xref: /third_party/python/Doc/library/gettext.rst

:mod:`gettext` --- Multilingual internationalization services
=============================================================

.. module:: gettext
   :synopsis: Multilingual internationalization services.

.. moduleauthor:: Barry A. Warsaw <barry@python.org>
.. sectionauthor:: Barry A. Warsaw <barry@python.org>

**Source code:** :source:`Lib/gettext.py`

--------------

The :mod:`gettext` module provides internationalization (I18N) and localization
(L10N) services for your Python modules and applications. It supports both the
GNU :program:`gettext` message catalog API and a higher level, class-based API that may
be more appropriate for Python files.  The interface described below allows you
to write your module and application messages in one natural language, and
provide a catalog of translated messages for running under different natural
languages.

Some hints on localizing your Python modules and applications are also given.


GNU :program:`gettext` API
--------------------------

The :mod:`gettext` module defines the following API, which is very similar to
the GNU :program:`gettext` API.  If you use this API you will affect the
translation of your entire application globally.  Often this is what you want if
your application is monolingual, with the choice of language dependent on the
locale of your user.  If you are localizing a Python module, or if your
application needs to switch languages on the fly, you probably want to use the
class-based API instead.


.. function:: bindtextdomain(domain, localedir=None)

   Bind the *domain* to the locale directory *localedir*.  More concretely,
   :mod:`gettext` will look for binary :file:`.mo` files for the given domain using
   the path (on Unix): :file:`{localedir}/{language}/LC_MESSAGES/{domain}.mo`, where
   *language* is searched for in the environment variables :envvar:`LANGUAGE`,
   :envvar:`LC_ALL`, :envvar:`LC_MESSAGES`, and :envvar:`LANG` respectively.

   If *localedir* is omitted or ``None``, then the current binding for *domain* is
   returned. [#]_


.. function:: textdomain(domain=None)

   Change or query the current global domain.  If *domain* is ``None``, then the
   current global domain is returned, otherwise the global domain is set to
   *domain*, which is returned.


.. index:: single: _ (underscore); gettext
.. function:: gettext(message)

   Return the localized translation of *message*, based on the current global
   domain, language, and locale directory.  This function is usually aliased as
   :func:`_` in the local namespace (see examples below).


.. function:: dgettext(domain, message)

   Like :func:`.gettext`, but look the message up in the specified *domain*.


.. function:: ngettext(singular, plural, n)

   Like :func:`.gettext`, but consider plural forms. If a translation is found,
   apply the plural formula to *n*, and return the resulting message (some
   languages have more than two plural forms). If no translation is found, return
   *singular* if *n* is 1; return *plural* otherwise.

   The Plural formula is taken from the catalog header. It is a C or Python
   expression that has a free variable *n*; the expression evaluates to the index
   of the plural in the catalog. See
   `the GNU gettext documentation <https://www.gnu.org/software/gettext/manual/gettext.html>`__
   for the precise syntax to be used in :file:`.po` files and the
   formulas for a variety of languages.


.. function:: dngettext(domain, singular, plural, n)

   Like :func:`ngettext`, but look the message up in the specified *domain*.


.. function:: pgettext(context, message)
.. function:: dpgettext(domain, context, message)
.. function:: npgettext(context, singular, plural, n)
.. function:: dnpgettext(domain, context, singular, plural, n)

   Similar to the corresponding functions without the ``p`` in the prefix (that
   is, :func:`gettext`, :func:`dgettext`, :func:`ngettext`, :func:`dngettext`),
   but the translation is restricted to the given message *context*.

   .. versionadded:: 3.8


Note that GNU :program:`gettext` also defines a :func:`dcgettext` method, but
this was deemed not useful and so it is currently unimplemented.

Here's an example of typical usage for this API::

   import gettext
   gettext.bindtextdomain('myapplication', '/path/to/my/language/directory')
   gettext.textdomain('myapplication')
   _ = gettext.gettext
   # ...
   print(_('This is a translatable string.'))


Class-based API
---------------

The class-based API of the :mod:`gettext` module gives you more flexibility and
greater convenience than the GNU :program:`gettext` API.  It is the recommended
way of localizing your Python applications and modules.  :mod:`!gettext` defines
a :class:`GNUTranslations` class which implements the parsing of GNU :file:`.mo` format
files, and has methods for returning strings. Instances of this class can also
install themselves in the built-in namespace as the function :func:`_`.


