameter. If you used positional interpolation,
translations wouldn't be able to reorder placeholder text.

Marking strings as no-op
------------------------

Use the function ``django.utils.translation.ugettext_noop()`` to mark a string
as a translation string without translating it. The string is later translated
from a variable.

Use this if you have constant strings that should be stored in the source
language because they are exchanged over systems or users -- such as strings in
a database -- but should be translated at the last possible point in time, such
as when the string is presented to the user.

Pluralization
-------------

Use the function ``django.utils.translation.ungettext()`` to specify pluralized
messages.

``ungettext`` takes three arguments: the singular translation string, the plural
translation string and the number of objects.

This function is useful when you need your Django application to be localizable
to languages where the number and complexity of `plural forms
<http://www.gnu.org/software/gettext/manual/gettext.html#Plural-forms>`_ is
greater than the two forms used in English ('object' for the singular and
'objects' for all the cases where ``count`` is different from zero, irrespective
of its value.)

For example::

    from django.utils.translation import ungettext
    def hello_world(request, count):
        page = ungettext('there is %(count)d object', 'there are %(count)d objects', count) % {
            'count': count,
        }
        return HttpResponse(page)

In this example the number of objects is passed to the translation languages as
the ``count`` variable.

Lets see a slightly more complex usage example::

    from django.utils.translation import ungettext

    count = Report.objects.count()
    if count == 1:
        name = Report._meta.verbose_name
    else:
        name = Report._meta.verbose_name_plural

    text = ungettext(
            'There is %(count)d %(name)s available.',
            'There are %(count)d %(name)s available.',
            count
    ) % {
        'count': count,
        'name': name
    }

Here we reuse localizable, hopefully already translated literals (contained in
the ``verbose_name`` and ``verbose_name_plural`` model ``Meta`` options) for
other parts of the sentence so all of it is consistently based on the
cardinality of the elements at play.

.. _pluralization-var-notes:

.. note::

    When using this technique, make sure you use a single name for every
    extrapolated variable included in the literal. In the example above note how
    we used the ``name`` Python variable in both translation strings. This
    example would fail::

        from django.utils.translation import ungettext
        from myapp.models import Report

        count = Report.objects.count()
        d = {
            'count': count,
            'name': Report._meta.verbose_name
            'plural_name': Report._meta.verbose_name_plural
        }
        text = ungettext(
                'There is %(count)d %(name)s available.',
                'There are %(count)d %(plural_name)s available.',
                count
        ) % d

    You would get a ``a format specification for argument 'name', as in
    'msgstr[0]', doesn't exist in 'msgid'`` error when running
    ``django-admin.py compilemessages`` or a ``KeyError`` Python exception at
    runtime.

.. _lazy-translations:

Lazy translation
----------------

Use the function ``django.utils.translation.ugettext_lazy()`` to translate
strings lazily -- when the value is accessed rather than when the
``ugettext_lazy()`` function is called.

For example, to translate a model's ``help_text``, do the following::

    from django.utils.translation import ugettext_lazy

    class MyThing(models.Model):
        name = models.CharField(help_text=ugettext_lazy('This is the help text'))

In this example, ``ugettext_lazy()`` stores a lazy reference to the string --
not the actual translation. The translation itself will be done when the string
is used in a string context, such as template rendering on the Django admin
site.

The result of a ``ugettext_lazy()`` call can be used wherever you would use a
unicode string (an object with type ``unicode``) in Python. If you try to use
it where a bytestring (a ``str`` object) is expected, things will not work as
expected, since a ``ugettext_lazy()`` object doesn't know how to convert
itself to a bytestring.  You can't use a unicode string inside a bytestring,
either, so this is consistent with normal Python behavior. For example::

    # This is fine: putting a unicode proxy into a unicode string.
    u"Hello %s" % ugettext_lazy("people")

    # This will not work, since you cannot insert a unicode object
    # into a bytestring (nor can you insert our unicode proxy there)
    "Hello %s" % ugettext_lazy("people")

If you ever see output that looks like ``"hello
<django.utils.functional...>"``, you have tried to insert the result of
``ugettext_lazy()`` into a bytestring. That's a bug in your code.

