Flask-BabelEx¶
Flask-BabelEx is an extension to Flask that adds i18n and l10n support to
any Flask application with the help of babel, pytz and
speaklater. It has builtin support for date formatting with timezone
support as well as a very simple and friendly interface to gettext
translations.
Installation¶
Install the extension with one of the following commands:
$ easy_install Flask-BabelEx
or alternatively if you have pip installed:
$ pip install Flask-BabelEx
Please note that Flask-BabelEx requires Jinja 2.5. If you are using an older version you will have to upgrade or disable the Jinja support.
Configuration¶
To get started all you need to do is to instanciate a Babel
object after configuring the application:
from flask import Flask
from flask_babelex import Babel
app = Flask(__name__)
app.config.from_pyfile('mysettings.cfg')
babel = Babel(app)
The babel object itself can be used to configure the babel support further. Babel has two configuration values that can be used to change some internal defaults:
BABEL_DEFAULT_LOCALE |
The default locale to use if no locale
selector is registered. This defaults
to |
BABEL_DEFAULT_TIMEZONE |
The timezone to use for user facing dates.
This defaults to |
For more complex applications you might want to have multiple applications
for different users which is where selector functions come in handy. The
first time the babel extension needs the locale (language code) of the
current user it will call a localeselector()
function, and
the first time the timezone is needed it will call a
timezoneselector()
function.
If any of these methods return None the extension will automatically
fall back to what’s in the config. Furthermore for efficiency that
function is called only once and the return value then cached. If you
need to switch the language between a request, you can refresh()
the
cache.
Example selector functions:
from flask import g, request
@babel.localeselector
def get_locale():
# if a user is logged in, use the locale from the user settings
user = getattr(g, 'user', None)
if user is not None:
return user.locale
# otherwise try to guess the language from the user accept
# header the browser transmits. We support de/fr/en in this
# example. The best match wins.
return request.accept_languages.best_match(['de', 'fr', 'en'])
@babel.timezoneselector
def get_timezone():
user = getattr(g, 'user', None)
if user is not None:
return user.timezone
The example above assumes that the current user is stored on the
flask.g
object.
Formatting Dates¶
To format dates you can use the format_datetime()
,
format_date()
, format_time()
and format_timedelta()
functions. They all accept a datetime.datetime
(or
datetime.date
, datetime.time
and
datetime.timedelta
) object as first parameter and then optionally
a format string. The application should use naive datetime objects
internally that use UTC as timezone. On formatting it will automatically
convert into the user’s timezone in case it differs from UTC.
To play with the date formatting from the console, you can use the
test_request_context()
method:
>>> app.test_request_context().push()
Here some examples:
>>> from flask_babelex import format_datetime
>>> from datetime import datetime
>>> format_datetime(datetime(1987, 3, 5, 17, 12))
u'Mar 5, 1987 5:12:00 PM'
>>> format_datetime(datetime(1987, 3, 5, 17, 12), 'full')
u'Thursday, March 5, 1987 5:12:00 PM World (GMT) Time'
>>> format_datetime(datetime(1987, 3, 5, 17, 12), 'short')
u'3/5/87 5:12 PM'
>>> format_datetime(datetime(1987, 3, 5, 17, 12), 'dd mm yyy')
u'05 12 1987'
>>> format_datetime(datetime(1987, 3, 5, 17, 12), 'dd mm yyyy')
u'05 12 1987'
And again with a different language:
>>> app.config['BABEL_DEFAULT_LOCALE'] = 'de'
>>> from flask_babelex import refresh; refresh()
>>> format_datetime(datetime(1987, 3, 5, 17, 12), 'EEEE, d. MMMM yyyy H:mm')
u'Donnerstag, 5. M\xe4rz 1987 17:12'
For more format examples head over to the babel documentation.
Using Translations¶
The other big part next to date formatting are translations. For that,
Flask 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 app, 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:
from flask_babelex import gettext, ngettext
gettext(u'A simple string')
gettext(u'Value: %(value)s', value=42)
ngettext(u'%(num)s Apple', u'%(num)s Apples', number_of_apples)
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:
from flask_babelex import lazy_gettext
class MyForm(formlibrary.FormBase):
success_message = lazy_gettext(u'The form was successfully saved.')
So how does Flask-BabelEx find the translations? Well first you have to create some. Here is how you do it:
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:
$ pybabel init -i messages.pot -d translations -l de
-d translations
tells pybabel to store the translations in this
folder. This is where Flask-BabelEx 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.
Flask-BabelEx looks for message catalogs in translations
directory
which should be located under Flask 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, Flask-BabelEx will use “messages” domain, which will make it use translations
from the messages.mo
file. It is not very convenient for third-party Flask extensions,
which might want to localize themselves without requiring user to merge their translations
into “messages” domain.
Flask-BabelEx allows extension developers to specify which translation domain to use:
from flask_babelex 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 flask_babelex import Domain
from flask.ext.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 flask import Flask
from flask_babelex import Babel, Domain
app = Flask(__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 flask import Flask
from flask_babelex import Babel, Domain, gettext
app = Flask(__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.
Troubleshooting¶
On Snow Leopard pybabel will most likely fail with an exception. If this happens, check if this command outputs UTF-8:
$ echo $LC_CTYPE
UTF-8
This is a OS X bug unfortunately. To fix it, put the following lines into
your ~/.profile
file:
export LC_CTYPE=en_US.utf-8
Then restart your terminal.
API¶
This part of the documentation documents each and every public class or function from Flask-BabelEx.