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649 lines
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ReStructuredText
649 lines
25 KiB
ReStructuredText
:mod:`gettext` --- Multilingual internationalization services
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=============================================================
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.. module:: gettext
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:synopsis: Multilingual internationalization services.
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.. moduleauthor:: Barry A. Warsaw <barry@python.org>
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.. sectionauthor:: Barry A. Warsaw <barry@python.org>
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**Source code:** :source:`Lib/gettext.py`
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--------------
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The :mod:`gettext` module provides internationalization (I18N) and localization
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(L10N) services for your Python modules and applications. It supports both the
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GNU :program:`gettext` message catalog API and a higher level, class-based API that may
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be more appropriate for Python files. The interface described below allows you
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to write your module and application messages in one natural language, and
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provide a catalog of translated messages for running under different natural
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languages.
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Some hints on localizing your Python modules and applications are also given.
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GNU :program:`gettext` API
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--------------------------
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The :mod:`gettext` module defines the following API, which is very similar to
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the GNU :program:`gettext` API. If you use this API you will affect the
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translation of your entire application globally. Often this is what you want if
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your application is monolingual, with the choice of language dependent on the
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locale of your user. If you are localizing a Python module, or if your
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application needs to switch languages on the fly, you probably want to use the
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class-based API instead.
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.. function:: bindtextdomain(domain, localedir=None)
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Bind the *domain* to the locale directory *localedir*. More concretely,
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:mod:`gettext` will look for binary :file:`.mo` files for the given domain using
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the path (on Unix): :file:`{localedir}/{language}/LC_MESSAGES/{domain}.mo`, where
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*language* is searched for in the environment variables :envvar:`LANGUAGE`,
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:envvar:`LC_ALL`, :envvar:`LC_MESSAGES`, and :envvar:`LANG` respectively.
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If *localedir* is omitted or ``None``, then the current binding for *domain* is
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returned. [#]_
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.. function:: textdomain(domain=None)
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Change or query the current global domain. If *domain* is ``None``, then the
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current global domain is returned, otherwise the global domain is set to
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*domain*, which is returned.
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.. index:: single: _ (underscore); gettext
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.. function:: gettext(message)
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Return the localized translation of *message*, based on the current global
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domain, language, and locale directory. This function is usually aliased as
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:func:`_` in the local namespace (see examples below).
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.. function:: dgettext(domain, message)
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Like :func:`.gettext`, but look the message up in the specified *domain*.
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.. function:: ngettext(singular, plural, n)
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Like :func:`.gettext`, but consider plural forms. If a translation is found,
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apply the plural formula to *n*, and return the resulting message (some
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languages have more than two plural forms). If no translation is found, return
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*singular* if *n* is 1; return *plural* otherwise.
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The Plural formula is taken from the catalog header. It is a C or Python
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expression that has a free variable *n*; the expression evaluates to the index
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of the plural in the catalog. See
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`the GNU gettext documentation <https://www.gnu.org/software/gettext/manual/gettext.html>`__
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for the precise syntax to be used in :file:`.po` files and the
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formulas for a variety of languages.
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.. function:: dngettext(domain, singular, plural, n)
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Like :func:`ngettext`, but look the message up in the specified *domain*.
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.. function:: pgettext(context, message)
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.. function:: dpgettext(domain, context, message)
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.. function:: npgettext(context, singular, plural, n)
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.. function:: dnpgettext(domain, context, singular, plural, n)
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Similar to the corresponding functions without the ``p`` in the prefix (that
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is, :func:`gettext`, :func:`dgettext`, :func:`ngettext`, :func:`dngettext`),
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but the translation is restricted to the given message *context*.
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.. versionadded:: 3.8
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Note that GNU :program:`gettext` also defines a :func:`dcgettext` method, but
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this was deemed not useful and so it is currently unimplemented.
