oslo.i18n/doc/source/usage.rst

How to Use oslo.i18n in Your Application or Library

Installing

At the command line:

$ pip install oslo.i18n

Creating an Integration Module

To use oslo.i18n in a project, you will need to create a small integration module to hold an instance of ~oslo_i18n.TranslatorFactory and references to the marker functions the factory creates.

# app/i18n.py

import oslo_i18n

_translators = oslo_i18n.TranslatorFactory(domain='myapp')

# The primary translation function using the well-known name "_"
_ = _translators.primary

# Translators for log levels.
#
# The abbreviated names are meant to reflect the usual use of a short
# name like '_'. The "L" is for "log" and the other letter comes from
# the level.
_LI = _translators.log_info
_LW = _translators.log_warning
_LE = _translators.log_error
_LC = _translators.log_critical

Then, in the rest of your code, use the appropriate marker function for each message:

from myapp.i18n import _, _LW

# ...

LOG.warn(_LW('warning message: %s'), var)

# ...

try:

    # ...

except AnException1:

    # Log only
    LOG.exception(_LE('exception message'))

except AnException2:

    # Raise only
    raise RuntimeError(_('exception message'))

else:

    # Log and Raise
    msg = _('Unexpected error message')
    LOG.exception(msg)
    raise RuntimeError(msg)

Note

Libraries probably do not want to expose the new integration module as part of their public API, so rather than naming it mylib.i18n it should be called mylib._i18n to indicate that it is a private implementation detail, and not meant to be used outside of the library's own code.

Warning

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:

# WRONG
from foo import bar

bar()

# RIGHT

import foo

foo.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 the integration module, add an import exception to tox.ini.

For example:

# tox.ini
[hacking]
import_exceptions =
  app.i18n

Lazy Translation

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 enable_lazy.

import oslo_i18n

oslo_i18n.enable_lazy()

Translating Messages

Use ~oslo_i18n.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 emitted.

import oslo_i18n

trans_msg = oslo_i18n.translate(msg, desired_locale=my_locale)

if desired_locale is not specified then the default locale is used.

Available Languages

Only the languages that have translations provided are available for translation. To determine which languages are available the ~oslo_i18n.get_available_languages is provided. Since different languages can be installed for each domain, the domain must be specified.

import oslo_i18n

avail_lang = oslo_i18n.get_available_languages('myapp')

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