How to Use oslo.i18n in Your Application or Library¶
At the command line:
$ pip install oslo.i18n
Creating an Integration Module¶
To use oslo.i18n in a project (e.g. myapp), you will need to create a
small integration module to hold an instance of
TranslatorFactory and references to
the marker functions the factory creates.
Libraries probably do not want to expose the new integration module
as part of their public API, so rather than naming it
myapp.i18n it should be called
myapp._i18n to indicate that
it is a private implementation detail, and not meant to be used
outside of the library’s own code.
Starting with the Pike series, OpenStack no longer supports log translation. It is not necessary to add translation instructions to new code, and the instructions can be removed from old code. Refer to the email thread understanding log domain change on the openstack-dev mailing list for more details.
DOMAIN = "myapp"
_translators = oslo_i18n.TranslatorFactory(domain=DOMAIN)
# The primary translation function using the well-known name "_"
_ = _translators.primary
# The contextual translation function using the name "_C"
# requires oslo.i18n >=2.1.0
_C = _translators.contextual_form
# The plural translation function using the name "_P"
# requires oslo.i18n >=2.1.0
_P = _translators.plural_form
Then, in the rest of your code, use the appropriate marker function for each message:
from myapp._i18n import _
variable = "openstack"
some_object.name_msg = _('my name is: %s') % variable
# Log only, log messages are no longer translated
# Raise only
raise RuntimeError(_('exception message'))
# Log and Raise
msg = _('Unexpected error message')
The import of multiple modules from _i18n on a single line is a valid exception to OpenStack Style Guidelines for import statements.
It is important to use the marker functions (e.g. _), rather than the longer form of the name, because the tool that scans the source code for translatable strings looks for the marker function names.
The old method of installing a version of
_() in the builtins
namespace is deprecated. Modifying the global namespace affects
libraries as well as the application, so it may interfere with
proper message catalog lookups. Calls to
gettextutils.install() should be replaced with the
application or library integration module described here.
Handling hacking Objections to Imports¶
The OpenStack Style Guidelines prefer importing modules and accessing names from those modules after import, rather than importing the names directly. For example:
from foo import bar
The linting tool hacking will typically complain about importing
names from within modules. It is acceptable to bypass this for the
translation marker functions, because they must have specific names
and their use pattern is dictated by the message catalog extraction
tools rather than our style guidelines. To bypass the hacking check
for imports from this integration module, add an import exception to
import_exceptions = myapp._i18n
Lazy translation delays converting a message string to the translated form as long as possible, including possibly never if the message is not logged or delivered to the user in some other way. It also supports logging translated messages in multiple languages, by configuring separate log handlers.
Lazy translation is implemented by returning a special object from the translation function, instead of a unicode string. That special message object supports some, but not all, string manipulation APIs. For example, concatenation with addition is not supported, but interpolation of variables is supported. Depending on how translated strings are used in an application, these restrictions may mean that lazy translation cannot be used, and so it is not enabled by default.
To enable lazy translation, call
translate() to translate strings to
a specific locale.
translate() handles delayed translation and
strings that have already been translated immediately. It should be
used at the point where the locale to be used is known, which is often
just prior to the message being returned or a log message being
trans_msg = oslo_i18n.translate(msg, my_locale)
If a locale is not specified the default locale is used.
Only the languages that have translations provided are available for
translation. To determine which languages are available the
get_available_languages() is provided. The integration
module provides a domain defined specific function.
languages = myapp._i18n.get_available_languages()
Displaying translated messages¶
Several preparations are required to display translated messages in your running application.
- Preferred language
You need to specify your preferred language through an environment variable. The preferred language can be specified by
LANGUAGE(A former one has a priority).
oslo_i18n.translate()can be used to translate a string to override the preferred language.
You need to use
enable_lazy()to override the preferred language by using
- Locale directory
gettextlooks for binary
mofiles for the given domain using the path
<localedir>/<language>/LC_MESSAGES/<domain>.mo. The default locale directory varies on distributions, and it is
/usr/share/localein most cases.
If you store message catalogs in a different location, you need to specify the location via an environment variable named
<DOMAIN>is an upper-case domain name with replacing
-. For example,
NEUTRON_LOCALEDIRfor a domain
OSLO_I18N_LOCALEDIRfor a domain
When you specify locale directories via
<DOMAIN>_LOCALEDIRenvironment variables, you need to specify an environment variable per domain. More concretely, if your application using a domain
myapp` uses oslo.policy, you need to specify both ``MYAPP_LOCALEDIRand
OSLO_POLICY_LOCALEDIRto ensure that translation messages from both your application and oslo.policy are displayed.