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Here's an example of typical usage for this API::
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import gettext
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gettext.bindtextdomain('myapplication', '/path/to/my/language/directory')
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gettext.textdomain('myapplication')
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_ = gettext.gettext
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# ...
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print(_('This is a translatable string.'))
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Class-based API
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---------------
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The class-based API of the :mod:`gettext` module gives you more flexibility and
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greater convenience than the GNU :program:`gettext` API. It is the recommended
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way of localizing your Python applications and modules. :mod:`!gettext` defines
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a :class:`GNUTranslations` class which implements the parsing of GNU :file:`.mo` format
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files, and has methods for returning strings. Instances of this class can also
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install themselves in the built-in namespace as the function :func:`_`.
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.. function:: find(domain, localedir=None, languages=None, all=False)
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This function implements the standard :file:`.mo` file search algorithm. It
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takes a *domain*, identical to what :func:`textdomain` takes. Optional
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*localedir* is as in :func:`bindtextdomain`. Optional *languages* is a list of
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strings, where each string is a language code.
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If *localedir* is not given, then the default system locale directory is used.
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[#]_ If *languages* is not given, then the following environment variables are
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searched: :envvar:`LANGUAGE`, :envvar:`LC_ALL`, :envvar:`LC_MESSAGES`, and
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:envvar:`LANG`. The first one returning a non-empty value is used for the
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*languages* variable. The environment variables should contain a colon separated
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list of languages, which will be split on the colon to produce the expected list
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of language code strings.
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:func:`find` then expands and normalizes the languages, and then iterates
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through them, searching for an existing file built of these components:
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:file:`{localedir}/{language}/LC_MESSAGES/{domain}.mo`
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The first such file name that exists is returned by :func:`find`. If no such
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file is found, then ``None`` is returned. If *all* is given, it returns a list
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of all file names, in the order in which they appear in the languages list or
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the environment variables.
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.. function:: translation(domain, localedir=None, languages=None, class_=None, fallback=False)
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Return a :class:`*Translations` instance based on the *domain*, *localedir*,
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and *languages*, which are first passed to :func:`find` to get a list of the
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associated :file:`.mo` file paths. Instances with identical :file:`.mo` file
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names are cached. The actual class instantiated is *class_* if
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provided, otherwise :class:`GNUTranslations`. The class's constructor must
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take a single :term:`file object` argument. If provided, *codeset* will change
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the charset used to encode translated strings in the
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:meth:`~NullTranslations.lgettext` and :meth:`~NullTranslations.lngettext`
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methods.
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If multiple files are found, later files are used as fallbacks for earlier ones.
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To allow setting the fallback, :func:`copy.copy` is used to clone each
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translation object from the cache; the actual instance data is still shared with
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the cache.
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If no :file:`.mo` file is found, this function raises :exc:`OSError` if
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*fallback* is false (which is the default), and returns a
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:class:`NullTranslations` instance if *fallback* is true.
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.. versionchanged:: 3.3
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:exc:`IOError` used to be raised instead of :exc:`OSError`.
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.. versionchanged:: 3.11
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*codeset* parameter is removed.
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.. function:: install(domain, localedir=None, *, names=None)
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This installs the function :func:`_` in Python's builtins namespace, based on
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*domain* and *localedir* which are passed to the function :func:`translation`.
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For the *names* parameter, please see the description of the translation
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object's :meth:`~NullTranslations.install` method.
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As seen below, you usually mark the strings in your application that are
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candidates for translation, by wrapping them in a call to the :func:`_`
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function, like this::
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print(_('This string will be translated.'))
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For convenience, you want the :func:`_` function to be installed in Python's
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builtins namespace, so it is easily accessible in all modules of your
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application.
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.. versionchanged:: 3.11
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*names* is now a keyword-only parameter.