.. function:: find(domain, localedir=None, languages=None, all=False)

   This function implements the standard :file:`.mo` file search algorithm.  It
   takes a *domain*, identical to what :func:`textdomain` takes.  Optional
   *localedir* is as in :func:`bindtextdomain`. Optional *languages* is a list of
   strings, where each string is a language code.

   If *localedir* is not given, then the default system locale directory is used.
   [#]_  If *languages* is not given, then the following environment variables are
   searched: :envvar:`LANGUAGE`, :envvar:`LC_ALL`, :envvar:`LC_MESSAGES`, and
   :envvar:`LANG`.  The first one returning a non-empty value is used for the
   *languages* variable. The environment variables should contain a colon separated
   list of languages, which will be split on the colon to produce the expected list
   of language code strings.

   :func:`find` then expands and normalizes the languages, and then iterates
   through them, searching for an existing file built of these components:

   :file:`{localedir}/{language}/LC_MESSAGES/{domain}.mo`

   The first such file name that exists is returned by :func:`find`. If no such
   file is found, then ``None`` is returned. If *all* is given, it returns a list
   of all file names, in the order in which they appear in the languages list or
   the environment variables.


.. function:: translation(domain, localedir=None, languages=None, class_=None, fallback=False)

   Return a :class:`*Translations` instance based on the *domain*, *localedir*,
   and *languages*, which are first passed to :func:`find` to get a list of the
   associated :file:`.mo` file paths.  Instances with identical :file:`.mo` file
   names are cached.  The actual class instantiated is *class_* if
   provided, otherwise :class:`GNUTranslations`.  The class's constructor must
   take a single :term:`file object` argument.  If provided, *codeset* will change
   the charset used to encode translated strings in the
   :meth:`~NullTranslations.lgettext` and :meth:`~NullTranslations.lngettext`
   methods.

   If multiple files are found, later files are used as fallbacks for earlier ones.
   To allow setting the fallback, :func:`copy.copy` is used to clone each
   translation object from the cache; the actual instance data is still shared with
   the cache.

   If no :file:`.mo` file is found, this function raises :exc:`OSError` if
   *fallback* is false (which is the default), and returns a
   :class:`NullTranslations` instance if *fallback* is true.

   .. versionchanged:: 3.3
      :exc:`IOError` used to be raised instead of :exc:`OSError`.

   .. versionchanged:: 3.11
      *codeset* parameter is removed.

.. function:: install(domain, localedir=None, *, names=None)

   This installs the function :func:`_` in Python's builtins namespace, based on
   *domain* and *localedir* which are passed to the function :func:`translation`.

   For the *names* parameter, please see the description of the translation
   object's :meth:`~NullTranslations.install` method.

   As seen below, you usually mark the strings in your application that are
   candidates for translation, by wrapping them in a call to the :func:`_`
   function, like this::

      print(_('This string will be translated.'))

   For convenience, you want the :func:`_` function to be installed in Python's
   builtins namespace, so it is easily accessible in all modules of your
   application.

   .. versionchanged:: 3.11
      *names* is now a keyword-only parameter.

The :class:`NullTranslations` class
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Translation classes are what actually implement the translation of original
source file message strings to translated message strings. The base class used
by all translation classes is :class:`NullTranslations`; this provides the basic
interface you can use to write your own specialized translation classes.  Here
are the methods of :class:`!NullTranslations`:


.. class:: NullTranslations(fp=None)

   Takes an optional :term:`file object` *fp*, which is ignored by the base class.
   Initializes "protected" instance variables *_info* and *_charset* which are set
   by derived classes, as well as *_fallback*, which is set through
   :meth:`add_fallback`.  It then calls ``self._parse(fp)`` if *fp* is not
   ``None``.

   .. method:: _parse(fp)

      No-op in the base class, this method takes file object *fp*, and reads
      the data from the file, initializing its message catalog.  If you have an
      unsupported message catalog file format, you should override this method
      to parse your format.


   .. method:: add_fallback(fallback)

      Add *fallback* as the fallback object for the current translation object.
      A translation object should consult the fallback if it cannot provide a
      translation for a given message.