If you don't like the verbose name ``ugettext_lazy``, you can just alias it as
``_`` (underscore), like so::

    from django.utils.translation import ugettext_lazy as _

    class MyThing(models.Model):
        name = models.CharField(help_text=_('This is the help text'))

Always use lazy translations in :ref:`Django models <topics-db-models>`.
Field names and table names should be marked for translation (otherwise, they
won't be translated in the admin interface). This means writing explicit
``verbose_name`` and ``verbose_name_plural`` options in the ``Meta`` class,
though, rather than relying on Django's default determination of
``verbose_name`` and ``verbose_name_plural`` by looking at the model's class
name::

    from django.utils.translation import ugettext_lazy as _

    class MyThing(models.Model):
        name = models.CharField(_('name'), help_text=_('This is the help text'))
        class Meta:
            verbose_name = _('my thing')
            verbose_name_plural = _('mythings')

Working with lazy translation objects
-------------------------------------

.. highlightlang:: python

Using ``ugettext_lazy()`` and ``ungettext_lazy()`` to mark strings in models
and utility functions is a common operation. When you're working with these
objects elsewhere in your code, you should ensure that you don't accidentally
convert them to strings, because they should be converted as late as possible
(so that the correct locale is in effect). This necessitates the use of a
couple of helper functions.

Joining strings: string_concat()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Standard Python string joins (``''.join([...])``) will not work on lists
containing lazy translation objects. Instead, you can use
``django.utils.translation.string_concat()``, which creates a lazy object that
concatenates its contents *and* converts them to strings only when the result
is included in a string. For example::

    from django.utils.translation import string_concat
    ...
    name = ugettext_lazy(u'John Lennon')
    instrument = ugettext_lazy(u'guitar')
    result = string_concat([name, ': ', instrument])

In this case, the lazy translations in ``result`` will only be converted to
strings when ``result`` itself is used in a string (usually at template
rendering time).