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The :class:`NullTranslations` class
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Translation classes are what actually implement the translation of original
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source file message strings to translated message strings. The base class used
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by all translation classes is :class:`NullTranslations`; this provides the basic
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interface you can use to write your own specialized translation classes. Here
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are the methods of :class:`!NullTranslations`:
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.. class:: NullTranslations(fp=None)
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Takes an optional :term:`file object` *fp*, which is ignored by the base class.
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Initializes "protected" instance variables *_info* and *_charset* which are set
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by derived classes, as well as *_fallback*, which is set through
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:meth:`add_fallback`. It then calls ``self._parse(fp)`` if *fp* is not
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``None``.
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.. method:: _parse(fp)
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No-op in the base class, this method takes file object *fp*, and reads
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the data from the file, initializing its message catalog. If you have an
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unsupported message catalog file format, you should override this method
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to parse your format.
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.. method:: add_fallback(fallback)
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Add *fallback* as the fallback object for the current translation object.
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A translation object should consult the fallback if it cannot provide a
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translation for a given message.
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.. method:: gettext(message)
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If a fallback has been set, forward :meth:`!gettext` to the fallback.
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Otherwise, return *message*. Overridden in derived classes.
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.. method:: ngettext(singular, plural, n)
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If a fallback has been set, forward :meth:`!ngettext` to the fallback.
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Otherwise, return *singular* if *n* is 1; return *plural* otherwise.
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Overridden in derived classes.
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.. method:: pgettext(context, message)
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If a fallback has been set, forward :meth:`pgettext` to the fallback.
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Otherwise, return the translated message. Overridden in derived classes.
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.. versionadded:: 3.8
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.. method:: npgettext(context, singular, plural, n)
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If a fallback has been set, forward :meth:`npgettext` to the fallback.
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Otherwise, return the translated message. Overridden in derived classes.
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.. versionadded:: 3.8
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.. method:: info()
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Return the "protected" :attr:`_info` variable, a dictionary containing
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the metadata found in the message catalog file.
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.. method:: charset()
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Return the encoding of the message catalog file.
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.. method:: install(names=None)
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This method installs :meth:`.gettext` into the built-in namespace,
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binding it to ``_``.
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If the *names* parameter is given, it must be a sequence containing the
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names of functions you want to install in the builtins namespace in
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addition to :func:`_`. Supported names are ``'gettext'``, ``'ngettext'``,
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``'pgettext'``, ``'npgettext'``, ``'lgettext'``, and ``'lngettext'``.
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Note that this is only one way, albeit the most convenient way, to make
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the :func:`_` function available to your application. Because it affects
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the entire application globally, and specifically the built-in namespace,
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localized modules should never install :func:`_`. Instead, they should use
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this code to make :func:`_` available to their module::
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import gettext
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t = gettext.translation('mymodule', ...)
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_ = t.gettext
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This puts :func:`_` only in the module's global namespace and so only
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affects calls within this module.
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.. versionchanged:: 3.8
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Added ``'pgettext'`` and ``'npgettext'``.
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The :class:`GNUTranslations` class
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The :mod:`gettext` module provides one additional class derived from
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:class:`NullTranslations`: :class:`GNUTranslations`. This class overrides
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:meth:`_parse` to enable reading GNU :program:`gettext` format :file:`.mo` files
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in both big-endian and little-endian format.
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:class:`GNUTranslations` parses optional metadata out of the translation
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catalog. It is convention with GNU :program:`gettext` to include metadata as
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the translation for the empty string. This metadata is in :rfc:`822`\ -style
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``key: value`` pairs, and should contain the ``Project-Id-Version`` key. If the
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key ``Content-Type`` is found, then the ``charset`` property is used to
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initialize the "protected" :attr:`_charset` instance variable, defaulting to
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``None`` if not found. If the charset encoding is specified, then all message
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ids and message strings read from the catalog are converted to Unicode using
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this encoding, else ASCII is assumed.
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Since message ids are read as Unicode strings too, all :meth:`*gettext` methods
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will assume message ids as Unicode strings, not byte strings.
