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 Babel 0.9.4
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<h1 class="title">Working with Message Catalogs</h1>
<div class="contents topic">
<a id="contents" name="contents">Contents</a>
<ul class="auto-toc simple">
<li><a class="reference" href="#introduction" id="id2" name="id2">1   Introduction</a></li>
<li><a class="reference" href="#message-extraction" id="id3" name="id3">2   Message Extraction</a><ul class="auto-toc">
<li><a class="reference" href="#front-ends" id="id4" name="id4">2.1   Front-Ends</a></li>
<li><a class="reference" href="#extraction-method-mapping-and-configuration" id="id5" name="id5">2.2   Extraction Method Mapping and Configuration</a><ul class="auto-toc">
<li><a class="reference" href="#default-extraction-methods" id="id6" name="id6">2.2.1   Default Extraction Methods</a></li>
<li><a class="reference" href="#id1" id="id7" name="id7">2.2.2   Referencing Extraction Methods</a></li>
</ul>
</li>
<li><a class="reference" href="#writing-extraction-methods" id="id8" name="id8">2.3   Writing Extraction Methods</a></li>
<li><a class="reference" href="#translator-comments" id="id9" name="id9">2.4   Translator Comments</a></li>
</ul>
</li>
</ul>
</div>
<div class="section">
<h1><a id="introduction" name="introduction">1   Introduction</a></h1>
The <tt class="docutils literal">gettext</tt> translation system enables you to mark any strings used in your
application as subject to localization, by wrapping them in functions such as
<tt class="docutils literal">gettext(str)</tt> and <tt class="docutils literal">ngettext(singular, plural, num)</tt>. For brevity, the
<tt class="docutils literal">gettext</tt> function is often aliased to <tt class="docutils literal">_(str)</tt>, so you can write:
<div class="highlight"><pre>print _("Hello")
</pre></div>
instead of just:
<div class="highlight"><pre>print "Hello"
</pre></div>
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 <a class="reference" href="http://www.gnu.org/software/gettext/">gettext</a>
tools and the GNU <a class="reference" href="http://sourceforge.net/projects/translation">translation project</a>.
<blockquote>
</blockquote>
The general procedure for building message catalogs looks something like this:
<blockquote>
<ul class="simple">
<li>use a tool (such as <tt class="docutils literal">xgettext</tt>) to extract localizable strings from the
code base and write them to a POT (PO Template) file.</li>
<li>make a copy of the POT file for a specific locale (for example, "en_US")
and start translating the messages</li>
<li>use a tool such as <tt class="docutils literal">msgfmt</tt> to compile the locale PO file into an binary
MO file</li>
<li>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 <tt class="docutils literal">msgmerge</tt></li>
</ul>
</blockquote>
Python provides the <a class="reference" href="http://docs.python.org/lib/module-gettext.html">gettext module</a> as part of the standard library, which
enables applications to work with appropriately generated MO files.
<blockquote>
</blockquote>
As <tt class="docutils literal">gettext</tt> 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.
</div>
<div class="section">
<h1><a id="message-extraction" name="message-extraction">2   Message Extraction</a></h1>
Babel provides functionality similar to that of the <tt class="docutils literal">xgettext</tt> 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.
Unlike <tt class="docutils literal">xgettext</tt>, which is usually invoked once for every file, the routines
for message extraction in Babel operate on directories. While the per-file
approach of <tt class="docutils literal">xgettext</tt> works nicely with projects using a <tt class="docutils literal">Makefile</tt>,
Python projects rarely use <tt class="docutils literal">make</tt>, and thus a different mechanism is needed
for extracting messages from the heterogeneous collection of source files that
many Python projects are composed of.
When message extraction is based on directories instead of individual files,
there needs to be a way to configure which files should be treated in which
manner. For example, while many projects may contain <tt class="docutils literal">.html</tt> files, some of
those files may be static HTML files that don't contain localizable message,
while others may be <a class="reference" href="http://www.djangoproject.com/">Django</a> templates, and still others may contain <a class="reference" href="http://genshi.edgewall.org/">Genshi</a>
markup templates. Some projects may even mix HTML files for different templates
languages (for whatever reason). Therefore the way in which messages are
extracted from source files can not only depend on the file extension, but
needs to be controllable in a precise manner.
Babel accepts a configuration file to specify this mapping of files to
extraction methods, which is described below.
<div class="section">
<h2><a id="front-ends" name="front-ends">2.1   Front-Ends</a></h2>
Babel provides two different front-ends to access its functionality for working
with message catalogs:
<blockquote>
<ul class="simple">
<li>A <a class="reference" href="cmdline.html">Command-line interface</a>, and</li>
<li><a class="reference" href="setup.html">Integration with distutils/setuptools</a></li>
</ul>
</blockquote>
Which one you choose depends on the nature of your project. For most modern
Python projects, the distutils/setuptools integration is probably more
convenient.
</div>
<div class="section">
<h2><a id="extraction-method-mapping-and-configuration" name="extraction-method-mapping-and-configuration">2.2   Extraction Method Mapping and Configuration</a></h2>
The mapping of extraction methods to files in Babel is done via a configuration
file. This file maps extended glob patterns to the names of the extraction
methods, and can also set various options for each pattern (which options are
available depends on the specific extraction method).
For example, the following configuration adds extraction of messages from both
Genshi markup templates and text templates:
<div class="highlight"><pre># Extraction from Python source files

