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Chapter 18: Internationalization

Django was originally developed smack in the middle of the United States (literally; Lawrence, Kansas, is less than 40 miles from the geographic center of the continental United States). Like most open source projects, though, Django's community grew to include people from all over the globe. As Django's community became increasingly diverse, internationalization and localization became increasingly important. Since many developers have at best a fuzzy understanding of these terms, we'll define them briefly.

Internationalization refers to the process of designing programs for the potential use of any locale. This includes marking text (like UI elements and error messages) for future translation, abstracting the display of dates and times so that different local standards may be observed, providing support for differing time zones, and generally making sure that the code contains no assumptions about the location if its users. You'll often see "internationalization" abbreviated I18N (the number 18 refers to the number of letters omitted between the initial "I" and the terminal "N").

Localization refers to the process of actually translating an internationalized program for use in a particular locale. You'll sometimes see "localization" abbreviated as L10N.

Django itself is fully internationalized; all strings are marked for translation, and settings control the display of locale-dependent values like dates and times. Django also ships with over 40 different localization files. If you're not a native English speaker, there's a good chance that Django is already is translated into your primary language.

The same internationalization framework used for these localizations is available for you to use in your own code and templates.

In a nutshell, you'll need to add a minimal number of hooks to your Python code 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."

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

Essentially, Django does two things:

  • It lets developers and template authors specify which parts of their applications should be translatable.
  • It uses that information to translate Web applications for particular users according to their language preferences.

Note

Django's translation machinery uses GNU gettext (http://www.gnu.org/software/gettext/) via the standard gettext module that comes with Python.

If You Don't Need Internationalization:

Django's internationalization hooks are enabled by default, which incurs a small bit of overhead. If you don't use internationalization, you should set USE_I18N = False in your settings file. If USE_I18N is set to False, then Django will make some optimizations so as not to load the internationalization machinery.

You'll probably also want to remove 'django.core.context_processors.i18n' from your TEMPLATE_CONTEXT_PROCESSORS setting.

Specifying Translation Strings in Python Code

Translation strings specify "This text should be translated." These strings can appear in your Python code and templates. It's your responsibility to mark translatable strings; the system can only translate strings it knows about.

Standard Translation Functions

Specify a translation string by using the function _(). (Yes, the name of the function is the underscore character.) This function is available globally (i.e., as a built-in language); you don't have to import it.

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

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

The function django.utils.translation.gettext() is identical to _(). This example is identical to the previous one:

from django.utils.translation import gettext
def my_view(request):
    output = gettext("Welcome to my site.")
    return HttpResponse(output)

Most developers prefer to use _(), as it's shorter.

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, make-messages.py, won't be able to find these strings. More on make-messages later.)

The strings you pass to _() or gettext() can take placeholders, specified with Python's standard named-string interpolation syntax, for example:

def my_view(request, n):
    output = _('%(name)s is my name.') % {'name': n}
    return HttpResponse(output)

This technique lets language-specific translations reorder the placeholder text. For example, an English translation may be "Adrian is my name.", while a Spanish translation may be "Me llamo Adrian.", with the placeholder (the name) placed after the translated text instead of before it.

For this reason, you should use named-string interpolation (e.g., %(name)s) instead of positional interpolation (e.g., %s or %d). If you use positional interpolation, translations won't be able to reorder placeholder text.

Marking Strings As No-op

Use the function django.utils.translation.gettext_noop() to mark a string as a translation string without actually translating it at that moment. Strings thus marked aren't translated until the last possible moment.

Use this approach if you have constant strings that should be stored in the original language -- 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.

Lazy Translation

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

For example, to mark a fields's help_text attribute as translatable, do the following:

from django.utils.translation import gettext_lazy

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

In this example, gettext_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.

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

from django.utils.translation import gettext_lazy as _

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

Always use lazy translations in Django models (otherwise they won't be translated correctly on a per-user basis). And it's a good idea to add translations for the field names and table names, too. This means writing explicit verbose_name and verbose_name_plural options in the Meta class:

from django.utils.translation import gettext_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')

Pluralization

Use the function django.utils.translation.ngettext() to specify messages that have different singular and plural forms, for example:

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

ngettext takes three arguments: the singular translation string, the plural translation string, and the number of objects (which is passed to the translation languages as the count variable).

Specifying Translation Strings in Template Code

Using translations in Django 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 marks a string for translations:

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

If you only want to mark a value for translation, but translate it later, use the noop option:

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

It's not possible to use template variables in {% trans %} -- only constant strings, in single or double quotes, are allowed. If your translations require variables (placeholders), use {% blocktrans %}, for example:

{% blocktrans %}This 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 %}, for example:

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

Internally, all block and inline translations use the appropriate gettext/ngettext call.