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The entire set of key/value pairs are placed into a dictionary and set as the
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"protected" :attr:`_info` instance variable.
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If the :file:`.mo` file's magic number is invalid, the major version number is
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unexpected, or if other problems occur while reading the file, instantiating a
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:class:`GNUTranslations` class can raise :exc:`OSError`.
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.. class:: GNUTranslations
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The following methods are overridden from the base class implementation:
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.. method:: gettext(message)
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Look up the *message* id in the catalog and return the corresponding message
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string, as a Unicode string. If there is no entry in the catalog for the
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*message* id, and a fallback has been set, the look up is forwarded to the
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fallback's :meth:`~NullTranslations.gettext` method. Otherwise, the
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*message* id is returned.
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.. method:: ngettext(singular, plural, n)
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Do a plural-forms lookup of a message id. *singular* is used as the message id
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for purposes of lookup in the catalog, while *n* is used to determine which
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plural form to use. The returned message string is a Unicode string.
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If the message id is not found in the catalog, and a fallback is specified,
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the request is forwarded to the fallback's :meth:`~NullTranslations.ngettext`
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method. Otherwise, when *n* is 1 *singular* is returned, and *plural* is
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returned in all other cases.
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Here is an example::
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n = len(os.listdir('.'))
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cat = GNUTranslations(somefile)
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message = cat.ngettext(
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'There is %(num)d file in this directory',
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'There are %(num)d files in this directory',
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n) % {'num': n}
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.. method:: pgettext(context, message)
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Look up the *context* and *message* id in the catalog and return the
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corresponding message string, as a Unicode string. If there is no
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entry in the catalog for the *message* id and *context*, and a fallback
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has been set, the look up is forwarded to the fallback's
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:meth:`pgettext` method. Otherwise, the *message* id is returned.
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.. versionadded:: 3.8
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.. method:: npgettext(context, singular, plural, n)
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Do a plural-forms lookup of a message id. *singular* is used as the
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message id for purposes of lookup in the catalog, while *n* is used to
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determine which plural form to use.
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If the message id for *context* is not found in the catalog, and a
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fallback is specified, the request is forwarded to the fallback's
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:meth:`npgettext` method. Otherwise, when *n* is 1 *singular* is
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returned, and *plural* is returned in all other cases.
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.. versionadded:: 3.8
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Solaris message catalog support
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The Solaris operating system defines its own binary :file:`.mo` file format, but
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since no documentation can be found on this format, it is not supported at this
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time.
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The Catalog constructor
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^^^^^^^^^^^^^^^^^^^^^^^
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.. index:: single: GNOME
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GNOME uses a version of the :mod:`gettext` module by James Henstridge, but this
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version has a slightly different API. Its documented usage was::
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import gettext
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cat = gettext.Catalog(domain, localedir)
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_ = cat.gettext
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print(_('hello world'))
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For compatibility with this older module, the function :func:`Catalog` is an
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alias for the :func:`translation` function described above.
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One difference between this module and Henstridge's: his catalog objects
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supported access through a mapping API, but this appears to be unused and so is
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not currently supported.
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Internationalizing your programs and modules
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--------------------------------------------
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Internationalization (I18N) refers to the operation by which a program is made
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aware of multiple languages. Localization (L10N) refers to the adaptation of
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your program, once internationalized, to the local language and cultural habits.
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In order to provide multilingual messages for your Python programs, you need to
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take the following steps:
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#. prepare your program or module by specially marking translatable strings
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#. run a suite of tools over your marked files to generate raw messages catalogs
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#. create language-specific translations of the message catalogs
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#. use the :mod:`gettext` module so that message strings are properly translated
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In order to prepare your code for I18N, you need to look at all the strings in
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your files. Any string that needs to be translated should be marked by wrapping
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it in ``_('...')`` --- that is, a call to the function :func:`_`. For example::
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filename = 'mylog.txt'
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message = _('writing a log message')
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with open(filename, 'w') as fp:
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fp.write(message)
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In this example, the string ``'writing a log message'`` is marked as a candidate
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for translation, while the strings ``'mylog.txt'`` and ``'w'`` are not.
