cmlenz@2: .. -*- mode: rst; encoding: utf-8 -*-
cmlenz@2: 
cmlenz@2: =============================
cmlenz@2: Working with Message Catalogs
cmlenz@2: =============================
cmlenz@2: 
cmlenz@2: .. contents:: Contents
cmlenz@2:    :depth: 2
cmlenz@2: .. sectnum::
cmlenz@2: 
cmlenz@2: 
cmlenz@2: Introduction
cmlenz@2: ============
cmlenz@2: 
cmlenz@2: The ``gettext`` translation system enables you to mark any strings used in your
cmlenz@2: application as subject to localization, by wrapping them in functions such as
cmlenz@2: ``gettext(str)`` and ``ngettext(singular, plural, num)``. For brevity, the
cmlenz@40: ``gettext`` function is often aliased to ``_(str)``, so you can write:
cmlenz@40: 
cmlenz@40: .. code-block:: python
cmlenz@2: 
cmlenz@2:     print _("Hello")
cmlenz@2: 
cmlenz@40: instead of just:
cmlenz@40: 
cmlenz@40: .. code-block:: python
cmlenz@2: 
cmlenz@2:     print "Hello"
cmlenz@2: 
cmlenz@2: to make the string "Hello" localizable.
cmlenz@2: 
cmlenz@2: Message catalogs are collections of translations for such localizable messages
cmlenz@2: used in an application. They are commonly stored in PO (Portable Object) and MO
cmlenz@2: (Machine Object) files, the formats of which are defined by the GNU `gettext`_
cmlenz@2: tools and the GNU `translation project`_.
cmlenz@2: 
cmlenz@2:  .. _`gettext`: http://www.gnu.org/software/gettext/
cmlenz@2:  .. _`translation project`: http://sourceforge.net/projects/translation
cmlenz@2: 
cmlenz@2: The general procedure for building message catalogs looks something like this:
cmlenz@2: 
cmlenz@2:  * use a tool (such as ``xgettext``) to extract localizable strings from the
cmlenz@2:    code base and write them to a POT (PO Template) file.
cmlenz@2:  * make a copy of the POT file for a specific locale (for example, "en_US")
cmlenz@2:    and start translating the messages
cmlenz@2:  * use a tool such as ``msgfmt`` to compile the locale PO file into an binary
cmlenz@2:    MO file
cmlenz@2:  * later, when code changes make it necessary to update the translations, you
cmlenz@2:    regenerate the POT file and merge the changes into the various
cmlenz@2:    locale-specific PO files, for example using ``msgmerge``
cmlenz@2: 
cmlenz@2: Python provides the `gettext module`_ as part of the standard library, which
cmlenz@2: enables applications to work with appropriately generated MO files.
cmlenz@2: 
cmlenz@2:  .. _`gettext module`: http://docs.python.org/lib/module-gettext.html
cmlenz@2: 
cmlenz@2: As ``gettext`` provides a solid and well supported foundation for translating
cmlenz@2: application messages, Babel does not reinvent the wheel, but rather reuses this
cmlenz@2: infrastructure, and makes it easier to build message catalogs for Python
cmlenz@2: applications.
cmlenz@2: 
cmlenz@2: 
cmlenz@2: Message Extraction
cmlenz@2: ==================
cmlenz@2: 
cmlenz@2: Babel provides functionality similar to that of the ``xgettext`` program,
cmlenz@2: except that only extraction from Python source files is built-in, while support
cmlenz@2: for other file formats can be added using a simple extension mechanism.
cmlenz@2: 
cmlenz@48: Unlike ``xgettext``, which is usually invoked once for every file, the routines
cmlenz@48: for message extraction in Babel operate on directories. While the per-file
cmlenz@48: approach of ``xgettext`` works nicely with projects using a ``Makefile``,
cmlenz@48: Python projects rarely use ``make``, and thus a different mechanism is needed
cmlenz@48: for extracting messages from the heterogeneous collection of source files that
cmlenz@48: many Python projects are composed of.
cmlenz@48: 
cmlenz@48: When message extraction is based on directories instead of individual files,
cmlenz@48: there needs to be a way to configure which files should be treated in which
cmlenz@48: manner. For example, while many projects may contain ``.html`` files, some of
cmlenz@48: those files may be static HTML files that don't contain localizable message,
cmlenz@48: while others may be `Django`_ templates, and still others may contain `Genshi`_
cmlenz@48: markup templates. Some projects may even mix HTML files for different templates
cmlenz@48: languages (for whatever reason). Therefore the way in which messages are
cmlenz@48: extracted from source files can not only depend on the file extension, but
cmlenz@48: needs to be controllable in a precise manner.
cmlenz@48: 
cmlenz@48: .. _`Django`: http://www.djangoproject.com/
cmlenz@48: .. _`Genshi`: http://genshi.edgewall.org/
cmlenz@48: 
cmlenz@48: Babel accepts a configuration file to specify this mapping of files to
cmlenz@48: extraction methods, which is described below.