[python: **.py]

# Extraction from Genshi HTML and text templates

[genshi: **/templates/**.html]
ignore_tags = script,style
include_attrs = alt title summary

[genshi: **/templates/**.txt]
template_class = genshi.template:TextTemplate
encoding = ISO-8819-15
</pre></div>
The configuration file syntax is based on the format commonly found in <tt class="docutils literal">.INI</tt>
files on Windows systems, and as supported by the <tt class="docutils literal">ConfigParser</tt> module in
the Python standard library. Section names (the strings enclosed in square
brackets) specify both the name of the extraction method, and the extended glob
pattern to specify the files that this extraction method should be used for,
separated by a colon. The options in the sections are passed to the extraction
method. Which options are available is specific to the extraction method used.
The extended glob patterns used in this configuration are similar to the glob
patterns provided by most shells. A single asterisk (<tt class="docutils literal">*</tt>) is a wildcard for
any number of characters (except for the pathname component separator "/"),
while a question mark (<tt class="docutils literal">?</tt>) only matches a single character. In addition,
two subsequent asterisk characters (<tt class="docutils literal">**</tt>) can be used to make the wildcard
match any directory level, so the pattern <tt class="docutils literal">**.txt</tt> matches any file with the
extension <tt class="docutils literal">.txt</tt> in any directory.
Lines that start with a <tt class="docutils literal">#</tt> or <tt class="docutils literal">;</tt> character are ignored and can be used
for comments. Empty lines are ignored, too.
<div class="note">
Note
if you're performing message extraction using the command Babel
provides for integration into <tt class="docutils literal">setup.py</tt> scripts, you can also
provide this configuration in a different way, namely as a keyword
argument to the <tt class="docutils literal">setup()</tt> function. See <a class="reference" href="setup.html">Distutils/Setuptools
Integration</a> for more information.
</div>
<div class="section">
<h3><a id="default-extraction-methods" name="default-extraction-methods">2.2.1   Default Extraction Methods</a></h3>
Babel comes with only two builtin extractors: <tt class="docutils literal">python</tt> (which extracts
messages from Python source files) and <tt class="docutils literal">ignore</tt> (which extracts nothing).
The <tt class="docutils literal">python</tt> extractor is by default mapped to the glob pattern <tt class="docutils literal">**.py</tt>,
meaning it'll be applied to all files with the <tt class="docutils literal">.py</tt> extension in any
directory. If you specify your own mapping configuration, this default mapping
is discarded, so you need to explicitly add it to your mapping (as shown in the
example above.)
</div>
<div class="section">
<h3><a id="id1" name="id1">2.2.2   Referencing Extraction Methods</a></h3>
To be able to use short extraction method names such as “genshi”, you need to
have <a class="reference" href="http://peak.telecommunity.com/DevCenter/PkgResources">pkg_resources</a> installed, and the package implementing that extraction
method needs to have been installed with its meta data (the <a class="reference" href="http://peak.telecommunity.com/DevCenter/PythonEggs">egg-info</a>).
If this is not possible for some reason, you need to map the short names to
fully qualified function names in an extract section in the mapping
configuration. For example:
<div class="highlight"><pre># Some custom extraction method

