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Working with Message Catalogs
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Introduction
============

The ``gettext`` translation system enables you to mark any strings used in your
application as subject to localization, by wrapping them in functions such as
``gettext(str)`` and ``ngettext(singular, plural, num)``. For brevity, the
``gettext`` function is often aliased to ``_(str)``, so you can write::

    print _("Hello")

instead of just::

    print "Hello"

to make the string "Hello" localizable.

Message catalogs are collections of translations for such localizable messages
used in an application. They are commonly stored in PO (Portable Object) and MO
(Machine Object) files, the formats of which are defined by the GNU `gettext`_
tools and the GNU `translation project`_.

 .. _`gettext`: http://www.gnu.org/software/gettext/
 .. _`translation project`: http://sourceforge.net/projects/translation

The general procedure for building message catalogs looks something like this:

 * use a tool (such as ``xgettext``) to extract localizable strings from the
   code base and write them to a POT (PO Template) file.
 * make a copy of the POT file for a specific locale (for example, "en_US")
   and start translating the messages
 * use a tool such as ``msgfmt`` to compile the locale PO file into an binary
   MO file
 * later, when code changes make it necessary to update the translations, you
   regenerate the POT file and merge the changes into the various
   locale-specific PO files, for example using ``msgmerge``

Python provides the `gettext module`_ as part of the standard library, which
enables applications to work with appropriately generated MO files.

 .. _`gettext module`: http://docs.python.org/lib/module-gettext.html

As ``gettext`` provides a solid and well supported foundation for translating
application messages, Babel does not reinvent the wheel, but rather reuses this
infrastructure, and makes it easier to build message catalogs for Python
applications.


Message Extraction
==================

Babel provides functionality similar to that of the ``xgettext`` program,
except that only extraction from Python source files is built-in, while support
for other file formats can be added using a simple extension mechanism.

(TODO: more)


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Writing Extraction Methods
--------------------------

(TODO: write)

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``setup.py`` Commands
---------------------

(TODO: overview)

See `Distutils/Setuptools Integration <setup.html>`_ for more information.

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Command-line script
-------------------

(TODO: overview)

See `Command-Line Interface <cmdline.html>`_ for more information.


Extended ``Translations`` Class
===============================

Many web-based applications are composed of a variety of different components
(possibly using some kind of plugin system), and some of those components may
provide their own message catalogs that need to be integrated into the larger
system.

To support this usage pattern, Babel provides a ``Translations`` class that is
derived from the ``GNUTranslations`` class in the ``gettext`` module. This
class adds a ``merge()`` method that takes another ``Translations`` instance,
and merges its contents into the catalog::

    translations = Translations.load('main')
    translations.merge(Translations.load('plugin1'))