   .. method:: gettext(message)

      If a fallback has been set, forward :meth:`!gettext` to the fallback.
      Otherwise, return *message*.  Overridden in derived classes.


   .. method:: ngettext(singular, plural, n)

      If a fallback has been set, forward :meth:`!ngettext` to the fallback.
      Otherwise, return *singular* if *n* is 1; return *plural* otherwise.
      Overridden in derived classes.


   .. method:: pgettext(context, message)

      If a fallback has been set, forward :meth:`pgettext` to the fallback.
      Otherwise, return the translated message.  Overridden in derived classes.

      .. versionadded:: 3.8


   .. method:: npgettext(context, singular, plural, n)

      If a fallback has been set, forward :meth:`npgettext` to the fallback.
      Otherwise, return the translated message.  Overridden in derived classes.

      .. versionadded:: 3.8


   .. method:: info()

      Return the "protected" :attr:`_info` variable, a dictionary containing
      the metadata found in the message catalog file.


   .. method:: charset()

      Return the encoding of the message catalog file.


   .. method:: install(names=None)

      This method installs :meth:`.gettext` into the built-in namespace,
      binding it to ``_``.

      If the *names* parameter is given, it must be a sequence containing the
      names of functions you want to install in the builtins namespace in
      addition to :func:`_`.  Supported names are ``'gettext'``, ``'ngettext'``,
      ``'pgettext'``, ``'npgettext'``, ``'lgettext'``, and ``'lngettext'``.

      Note that this is only one way, albeit the most convenient way, to make
      the :func:`_` function available to your application.  Because it affects
      the entire application globally, and specifically the built-in namespace,
      localized modules should never install :func:`_`. Instead, they should use
      this code to make :func:`_` available to their module::

         import gettext
         t = gettext.translation('mymodule', ...)
         _ = t.gettext

      This puts :func:`_` only in the module's global namespace and so only
      affects calls within this module.

      .. versionchanged:: 3.8
         Added ``'pgettext'`` and ``'npgettext'``.


The :class:`GNUTranslations` class
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The :mod:`gettext` module provides one additional class derived from
:class:`NullTranslations`: :class:`GNUTranslations`.  This class overrides
:meth:`_parse` to enable reading GNU :program:`gettext` format :file:`.mo` files
in both big-endian and little-endian format.

:class:`GNUTranslations` parses optional metadata out of the translation
catalog. It is convention with GNU :program:`gettext` to include metadata as
the translation for the empty string. This metadata is in :rfc:`822`\ -style
``key: value`` pairs, and should contain the ``Project-Id-Version`` key.  If the
key ``Content-Type`` is found, then the ``charset`` property is used to
initialize the "protected" :attr:`_charset` instance variable, defaulting to
``None`` if not found.  If the charset encoding is specified, then all message
ids and message strings read from the catalog are converted to Unicode using
this encoding, else ASCII is assumed.

Since message ids are read as Unicode strings too, all :meth:`*gettext` methods
will assume message ids as Unicode strings, not byte strings.

The entire set of key/value pairs are placed into a dictionary and set as the
"protected" :attr:`_info` instance variable.

If the :file:`.mo` file's magic number is invalid, the major version number is
unexpected, or if other problems occur while reading the file, instantiating a
:class:`GNUTranslations` class can raise :exc:`OSError`.

.. class:: GNUTranslations

   The following methods are overridden from the base class implementation:

   .. method:: gettext(message)

      Look up the *message* id in the catalog and return the corresponding message
      string, as a Unicode string.  If there is no entry in the catalog for the
      *message* id, and a fallback has been set, the look up is forwarded to the
      fallback's :meth:`~NullTranslations.gettext` method.  Otherwise, the
      *message* id is returned.


   .. method:: ngettext(singular, plural, n)

      Do a plural-forms lookup of a message id.  *singular* is used as the message id
      for purposes of lookup in the catalog, while *n* is used to determine which
      plural form to use.  The returned message string is a Unicode string.

      If the message id is not found in the catalog, and a fallback is specified,
      the request is forwarded to the fallback's :meth:`~NullTranslations.ngettext`
      method.  Otherwise, when *n* is 1 *singular* is returned, and *plural* is
      returned in all other cases.

