Using Translations

The other big part next to date formatting are translations. For that, Quart uses gettext together with Babel. The idea of gettext is that you can mark certain strings as translatable and a tool will pick all those up, collect them in a separate file for you to translate. At runtime the original strings (which should be English) will be replaced by the language you selected.

There are two functions responsible for translating: gettext() and ngettext(). The first to translate singular strings and the second to translate strings that might become plural. Here some examples:

gettext and ngettext examples

from datetime import datetime
from quart import quart
from quart_babel import Babel, gettext, ngettext

app = Quart(__name__)
Babel(app)

@app.route('/')
async def index():
    # Simple string
    simple_string = gettext(u'A simple string')

    # String with value
    value_string = gettext(u'Value: %(value)s', value=42)

    # Plural string
    p_string = ngettext(u'%(num)s Apple', u'%(num)s Apples', number_of_apples)

    # .... Additional route code here, such as return.

Additionally if you want to use constant strings somewhere in your application and define them outside of a request, you can use a lazy strings. Lazy strings will not be evaluated until they are actually used. To use such a lazy string, use the lazy_gettext() function:

lazy_gettext example

from quart_babel import lazy_gettext

class MyForm(formlibrary.FormBase):
    success_message = lazy_gettext(u'The form was successfully saved.')

Translating Applications

First you need to mark all the strings you want to translate in your application with gettext() or ngettext(). After that, it’s time to create a .pot file. A .pot file contains all the strings and is the template for a .po file which contains the translated strings. Babel can do all that for you.

First of all you have to get into the folder where you have your application and create a mapping file. For typical Flask applications, this is what you want in there:

[python: **.py]
[jinja2: **/templates/**.html]
extensions=jinja2.ext.autoescape,jinja2.ext.with_

Save it as babel.cfg or something similar next to your application. Then it’s time to run the pybabel command that comes with Babel to extract your strings:

$ pybabel extract -F babel.cfg -o messages.pot .

If you are using the lazy_gettext() function you should tell pybabel that it should also look for such function calls:

$ pybabel extract -F babel.cfg -k lazy_gettext -o messages.pot .

This will use the mapping from the babel.cfg file and store the generated template in messages.pot. Now we can create the first translation. For example to translate to German use this command:

.. code-block:: console

$ pybabel init -i messages.pot -d translations -l de

-d translations tells pybabel to store the translations in this folder. This is where Flask-BabelPlus will look for translations. Put it next to your template folder.

Now edit the translations/de/LC_MESSAGES/messages.po file as needed. Check out some gettext tutorials if you feel lost.

To compile the translations for use, pybabel helps again:

$ pybabel compile -d translations

What if the strings change? Create a new messages.pot like above and then let pybabel merge the changes:

$ pybabel update -i messages.pot -d translations

Afterwards some strings might be marked as fuzzy (where it tried to figure out if a translation matched a changed key). If you have fuzzy entries, make sure to check them by hand and remove the fuzzy flag before compiling.

Quart-Babel looks for message catalogs in translations directory which should be located under Quart application directory. Default domain is “messages”.

For example, if you want to have translations for German, Spanish and French, directory structure should look like this:

translations/de/LC_MESSAGES/messages.mo translations/sp/LC_MESSAGES/messages.mo translations/fr/LC_MESSAGES/messages.mo

Translation Domains

By default, Quart-Babel will use “messages” domain, which will make it use translations from the messages.mo file. It is not very convenient for third-party Quart extensions, which might want to localize themselves without requiring user to merge their translations into “messages” domain.

Quart-Babel allows extension developers to specify which translation domain to use:

from quart_babel import Domain

mydomain = Domain(domain='myext')

mydomain.lazy_gettext('Hello World!')

Domain contains all gettext-related methods (gettext(), ngettext(), etc).

In previous example, localizations will be read from the myext.mo files, but they have to be located in translations directory under users Flask application. If extension is distributed with the localizations, it is possible to specify their location:

from quart_babel import Domain

from quart_myext import translations
mydomain = Domain(translations.__path__[0])

mydomain will look for translations in extension directory with default (messages) domain.

It is also possible to change the translation domain used by default, either for each app or per request.

To set the Domain that will be used in an app, pass it to Babel on initialization:

from quart import Quart
from quart_babel import Babel, Domain

app = Quart(__name__)
domain = Domain(domain='myext')
babel = Babel(app, default_domain=domain)

Translations will then come from the myext.mo files by default.

To change the default domain in a request context, call the as_default() method from within the request context:

from quart import Quart
from quart_babel import Babel, Domain, gettext

app = Quart(__name__)
domain = Domain(domain='myext')
babel = Babel(app)

@app.route('/path')
def demopage():
    domain.as_default()

    return gettext('Hello World!')

Hello World! will get translated using the myext.mo files, but other requests will use the default messages.mo. Note that a Babel must be initialized for the app for translations to work at all.