"gettext" — Multilingual internationalization services
******************************************************

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

======================================================================

The "gettext" module provides internationalization (I18N) and
localization (L10N) services for your Python modules and applications.
It supports both the GNU **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 **gettext** API
===================

The "gettext" module defines the following API, which is very similar
to the GNU **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.

gettext.bindtextdomain(domain, localedir=None)

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

   If *localedir* is omitted or "None", then the current binding for
   *domain* is returned. [1]

gettext.bind_textdomain_codeset(domain, codeset=None)

   Bind the *domain* to *codeset*, changing the encoding of byte
   strings returned by the "lgettext()", "ldgettext()", "lngettext()"
   and "ldngettext()" functions. If *codeset* is omitted, then the
   current binding is returned.

gettext.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.

gettext.gettext(message)

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

gettext.dgettext(domain, message)

   Like "gettext()", but look the message up in the specified
   *domain*.

gettext.ngettext(singular, plural, n)

   Like "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 for the precise syntax to be used in ".po"
   files and the formulas for a variety of languages.

gettext.dngettext(domain, singular, plural, n)

   Like "ngettext()", but look the message up in the specified
   *domain*.

gettext.lgettext(message)

gettext.ldgettext(domain, message)

gettext.lngettext(singular, plural, n)

gettext.ldngettext(domain, singular, plural, n)

   Equivalent to the corresponding functions without the "l" prefix
   ("gettext()", "dgettext()", "ngettext()" and "dngettext()"), but
   the translation is returned as a byte string encoded in the
   preferred system encoding if no other encoding was explicitly set
   with "bind_textdomain_codeset()".

   Warning: These functions should be avoided in Python 3, because
     they return encoded bytes.  It’s much better to use alternatives
     which return Unicode strings instead, since most Python
     applications will want to manipulate human readable text as
     strings instead of bytes.  Further, it’s possible that you may
     get unexpected Unicode-related exceptions if there are encoding
     problems with the translated strings.  It is possible that the
     "l*()" functions will be deprecated in future Python versions due
     to their inherent problems and limitations.

Note that GNU **gettext** also defines a "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 "gettext" module gives you more flexibility
and greater convenience than the GNU **gettext** API.  It is the
recommended way of localizing your Python applications and modules.
"gettext" defines a "GNUTranslations" class which implements the
parsing of GNU ".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 "_()".

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

   This function implements the standard ".mo" file search algorithm.
   It takes a *domain*, identical to what "textdomain()" takes.
   Optional *localedir* is as in "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. [2]  If *languages* is not given, then the
   following environment variables are searched: "LANGUAGE", "LC_ALL",
   "LC_MESSAGES", and "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.

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

   "*localedir*/*language*/LC_MESSAGES/*domain*.mo"

   The first such file name that exists is returned by "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.

gettext.translation(domain, localedir=None, languages=None, class_=None, fallback=False, codeset=None)

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

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

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

   Changed in version 3.3: "IOError" used to be raised instead of
   "OSError".

gettext.install(domain, localedir=None, codeset=None, names=None)

   This installs the function "_()" in Python’s builtins namespace,
   based on *domain*, *localedir*, and *codeset* which are passed to
   the function "translation()".

   For the *names* parameter, please see the description of the
   translation object’s "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 "_()" function, like this:

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

   For convenience, you want the "_()" function to be installed in
   Python’s builtins namespace, so it is easily accessible in all
   modules of your application.


The "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 "NullTranslations";
this provides the basic interface you can use to write your own
specialized translation classes.  Here are the methods of
"NullTranslations":

class gettext.NullTranslations(fp=None)

   Takes an optional *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 "add_fallback()".  It then calls
   "self._parse(fp)" if *fp* is not "None".

   _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.

   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.

   gettext(message)

      If a fallback has been set, forward "gettext()" to the fallback.
      Otherwise, return *message*.  Overridden in derived classes.

   ngettext(singular, plural, n)

      If a fallback has been set, forward "ngettext()" to the
      fallback. Otherwise, return *singular* if *n* is 1; return
      *plural* otherwise. Overridden in derived classes.

   lgettext(message)

   lngettext(singular, plural, n)

      Equivalent to "gettext()" and "ngettext()", but the translation
      is returned as a byte string encoded in the preferred system
      encoding if no encoding was explicitly set with
      "set_output_charset()". Overridden in derived classes.