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There are a few tools to extract the strings meant for translation.
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The original GNU :program:`gettext` only supported C or C++ source
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code but its extended version :program:`xgettext` scans code written
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in a number of languages, including Python, to find strings marked as
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translatable. `Babel <https://babel.pocoo.org/>`__ is a Python
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internationalization library that includes a :file:`pybabel` script to
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extract and compile message catalogs. François Pinard's program
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called :program:`xpot` does a similar job and is available as part of
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his `po-utils package <https://github.com/pinard/po-utils>`__.
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(Python also includes pure-Python versions of these programs, called
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:program:`pygettext.py` and :program:`msgfmt.py`; some Python distributions
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will install them for you. :program:`pygettext.py` is similar to
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:program:`xgettext`, but only understands Python source code and
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cannot handle other programming languages such as C or C++.
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:program:`pygettext.py` supports a command-line interface similar to
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:program:`xgettext`; for details on its use, run ``pygettext.py
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--help``. :program:`msgfmt.py` is binary compatible with GNU
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:program:`msgfmt`. With these two programs, you may not need the GNU
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:program:`gettext` package to internationalize your Python
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applications.)
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:program:`xgettext`, :program:`pygettext`, and similar tools generate
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:file:`.po` files that are message catalogs. They are structured
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human-readable files that contain every marked string in the source
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code, along with a placeholder for the translated versions of these
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strings.
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Copies of these :file:`.po` files are then handed over to the
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individual human translators who write translations for every
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supported natural language. They send back the completed
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language-specific versions as a :file:`<language-name>.po` file that's
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compiled into a machine-readable :file:`.mo` binary catalog file using
|
|
the :program:`msgfmt` program. The :file:`.mo` files are used by the
|
|
:mod:`gettext` module for the actual translation processing at
|
|
run-time.
|
|
|
|
How you use the :mod:`gettext` module in your code depends on whether you are
|
|
internationalizing a single module or your entire application. The next two
|
|
sections will discuss each case.
|
|
|
|
|
|
Localizing your module
|
|
^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
If you are localizing your module, you must take care not to make global
|
|
changes, e.g. to the built-in namespace. You should not use the GNU :program:`gettext`
|
|
API but instead the class-based API.
|
|
|
|
Let's say your module is called "spam" and the module's various natural language
|
|
translation :file:`.mo` files reside in :file:`/usr/share/locale` in GNU
|
|
:program:`gettext` format. Here's what you would put at the top of your
|
|
module::
|
|
|
|
import gettext
|
|
t = gettext.translation('spam', '/usr/share/locale')
|
|
_ = t.gettext
|
|
|
|
|
|
Localizing your application
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
If you are localizing your application, you can install the :func:`_` function
|
|
globally into the built-in namespace, usually in the main driver file of your
|
|
application. This will let all your application-specific files just use
|
|
``_('...')`` without having to explicitly install it in each file.
|
|
|
|
In the simple case then, you need only add the following bit of code to the main
|
|
driver file of your application::
|
|
|
|
import gettext
|
|
gettext.install('myapplication')
|
|
|
|
If you need to set the locale directory, you can pass it into the
|
|
:func:`install` function::
|
|
|
|
import gettext
|
|
gettext.install('myapplication', '/usr/share/locale')
|
|
|
|
|
|
Changing languages on the fly
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
If your program needs to support many languages at the same time, you may want
|
|
to create multiple translation instances and then switch between them
|
|
explicitly, like so::
|
|
|
|
import gettext
|
|
|
|
lang1 = gettext.translation('myapplication', languages=['en'])
|
|
lang2 = gettext.translation('myapplication', languages=['fr'])
|
|
lang3 = gettext.translation('myapplication', languages=['de'])
|
|
|
|
# start by using language1
|
|
lang1.install()
|
|
|
|
# ... time goes by, user selects language 2
|
|
lang2.install()
|
|
|
|
# ... more time goes by, user selects language 3
|
|
lang3.install()
|
|
|
|
|
|
Deferred translations
|
|
^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
In most coding situations, strings are translated where they are coded.