When you use RequestContext (see Chapter 10), your templates have access to three translation-specific variables:

  • {{ LANGUAGES }} is a list of tuples in which the first element is the language code and the second is the language name (in that language).
  • {{ LANGUAGE_CODE }} is the current user's preferred language, as a string (e.g., en-us). (See the "How Django Discovers Language Preference" section for more information.)
  • {{ LANGUAGE_BIDI }} is the current language's writing system. 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).

You can also load these values using template tags:

{% load i18n %}
{% get_current_language as LANGUAGE_CODE %}
{% get_available_languages as LANGUAGES %}
{% get_current_language_bidi as LANGUAGE_BIDI %}

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, for example:

{% 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 (i.e., the string is translated before being passed to the tag handler functions), so they don't need to be aware of translations.

Creating Language Files

Once you've tagged your strings for later translation, you need to write (or obtain) the language translations themselves. In this section we explain how that works.

Creating Message Files

The first step is to create a message file for a new language. A message file is a plain-text file representing a single language that contains all available translation strings and how they should be represented in the given language. Message files have a .po file extension.

Django comes with a tool, bin/make-messages.py, that automates the creation and maintenance of these files.

To create or update a message file, run this command:

bin/make-messages.py -l de

where de is the language code for the message file you want to create. The language code, in this case, is in locale format. For example, it's pt_BR for Brazilian Portuguese and de_AT for Austrian German. Take a look at thelanguage codes in the django/conf/locale/ directory to see which languages are currently supported.

The script should be run from one of three places:

  • The root django directory (not a Subversion checkout, but the one that is linked to via $PYTHONPATH or is located somewhere on that path)
  • The root directory of your Django project
  • The root directory of your Django application

The script runs over the entire tree it is run on and pulls out all strings marked for translation. It creates (or updates) a message file in the directory conf/locale. In the de example, the file will be conf/locale/de/LC_MESSAGES/django.po.

If run over your project source tree or your application source tree, it will do the same, but the location of the locale directory is locale/LANG/LC_MESSAGES (note the missing conf prefix). The first time you run it on your tree you'll need to create the locale directory.

No gettext?

If you don't have the gettext utilities installed, make-messages.py will create empty files. If that's the case, either install the gettext utilities or just copy the English message file (conf/locale/en/LC_MESSAGES/django.po) and use it as a starting point; it's just an empty translation file.

The format of .po files is straightforward. Each .po file contains a small bit of metadata, such as the translation maintainer's contact information, but the bulk of the file is a list of messages -- simple mappings between translation strings and the actual translated text for the particular language.

For example, if your Django application contains a translation string for the text "Welcome to my site.", like so:

_("Welcome to my site.")

then make-messages.py will have created a .po file containing the following snippet -- a message:

#: path/to/python/module.py:23
msgid "Welcome to my site."
msgstr ""

A quick explanation is in order:

  • msgid is the translation string, which appears in the source. Don't change it.
  • msgstr is where you put the language-specific translation. It starts out empty, so it's your responsibility to change it. Make sure you keep the quotes around your translation.
  • As a convenience, each message includes the file name and line number from which the translation string was gleaned.

Long messages are a special case. The first string directly after msgstr (or msgid) is an empty string. Then the content itself will be written over the next few lines as one string per line. Those strings are directly concatenated. Don't forget trailing spaces within the strings; otherwise, they'll be tacked together without whitespace!

For example, here's a multiline translation (taken from the Spanish localization that ships with Django):

msgid ""
"There's been an error. It's been reported to the site administrators via e-"
"mail and should be fixed shortly. Thanks for your patience."
msgstr ""
"Ha ocurrido un error. Se ha informado a los administradores del sitio "
"mediante correo electrónico y debería arreglarse en breve. Gracias por su "
"paciencia."

Note the trailing spaces.

Mind Your Charset

When creating a .po file with your favorite text editor, first edit the charset line (search for "CHARSET") and set it to the charset you'll be using to edit the content. Generally, UTF-8 should work for most languages, but gettext should handle any charset you throw at it.

To reexamine all source code and templates for new translation strings and update all message files for all languages, run this:

make-messages.py -a

Compiling Message Files

After you create your message file, and each time you make changes to it, you'll need to compile it into a more efficient form, for use by gettext. Do this with the bin/compile-messages.py utility.

This tool runs over all available .po files and creates .mo files, which are binary files optimized for use by gettext. In the same directory from which you ran make-messages.py, run compile-messages.py like this:

bin/compile-messages.py

That's it. Your translations are ready for use.