      Here is an example::

         n = len(os.listdir('.'))
         cat = GNUTranslations(somefile)
         message = cat.ngettext(
             'There is %(num)d file in this directory',
             'There are %(num)d files in this directory',
             n) % {'num': n}


   .. method:: pgettext(context, message)

      Look up the *context* and *message* id in the catalog and return the
      corresponding message string, as a Unicode string.  If there is no
      entry in the catalog for the *message* id and *context*, and a fallback
      has been set, the look up is forwarded to the fallback's
      :meth:`pgettext` method.  Otherwise, the *message* id is returned.

      .. versionadded:: 3.8


   .. method:: npgettext(context, singular, plural, n)

      Do a plural-forms lookup of a message id.  *singular* is used as the
      message id for purposes of lookup in the catalog, while *n* is used to
      determine which plural form to use.

      If the message id for *context* is not found in the catalog, and a
      fallback is specified, the request is forwarded to the fallback's
      :meth:`npgettext` method.  Otherwise, when *n* is 1 *singular* is
      returned, and *plural* is returned in all other cases.

      .. versionadded:: 3.8


Solaris message catalog support
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The Solaris operating system defines its own binary :file:`.mo` file format, but
since no documentation can be found on this format, it is not supported at this
time.


The Catalog constructor
^^^^^^^^^^^^^^^^^^^^^^^

.. index:: single: GNOME

GNOME uses a version of the :mod:`gettext` module by James Henstridge, but this
version has a slightly different API.  Its documented usage was::

   import gettext
   cat = gettext.Catalog(domain, localedir)
   _ = cat.gettext
   print(_('hello world'))

For compatibility with this older module, the function :func:`Catalog` is an
alias for the :func:`translation` function described above.

One difference between this module and Henstridge's: his catalog objects
supported access through a mapping API, but this appears to be unused and so is
not currently supported.


Internationalizing your programs and modules
--------------------------------------------

Internationalization (I18N) refers to the operation by which a program is made
aware of multiple languages.  Localization (L10N) refers to the adaptation of
your program, once internationalized, to the local language and cultural habits.
In order to provide multilingual messages for your Python programs, you need to
take the following steps:

#. prepare your program or module by specially marking translatable strings

#. run a suite of tools over your marked files to generate raw messages catalogs

#. create language-specific translations of the message catalogs

#. use the :mod:`gettext` module so that message strings are properly translated

In order to prepare your code for I18N, you need to look at all the strings in
your files.  Any string that needs to be translated should be marked by wrapping
it in ``_('...')`` --- that is, a call to the function :func:`_`.  For example::

   filename = 'mylog.txt'
   message = _('writing a log message')
   with open(filename, 'w') as fp:
       fp.write(message)

In this example, the string ``'writing a log message'`` is marked as a candidate
for translation, while the strings ``'mylog.txt'`` and ``'w'`` are not.

There are a few tools to extract the strings meant for translation.
The original GNU :program:`gettext` only supported C or C++ source
code but its extended version :program:`xgettext` scans code written
in a number of languages, including Python, to find strings marked as
translatable.  `Babel <https://babel.pocoo.org/>`__ is a Python
internationalization library that includes a :file:`pybabel` script to
extract and compile message catalogs.  François Pinard's program
called :program:`xpot` does a similar job and is available as part of
his `po-utils package <https://github.com/pinard/po-utils>`__.

(Python also includes pure-Python versions of these programs, called
:program:`pygettext.py` and :program:`msgfmt.py`; some Python distributions
will install them for you.  :program:`pygettext.py` is similar to
:program:`xgettext`, but only understands Python source code and
cannot handle other programming languages such as C or C++.
:program:`pygettext.py` supports a command-line interface similar to
:program:`xgettext`; for details on its use, run ``pygettext.py
--help``.  :program:`msgfmt.py` is binary compatible with GNU
:program:`msgfmt`.  With these two programs, you may not need the GNU
:program:`gettext` package to internationalize your Python
applications.)

:program:`xgettext`, :program:`pygettext`, and similar tools generate
:file:`.po` files that are message catalogs.  They are structured
human-readable files that contain every marked string in the source
code, along with a placeholder for the translated versions of these
strings.

Copies of these :file:`.po` files are then handed over to the
individual human translators who write translations for every
supported natural language.  They send back the completed
language-specific versions as a :file:`<language-name>.po` file that's
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.