|
|
Occasionally however, you need to mark strings for translation, but defer actual
|
|
translation until later. A classic example is::
|
|
|
|
animals = ['mollusk',
|
|
'albatross',
|
|
'rat',
|
|
'penguin',
|
|
'python', ]
|
|
# ...
|
|
for a in animals:
|
|
print(a)
|
|
|
|
Here, you want to mark the strings in the ``animals`` list as being
|
|
translatable, but you don't actually want to translate them until they are
|
|
printed.
|
|
|
|
Here is one way you can handle this situation::
|
|
|
|
def _(message): return message
|
|
|
|
animals = [_('mollusk'),
|
|
_('albatross'),
|
|
_('rat'),
|
|
_('penguin'),
|
|
_('python'), ]
|
|
|
|
del _
|
|
|
|
# ...
|
|
for a in animals:
|
|
print(_(a))
|
|
|
|
This works because the dummy definition of :func:`_` simply returns the string
|
|
unchanged. And this dummy definition will temporarily override any definition
|
|
of :func:`_` in the built-in namespace (until the :keyword:`del` command). Take
|
|
care, though if you have a previous definition of :func:`_` in the local
|
|
namespace.
|
|
|
|
Note that the second use of :func:`_` will not identify "a" as being
|
|
translatable to the :program:`gettext` program, because the parameter
|
|
is not a string literal.
|
|
|
|
Another way to handle this is with the following example::
|
|
|
|
def N_(message): return message
|
|
|
|
animals = [N_('mollusk'),
|
|
N_('albatross'),
|
|
N_('rat'),
|
|
N_('penguin'),
|
|
N_('python'), ]
|
|
|
|
# ...
|
|
for a in animals:
|
|
print(_(a))
|
|
|
|
In this case, you are marking translatable strings with the function
|
|
:func:`N_`, which won't conflict with any definition of :func:`_`.
|
|
However, you will need to teach your message extraction program to
|
|
look for translatable strings marked with :func:`N_`. :program:`xgettext`,
|
|
:program:`pygettext`, ``pybabel extract``, and :program:`xpot` all
|
|
support this through the use of the :option:`!-k` command-line switch.
|
|
The choice of :func:`N_` here is totally arbitrary; it could have just
|
|
as easily been :func:`MarkThisStringForTranslation`.
|
|
|
|
|
|
Acknowledgements
|
|
----------------
|
|
|
|
The following people contributed code, feedback, design suggestions, previous
|
|
implementations, and valuable experience to the creation of this module:
|
|
|
|
* Peter Funk
|
|
|
|
* James Henstridge
|
|
|
|
* Juan David Ibáñez Palomar
|
|
|
|
* Marc-André Lemburg
|
|
|
|
* Martin von Löwis
|
|
|
|
* François Pinard
|
|
|
|
* Barry Warsaw
|
|
|
|
* Gustavo Niemeyer
|
|
|
|
.. rubric:: Footnotes
|
|
|
|
.. [#] The default locale directory is system dependent; for example, on RedHat Linux
|
|
it is :file:`/usr/share/locale`, but on Solaris it is :file:`/usr/lib/locale`.
|
|
The :mod:`gettext` module does not try to support these system dependent
|
|
defaults; instead its default is :file:`{sys.base_prefix}/share/locale` (see
|
|
:data:`sys.base_prefix`). For this reason, it is always best to call
|
|
:func:`bindtextdomain` with an explicit absolute path at the start of your
|
|
application.
|
|
|
|
.. [#] See the footnote for :func:`bindtextdomain` above.
|