How Django Discovers Language Preference

Once you've prepared your translations -- or, if you just want to use the translations that are included with Django -- you'll just need to activate translation for your application.

Behind the scenes, Django has a very flexible model of deciding which language should be used -- installation-wide, for a particular user, or both.

To set an installation-wide language preference, set LANGUAGE_CODE in your settings file. Django uses this language as the default translation -- the final attempt if no other translator finds a translation.

If all you want to do is run Django with your native language, and a language file is available for your language, simply set LANGUAGE_CODE.

If you want to let each individual user specify the language he or she prefers, use LocaleMiddleware. LocaleMiddleware enables language selection based on data from the request. It customizes content for each user.

To use LocaleMiddleware, add 'django.middleware.locale.LocaleMiddleware' to your MIDDLEWARE_CLASSES setting. Because middleware order matters, you should follow these guidelines:

  • Make sure it's among the first middleware classes installed.
  • It should come after SessionMiddleware, because LocaleMiddleware makes use of session data.
  • If you use CacheMiddleware, put LocaleMiddleware after it (otherwise users could get cached content from the wrong locale).

For example, your MIDDLEWARE_CLASSES might look like this:

MIDDLEWARE_CLASSES = (
   'django.middleware.common.CommonMiddleware',
   'django.contrib.sessions.middleware.SessionMiddleware',
   'django.middleware.locale.LocaleMiddleware'
)

LocaleMiddleware tries to determine the user's language preference by following this algorithm:

  • First, it looks for a django_language key in the current user's session.
  • Failing that, it looks for a cookie called django_language.
  • Failing that, it looks at the Accept-Language HTTP header. This header is sent by your browser and tells the server which language(s) you prefer, in order of priority. Django tries each language in the header until it finds one with available translations.
  • Failing that, it uses the global LANGUAGE_CODE setting.

In each of these places, the language preference is expected to be in the standard language format, as a string. For example, Brazilian Portuguese is pt-br. If a base language is available but the sub-language specified is not, Django uses the base language. For example, if a user specifies de-at (Austrian German) but Django only has de available, Django uses de.

Only languages listed in the LANGUAGES setting can be selected. If you want to restrict the language selection to a subset of provided languages (because your application doesn't provide all those languages), set your LANGUAGES setting to a list of languages, for example:

LANGUAGES = (
    ('de', _('German')),
    ('en', _('English')),
)

This example restricts languages that are available for automatic selection to German and English (and any sub-language, like de-ch or en-us).

If you define a custom LANGUAGES, it's OK to mark the languages as translation strings -- but use a "dummy" gettext() function, not the one in django.utils.translation. You should never import django.utils.translation from within your settings file, because that module itself depends on the settings, and that would cause a circular import.

The solution is to use a "dummy" gettext() function. Here's a sample settings file:

_ = lambda s: s

LANGUAGES = (
      ('de', _('German')),
      ('en', _('English')),
)

With this arrangement, make-messages.py will still find and mark these strings for translation, but the translation won't happen at runtime, so you'll have to remember to wrap the languages in the real gettext() in any code that uses LANGUAGES at runtime.

The LocaleMiddleware can only select languages for which there is a Django-provided base translation. If you want to provide translations for your application that aren't already in the set of translations in Django's source tree, you'll want to provide at least basic translations for that language. For example, Django uses technical message IDs to translate date formats and time formats -- so you will need at least those translations for the system to work correctly.

A good starting point is to copy the English .po file and to translate at least the technical messages, and maybe the validator messages, too.

Technical message IDs are easily recognized; they're all uppercase. You don't translate the message ID as with other messages; rather, you provide the correct local variant on the provided English value. For example, with DATETIME_FORMAT (or DATE_FORMAT or TIME_FORMAT), this would be the format string that you want to use in your language. The format is identical to the format strings used by the now template tag.

Once LocaleMiddleware determines the user's preference, it makes this preference available as request.LANGUAGE_CODE for each request object. Feel free to read this value in your view code. Here's a simple example:

def hello_world(request, count):
    if request.LANGUAGE_CODE == 'de-at':
        return HttpResponse("You prefer to read Austrian German.")
    else:
        return HttpResponse("You prefer to read another language.")

Note that, with static (i.e. without middleware) translation, the language is in settings.LANGUAGE_CODE, while with dynamic (middleware) translation, it's in request.LANGUAGE_CODE.

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 GET method, with a language parameter set in the query string. If session support is enabled, the view saves the language choice in the user's session. Otherwise, it saves the language choice in a django_language cookie.