The allow_lazy() decorator
~~~~~~~~~~~~~~~~~~~~~~~~~~

Django offers many utility functions (particularly in ``django.utils``) that
take a string as their first argument and do something to that string. These
functions are used by template filters as well as directly in other code.

If you write your own similar functions and deal with translations, you'll
face the problem of what to do when the first argument is a lazy translation
object. You don't want to convert it to a string immediately, because you might
be using this function outside of a view (and hence the current thread's locale
setting will not be correct).

For cases like this, use the ``django.utils.functional.allow_lazy()``
decorator. It modifies the function so that *if* it's called with a lazy
translation as the first argument, the function evaluation is delayed until it
needs to be converted to a string.

For example::

    from django.utils.functional import allow_lazy

    def fancy_utility_function(s, ...):
        # Do some conversion on string 's'
        ...
    fancy_utility_function = allow_lazy(fancy_utility_function, unicode)

The ``allow_lazy()`` decorator takes, in addition to the function to decorate,
a number of extra arguments (``*args``) specifying the type(s) that the
original function can return. Usually, it's enough to include ``unicode`` here
and ensure that your function returns only Unicode strings.

Using this decorator means you can write your function and assume that the
input is a proper string, then add support for lazy translation objects at the
end.

Specifying translation strings: In template code
================================================

.. highlightlang:: html+django

Translations in :ref:`Django templates <topics-templates>` uses two template
tags and a slightly different syntax than in Python code. To give your template
access to these tags, put ``{% load i18n %}`` toward the top of your template.

The ``{% trans %}`` template tag translates either a constant string
(enclosed in single or double quotes) or variable content::

    <title>{% trans "This is the title." %}</title>
    <title>{% trans myvar %}</title>

If the ``noop`` option is present, variable lookup still takes place but the
translation is skipped. This is useful when "stubbing out" content that will
require translation in the future::

    <title>{% trans "myvar" noop %}</title>

Internally, inline translations use an ``ugettext`` call.

It's not possible to mix a template variable inside a string within ``{% trans
%}``. If your translations require strings with variables (placeholders), use
``{% blocktrans %}``::

    {% blocktrans %}This string will have {{ value }} inside.{% endblocktrans %}

To translate a template expression -- say, using template filters -- you need
to bind the expression to a local variable for use within the translation
block::

    {% blocktrans with value|filter as myvar %}
    This will have {{ myvar }} inside.
    {% endblocktrans %}

If you need to bind more than one expression inside a ``blocktrans`` tag,
separate the pieces with ``and``::

    {% blocktrans with book|title as book_t and author|title as author_t %}
    This is {{ book_t }} by {{ author_t }}
    {% endblocktrans %}

To pluralize, specify both the singular and plural forms with the
``{% plural %}`` tag, which appears within ``{% blocktrans %}`` and
``{% endblocktrans %}``. Example::

    {% blocktrans count list|length as counter %}
    There is only one {{ name }} object.
    {% plural %}
    There are {{ counter }} {{ name }} objects.
    {% endblocktrans %}

When you use the pluralization feature and bind additional values to local
variables apart from the counter value that selects the translated literal to be
used, have in mind that the ``blocktrans`` construct is internally converted
to an ``ungettext`` call. This means the same :ref:`notes regarding ungettext
variables <pluralization-var-notes>` apply.

Each ``RequestContext`` has access to three translation-specific variables:

    * ``LANGUAGES`` is a list of tuples in which the first element is the
      :term:`language code` and the second is the language name (translated into
      the currently active locale).

    * ``LANGUAGE_CODE`` is the current user's preferred language, as a string.
      Example: ``en-us``. (See :ref:`how-django-discovers-language-preference`.)

    * ``LANGUAGE_BIDI`` is the current locale's direction. If True, it's a
      right-to-left language, e.g.: Hebrew, Arabic. If False it's a
      left-to-right language, e.g.: English, French, German etc.


If you don't use the ``RequestContext`` extension, you can get those values with
three tags::

    {% get_current_language as LANGUAGE_CODE %}
    {% get_available_languages as LANGUAGES %}
    {% get_current_language_bidi as LANGUAGE_BIDI %}

These tags also require a ``{% load i18n %}``.

Translation hooks are also available within any template block tag that accepts
constant strings. In those cases, just use ``_()`` syntax to specify a
translation string::

    {% some_special_tag _("Page not found") value|yesno:_("yes,no") %}

In this case, both the tag and the filter will see the already-translated
string, so they don't need to be aware of translations.

.. note::
    In this example, the translation infrastructure will be passed the string
    ``"yes,no"``, not the individual strings ``"yes"`` and ``"no"``. The
    translated string will need to contain the comma so that the filter
    parsing code knows how to split up the arguments. For example, a German
    translator might translate the string ``"yes,no"`` as ``"ja,nein"``
    (keeping the comma intact).

.. _Django templates: ../templates_python/

Specifying translation strings: In JavaScript code
==================================================

