Guidelines for Use in OpenStack

The OpenStack I18N team has a limited capacity to translate messages, so we want to make their work as effective as possible by identifying the most useful text for them to translate. All text messages the user sees via exceptions or API calls should be marked for translation. However, some exceptions are used internally to signal error conditions between modules and are not intended to be presented to the user. Those do not need to be translated. Neither do log messages, as explained below.

Gettext Contextual Form and Plural Form

Sometimes under different contexts, the same word should be translated into different phrases using TranslatorFactory.contextual_form.

And recommend the following code to use contextual form:

# The contextual translation function using the name "_C"
_C = _translators.contextual_form

msg = _C('context', 'string')

In some languages, sometimes the translated strings are different with different item counts using TranslatorFactory.plural_form

And recommend the following code to use plural form:

# The plural translation function using the name "_P"
_P = _translators.plural_form

msg = _P('single', 'plural', count)

The contextual form and plural form are used only when needed. By default, the translation should use the _().


These two functions were only available in oslo.i18n >= 2.1.0.

Log Translation


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. The following documentation is retained to help developers understand existing usage and how to remove it.

Support was dropped primarily based on feedback from operators that they were not only not needed but also undesirable, because they fragmented the set of web pages providing helpful information about any particular log message, thereby reducing the chances of finding those web pages by doing a web search for the message. Refer to the email thread understanding log domain change on the openstack-dev mailing list for more details.

OpenStack previously supported translating some log levels using separate message catalogs, and so has separate marker functions. These well-known names were used by the build system jobs that extracted the messages from the source code and passed them to the translation tool.












Debug level log messages were never translated.

Using a Marker Function

The marker functions are used to mark the translatable strings in the code. The strings are extracted into catalogs using a tool that performs source inspection to look for these specific markers, so the function argument must just be a string.

For example: do not do this:

msg = _(variable_containing_msg)

Instead, use this style:

msg = _('My message.')

Choosing a Marker Function

The purpose of the different marker functions is to separate the translatable messages into different catalogs, which the translation teams can prioritize translating. It is important to choose the right marker function, to ensure that strings the user sees will be translated and to help the translation team manage their work load.

Everything marked with _() will be translated. Prioritizing the catalogs created from strings marked with the log marker functions is up to the individual translation teams and their users, but it is expected that they will work on critical and error messages before warning or info.

_() is preferred for any user facing message, even if it is also going to a log file. This ensures that the translated version of the message will be available to the user.

The log marker functions (_LI(), _LW(), _LE(), and _LC()) should no longer be used, and existing usages should be removed. Anytime that the message is passed outside of the current context (for example as part of an exception) the _() marker function must be used instead.

A common pattern used to be to define a single message object and use it more than once, for the log call and the exception. In that case, _() had to be used because the message was going to appear in an exception that may be presented to the user.

However, now that log messages are no longer translated, it is unfortunately necessary to use two separate strings: a plain one for the log message, and a translatable one for the exception.

For example, do not do this:

msg = _('There was an error.')
raise LocalExceptionClass(msg)

or this:

msg = _LE('There was an error.')
raise LocalExceptionClass(msg)

Instead, use this style:

LOG.error('There was an error.')
raise LocalExceptionClass(_('An error occurred.'))

Adding Variables to Translated Messages

Translated messages should not be combined with other literal strings to create partially translated messages. For example, do not do this:

raise ValueError(_('some message') + ': variable=%s' % variable)

Instead, use this style:

raise ValueError(_('some message: variable=%s') % variable)

Including the variable reference inside the translated message allows the translator to take into account grammar rules, differences in left-right vs. right-left rendering, and other factors to make the translated message more useful to the end user.

Any message with more than one variable should use named interpolation instead of positional, to allow translators to move the variables around in the string to account for differences in grammar and writing direction.

For example, do not do this:

raise ValueError(_('some message: v1=%s v2=%s') % (v1, v2))

Instead, use this style:

raise ValueError(_('some message: v1=%(v1)s v2=%(v2)s') % {'v1': v1, 'v2': v2})