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

  • Django looks for a next parameter in the query string.
  • If that doesn't exist or is empty, Django tries the URL in the Referer 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:

<form action="/i18n/setlang/" method="get">
<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>

Using Translations in Your Own Projects

Django looks for translations by following this algorithm:

  • First, it looks for a locale directory in the application directory of the view that's being called. If it finds a translation for the selected language, the translation will be installed.
  • Next, it looks for a locale directory in the project directory. If it finds a translation, the translation will be installed.
  • Finally, it checks the base translation in django/conf/locale.

This way, you can write applications that include their own translations, and you can override base translations in your project path. Or, you can just build a big project out of several applications and put all translations into one big project message file. The choice is yours.

Note

If you're using manually configured settings, the locale directory in the project directory will not be examined, since Django loses the ability to work out the location of the project directory. (Django normally uses the location of the settings file to determine this, and a settings file doesn't exist if you're manually configuring your settings.)

All message file repositories are structured the same way:

  • $APPPATH/locale/<language>/LC_MESSAGES/django.(po|mo)
  • $PROJECTPATH/locale/<language>/LC_MESSAGES/django.(po|mo)
  • All paths listed in LOCALE_PATHS in your settings file are searched in that order for <language>/LC_MESSAGES/django.(po|mo)
  • $PYTHONPATH/django/conf/locale/<language>/LC_MESSAGES/django.(po|mo)

To create message files, you use the same make-messages.py tool as with the Django message files. You only need to be in the right place -- in the directory where either the conf/locale (in case of the source tree) or the locale/ (in case of application messages or project messages) directory is located. And you use the same compile-messages.py to produce the binary django.mo files that are used by gettext.

Application message files are a bit complicated to discover -- they need the LocaleMiddleware. If you don't use the middleware, only the Django message files and project message files will be processed.

Finally, you should give some thought to the structure of your translation files. If your applications need to be delivered to other users and will be used in other projects, you might want to use application-specific translations. But using application-specific translations and project translations could produce weird problems with make-messages. make-messages will traverse all directories below the current path and so might put message IDs into the project message file that are already in application message files.

The easiest way out is to store applications that are not part of the project (and so carry their own translations) outside the project tree. That way, make-messages on the project level will only translate strings that are connected to your explicit project and not strings that are distributed independently.

Translations and JavaScript

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 and friends from within JavaScript.

The javascript_catalog View

The main solution to these problems is the javascript_catalog view, which generates 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're depending upon 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 plus signs (+) in the URL. This is especially useful if your pages use code from different applications, 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="/path/to/jsi18n/"></script>

This is how the admin site fetches the translation catalog from the server. 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 even is a ngettext interface and a string interpolation function:

d = {
    count: 10
};
s = interpolate(ngettext('this is %(count)s object', 'this are %(count)s objects', d.count), d);

The interpolate function supports both positional interpolation and named interpolation. So the preceding code could have been written as follows:

s = interpolate(ngettext('this is %s object', 'this are %s objects', 11), [11]);

The interpolation syntax is borrowed from Python. You shouldn't go over the top with string interpolation, though -- this is still JavaScript, so the code will have to do 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 (e.g., in conjunction with ngettext to produce proper pluralization).

Creating JavaScript Translation Catalogs

You create and update the translation catalogs the same way as the other Django translation catalogs: with the `make-messages.py` tool. The only difference is you need to provide a -d djangojs parameter, like this:

make-messages.py -d djangojs -l de

This creates or updates the translation catalog for JavaScript for German. After updating translation catalogs, just run compile-messages.py the same way as you do with normal Django translation catalogs.

Notes for Users Familiar with gettext

If you know gettext, you might note these special things in the way Django does translation:

  • The string domain is django or djangojs. The string domain is used to differentiate between different programs that store their data in a common message-file library (usually /usr/share/locale/). The django domain is used for Python and template translation strings, and is loaded into the global translation catalogs. The djangojs domain is only used for JavaScript translation catalogs to make sure that those are as small as possible.
  • Django only uses gettext and gettext_noop. That's because Django always uses DEFAULT_CHARSET strings internally. There isn't much benefit to using ugettext, because you'll always need to produce UTF-8 anyway.
  • Django doesn't use xgettext alone. It uses Python wrappers around xgettext and msgfmt. That's mostly for convenience.

What's Next?

This chapter mostly concludes our coverage of Django's features. You should now know enough to start producing your own Django sites.

However, writing the code is only the first step in deploying a successful Web site. The next two chapters cover the things you'll need to know if you want your site to survive in the real world. Chapter 19 discuses how you can secure your sites and your users from malicious attackers, and Chapter 20 details how to deploy a Django application onto one or many servers.

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