      Warning: These methods should be avoided in Python 3.  See the
        warning for the "lgettext()" function.

   info()

      Return the “protected” "_info" variable, a dictionary containing
      the metadata found in the message catalog file.

   charset()

      Return the encoding of the message catalog file.

   output_charset()

      Return the encoding used to return translated messages in
      "lgettext()" and "lngettext()".

   set_output_charset(charset)

      Change the encoding used to return translated messages.

   install(names=None)

      This method installs "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 "_()".  Supported names are
      "'gettext'", "'ngettext'", "'lgettext'" and "'lngettext'".

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

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

      This puts "_()" only in the module’s global namespace and so
      only affects calls within this module.


The "GNUTranslations" class
---------------------------

The "gettext" module provides one additional class derived from
"NullTranslations": "GNUTranslations".  This class overrides
"_parse()" to enable reading GNU **gettext** format ".mo" files in
both big-endian and little-endian format.

"GNUTranslations" parses optional metadata out of the translation
catalog. It is convention with GNU **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” "_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 "*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” "_info" instance variable.

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

class gettext.GNUTranslations

   The following methods are overridden from the base class
   implementation:

   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 "gettext()"
      method.  Otherwise, the *message* id is returned.

   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
      "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}

   lgettext(message)

   lngettext(singular, plural, n)

      Equivalent to "gettext()" and "ngettext()", but the translation
      is returned as a byte string encoded in the preferred system
      encoding if no encoding  was explicitly set with
      "set_output_charset()".

      Warning: These methods should be avoided in Python 3.  See the
        warning for the "lgettext()" function.


Solaris message catalog support
-------------------------------

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


The Catalog constructor
-----------------------

GNOME uses a version of the "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 "Catalog()" is
an alias for the "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:

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

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

3. create language-specific translations of the message catalogs

4. use the "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 "_()".  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 **gettext** only supported C or C++ source code but
its extended version **xgettext** scans code written in a number of
languages, including Python, to find strings marked as translatable.
Babel is a Python internationalization library that includes a
"pybabel" script to extract and compile message catalogs.  François
Pinard’s program called **xpot** does a similar job and is available
as part of his po-utils package.

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

**xgettext**, **pygettext**, and similar tools generate ".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 ".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 "<language-name>.po" file that’s compiled into a machine-readable
".mo" binary catalog file using the **msgfmt** program.  The ".mo"
files are used by the "gettext" module for the actual translation
processing at run-time.

How you use the "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 **gettext** API but instead the class-based API.

Let’s say your module is called “spam” and the module’s various
natural language translation ".mo" files reside in "/usr/share/locale"
in GNU **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 "_()"
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
"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 "_()" simply returns the
string unchanged.  And this dummy definition will temporarily override
any definition of "_()" in the built-in namespace (until the "del"
command). Take care, though if you have a previous definition of "_()"
in the local namespace.

Note that the second use of "_()" will not identify “a” as being
translatable to the **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
"N_()", which won’t conflict with any definition of "_()". However,
you will need to teach your message extraction program to look for
translatable strings marked with "N_()". **xgettext**, **pygettext**,
"pybabel extract", and **xpot** all support this through the use of
the "-k" command-line switch. The choice of "N_()" here is totally
arbitrary; it could have just as easily been
"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

-[ Footnotes ]-

[1] The default locale directory is system dependent; for example,
    on RedHat Linux it is "/usr/share/locale", but on Solaris it is
    "/usr/lib/locale". The "gettext" module does not try to support
    these system dependent defaults; instead its default is
    "*sys.prefix*/share/locale" (see "sys.prefix"). For this reason,
    it is always best to call "bindtextdomain()" with an explicit
    absolute path at the start of your application.

[2] See the footnote for "bindtextdomain()" above.