Adding translations to JavaScript poses some problems:

    * JavaScript code doesn't have access to a ``gettext`` implementation.

    * JavaScript code doesn't have access to .po or .mo files; they need to be
      delivered by the server.

    * The translation catalogs for JavaScript should be kept as small as
      possible.

Django provides an integrated solution for these problems: It passes the
translations into JavaScript, so you can call ``gettext``, etc., from within
JavaScript.

The ``javascript_catalog`` view
-------------------------------

The main solution to these problems is the ``javascript_catalog`` view, which
sends out a JavaScript code library with functions that mimic the ``gettext``
interface, plus an array of translation strings. Those translation strings are
taken from the application, project or Django core, according to what you
specify in either the info_dict or the URL.

You hook it up like this::

    js_info_dict = {
        'packages': ('your.app.package',),
    }

    urlpatterns = patterns('',
        (r'^jsi18n/$', 'django.views.i18n.javascript_catalog', js_info_dict),
    )

Each string in ``packages`` should be in Python dotted-package syntax (the
same format as the strings in ``INSTALLED_APPS``) and should refer to a package
that contains a ``locale`` directory. If you specify multiple packages, all
those catalogs are merged into one catalog. This is useful if you have
JavaScript that uses strings from different applications.

You can make the view dynamic by putting the packages into the URL pattern::

    urlpatterns = patterns('',
        (r'^jsi18n/(?P<packages>\S+?)/$', 'django.views.i18n.javascript_catalog'),
    )

With this, you specify the packages as a list of package names delimited by '+'
signs in the URL. This is especially useful if your pages use code from
different apps and this changes often and you don't want to pull in one big
catalog file. As a security measure, these values can only be either
``django.conf`` or any package from the ``INSTALLED_APPS`` setting.

Using the JavaScript translation catalog
----------------------------------------

To use the catalog, just pull in the dynamically generated script like this::

    <script type="text/javascript" src={% url django.views.i18n.javascript_catalog %}"></script>

This uses reverse URL lookup to find the URL of the JavaScript catalog view.
When the catalog is loaded, your JavaScript code can use the standard
``gettext`` interface to access it::

    document.write(gettext('this is to be translated'));

There is also an ``ngettext`` interface::

    var object_cnt = 1 // or 0, or 2, or 3, ...
    s = ngettext('literal for the singular case',
            'literal for the plural case', object_cnt);

and even a string interpolation function::

    function interpolate(fmt, obj, named);

The interpolation syntax is borrowed from Python, so the ``interpolate``
function supports both positional and named interpolation:

    * Positional interpolation: ``obj`` contains a JavaScript Array object
      whose elements values are then sequentially interpolated in their
      corresponding ``fmt`` placeholders in the same order they appear.
      For example::

        fmts = ngettext('There is %s object. Remaining: %s',
                'There are %s objects. Remaining: %s', 11);
        s = interpolate(fmts, [11, 20]);
        // s is 'There are 11 objects. Remaining: 20'

    * Named interpolation: This mode is selected by passing the optional
      boolean ``named`` parameter as true. ``obj`` contains a JavaScript
      object or associative array. For example::

        d = {
            count: 10
            total: 50
        };

        fmts = ngettext('Total: %(total)s, there is %(count)s object',
        'there are %(count)s of a total of %(total)s objects', d.count);
        s = interpolate(fmts, d, true);

You shouldn't go over the top with string interpolation, though: this is still
JavaScript, so the code has to make repeated regular-expression substitutions.
This isn't as fast as string interpolation in Python, so keep it to those
cases where you really need it (for example, in conjunction with ``ngettext``
to produce proper pluralizations).

The ``set_language`` redirect view
==================================

As a convenience, Django comes with a view, ``django.views.i18n.set_language``,
that sets a user's language preference and redirects back to the previous page.

Activate this view by adding the following line to your URLconf::

    (r'^i18n/', include('django.conf.urls.i18n')),

(Note that this example makes the view available at ``/i18n/setlang/``.)

The view expects to be called via the ``POST`` method, with a ``language``
parameter set in request. If session support is enabled, the view
saves the language choice in the user's session. Otherwise, it saves the
language choice in a cookie that is by default named ``django_language``.
(The name can be changed through the ``LANGUAGE_COOKIE_NAME`` setting.)

After setting the language choice, Django redirects the user, following this
algorithm:

    * Django looks for a ``next`` parameter in the ``POST`` data.
    * If that doesn't exist, or is empty, Django tries the URL in the
      ``Referrer`` header.
    * If that's empty -- say, if a user's browser suppresses that header --
      then the user will be redirected to ``/`` (the site root) as a fallback.

Here's example HTML template code:

.. code-block:: html+django

    <form action="/i18n/setlang/" method="post">
    {% csrf_token %}
    <input name="next" type="hidden" value="/next/page/" />
    <select name="language">
    {% for lang in LANGUAGES %}
    <option value="{{ lang.0 }}">{{ lang.1 }}</option>
    {% endfor %}
    </select>
    <input type="submit" value="Go" />
    </form>