cmlenz@2: 
cmlenz@2: 
cmlenz@48: .. _`mapping`:
cmlenz@2: 
cmlenz@48: -------------------------------------------
cmlenz@48: Extraction Method Mapping and Configuration
cmlenz@48: -------------------------------------------
cmlenz@48: 
cmlenz@48: The mapping of extraction methods to files in Babel is done via a configuration
cmlenz@48: file. This file maps extended glob patterns to the names of the extraction
cmlenz@48: methods, and can also set various options for each pattern (which options are
cmlenz@48: available depends on the specific extraction method).
cmlenz@48: 
cmlenz@48: For example, the following configuration adds extraction of messages from both
cmlenz@48: Genshi markup templates and text templates:
cmlenz@48: 
cmlenz@48: .. code-block:: ini
cmlenz@48: 
cmlenz@48:     # Extraction from Python source files
cmlenz@48:     
cmlenz@48:     [python: foobar/**.py]
cmlenz@48:     
cmlenz@48:     # Extraction from Genshi HTML and text templates
cmlenz@48:     
cmlenz@48:     [genshi: foobar/**/templates/**.html]
cmlenz@48:     ignore_tags = script,style
cmlenz@48:     include_attrs = alt title summary
cmlenz@48:     
cmlenz@48:     [genshi: foobar/**/templates/**.txt]
cmlenz@48:     template_class = genshi.template.text:TextTemplate
cmlenz@48:     encoding = ISO-8819-15
cmlenz@48: 
cmlenz@48: The configuration file syntax is based on the format commonly found in ``.INI``
cmlenz@48: files on Windows systems, and as supported by the ``ConfigParser`` module in
cmlenz@48: the Python standard libraries. Section names (the strings enclosed in square
cmlenz@48: brackets) specify both the name of the extraction method, and the extended glob
cmlenz@48: pattern to specify the files that this extraction method should be used for,
cmlenz@48: separated by a colon. The options in the sections are passed to the extraction
cmlenz@48: method. Which options are available is specific to the extraction method used.
cmlenz@48: 
cmlenz@48: The extended glob patterns used in this configuration are similar to the glob
cmlenz@48: patterns provided by most shells. A single asterisk (``*``) is a wildcard for
cmlenz@48: any number of characters (except for the pathname component separator "/"),
cmlenz@48: while a question mark (``?``) only matches a single character. In addition,
cmlenz@48: two subsequent asterisk characters (``**``) can be used to make the wildcard
cmlenz@48: match any directory level, so the pattern ``**.txt`` matches any file with the
cmlenz@48: extension ``.txt`` in any directory.
cmlenz@48: 
cmlenz@48: Lines that start with a ``#`` or ``;`` character are ignored and can be used
cmlenz@48: for comments. Empty lines are also ignored, too.
cmlenz@48: 
cmlenz@49: .. note:: if you're performing message extraction using the command Babel
cmlenz@49:           provides for integration into ``setup.py`` scripts (see below), you
cmlenz@49:           can also provide this configuration in a different way, namely as a
cmlenz@49:           keyword argument to the ``setup()`` function.
cmlenz@2: 
cmlenz@2: 
cmlenz@49: ----------
cmlenz@49: Front-Ends
cmlenz@49: ----------
cmlenz@2: 
cmlenz@49: Babel provides two different front-ends to access its functionality for working
cmlenz@49: with message catalogs:
cmlenz@2: 
cmlenz@49:  * A `Command-line interface <cmdline.html>`_, and
cmlenz@49:  * `Integeration with distutils/setuptools <setup.html>`_
cmlenz@49: 
cmlenz@49: Which one you choose depends on the nature of your project. For most modern
cmlenz@49: Python projects, the distutils/setuptools integration is probably more
cmlenz@49: convenient.
cmlenz@49: 
cmlenz@2: 
cmlenz@48: --------------------------
cmlenz@48: Writing Extraction Methods
cmlenz@48: --------------------------
cmlenz@48: 
cmlenz@48: (TODO: write)
cmlenz@48: 
cmlenz@48: 
cmlenz@2: 
cmlenz@2: Extended ``Translations`` Class
cmlenz@2: ===============================
cmlenz@2: 
cmlenz@2: Many web-based applications are composed of a variety of different components
cmlenz@2: (possibly using some kind of plugin system), and some of those components may
cmlenz@2: provide their own message catalogs that need to be integrated into the larger
cmlenz@2: system.
cmlenz@2: 
cmlenz@2: To support this usage pattern, Babel provides a ``Translations`` class that is
cmlenz@2: derived from the ``GNUTranslations`` class in the ``gettext`` module. This
cmlenz@2: class adds a ``merge()`` method that takes another ``Translations`` instance,
cmlenz@48: and merges its contents into the catalog:
cmlenz@2: 
cmlenz@40: .. code-block:: python
cmlenz@40: 
cmlenz@2:     translations = Translations.load('main')
cmlenz@2:     translations.merge(Translations.load('plugin1'))