[extractors]
custom = mypackage.module:extract_custom

[custom: **.ctm]
some_option = foo
</pre></div>
Note that the builtin extraction methods <tt class="docutils literal">python</tt> and <tt class="docutils literal">ignore</tt> are available
by default, even if <a class="reference" href="http://peak.telecommunity.com/DevCenter/PkgResources">pkg_resources</a> is not installed. You should never need to
explicitly define them in the <tt class="docutils literal">[extractors]</tt> section.
</div>
</div>
<div class="section">
<h2><a id="writing-extraction-methods" name="writing-extraction-methods">2.3   Writing Extraction Methods</a></h2>
Adding new methods for extracting localizable methods is easy. First, you'll
need to implement a function that complies with the following interface:
<div class="highlight"><pre>def extract_xxx(fileobj, keywords, comment_tags, options):
 """Extract messages from XXX files.

:param fileobj: the file-like object the messages should be extracted
 from
 :param keywords: a list of keywords (i.e. function names) that should
 be recognized as translation functions
 :param comment_tags: a list of translator tags to search for and
 include in the results
 :param options: a dictionary of additional options (optional)
 :return: an iterator over ``(lineno, funcname, message, comments)``
 tuples
 :rtype: ``iterator``
 """
</pre></div>
<div class="note">
Note
Any strings in the tuples produced by this function must be either
<tt class="docutils literal">unicode</tt> objects, or <tt class="docutils literal">str</tt> objects using plain ASCII characters.
That means that if sources contain strings using other encodings, it
is the job of the extractor implementation to do the decoding to
<tt class="docutils literal">unicode</tt> objects.
</div>
Next, you should register that function as an entry point. This requires your
<tt class="docutils literal">setup.py</tt> script to use <a class="reference" href="http://peak.telecommunity.com/DevCenter/setuptools">setuptools</a>, and your package to be installed with
the necessary metadata. If that's taken care of, add something like the
following to your <tt class="docutils literal">setup.py</tt> script:
<div class="highlight"><pre>def setup(...

entry_points = """
 [babel.extractors]
 xxx = your.package:extract_xxx
 """,
</pre></div>
That is, add your extraction method to the entry point group
<tt class="docutils literal">babel.extractors</tt>, where the name of the entry point is the name that people
will use to reference the extraction method, and the value being the module and
the name of the function (separated by a colon) implementing the actual
extraction.
<div class="note">
Note
As shown in <a class="reference" href="#referencing-extraction-methods">Referencing Extraction Methods</a>, declaring an entry
point is not strictly required, as users can still reference the
extraction function directly. But whenever possible, the entry point
should be declared to make configuration more convenient.
</div>
</div>
<div class="section">
<h2><a id="translator-comments" name="translator-comments">2.4   Translator Comments</a></h2>
First of all what are comments tags. Comments tags are excerpts of text to
search for in comments, only comments, right before the <a class="reference" href="http://docs.python.org/lib/module-gettext.html">python gettext</a>
calls, as shown on the following example:
<blockquote>
</blockquote>
<div class="highlight"><pre># NOTE: This is a comment about `Foo Bar`
_('Foo Bar')
</pre></div>
The comments tag for the above example would be <tt class="docutils literal">NOTE:</tt>, and the translator
comment for that tag would be <tt class="docutils literal">This is a comment about `Foo Bar`</tt>.
The resulting output in the catalog template would be something like:
<pre class="literal-block">
#. This is a comment about `Foo Bar`
#: main.py:2
msgid "Foo Bar"
msgstr ""
</pre>
Now, you might ask, why would I need that?
Consider this simple case; you have a menu item called “manual”. You know what
it means, but when the translator sees this they will wonder did you mean:
<ol class="arabic simple">
<li>a document or help manual, or</li>
<li>a manual process?</li>
</ol>
This is the simplest case where a translation comment such as
“The installation manual” helps to clarify the situation and makes a translator
more productive.
<div class="note">
Note
Whether translator comments can be extracted depends on the extraction
method in use. The Python extractor provided by Babel does implement
this feature, but others may not.
</div>
</div>
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