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<div class="section" id="module-gettext">
<span id="gettext-multilingual-internationalization-services"></span><h1>23.1. <a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-mod docutils literal"><span class="pre">gettext</span></code></a> — Multilingual internationalization services<a class="headerlink" href="#module-gettext" title="Permalink to this headline">¶</a></h1>
<p><strong>Source code:</strong> <a class="reference external" href="https://hg.python.org/cpython/file/3.5/Lib/gettext.py">Lib/gettext.py</a></p>
<hr class="docutils" />
<p>The <a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-mod docutils literal"><span class="pre">gettext</span></code></a> module provides internationalization (I18N) and localization
(L10N) services for your Python modules and applications. It supports both the
GNU <code class="docutils literal"><span class="pre">gettext</span></code> 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.</p>
<p>Some hints on localizing your Python modules and applications are also given.</p>
<div class="section" id="gnu-gettext-api">
<h2>23.1.1. GNU <strong class="program">gettext</strong> API<a class="headerlink" href="#gnu-gettext-api" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-mod docutils literal"><span class="pre">gettext</span></code></a> module defines the following API, which is very similar to
the GNU <strong class="program">gettext</strong> 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.</p>
<dl class="function">
<dt id="gettext.bindtextdomain">
<code class="descclassname">gettext.</code><code class="descname">bindtextdomain</code><span class="sig-paren">(</span><em>domain</em>, <em>localedir=None</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.bindtextdomain" title="Permalink to this definition">¶</a></dt>
<dd><p>Bind the <em>domain</em> to the locale directory <em>localedir</em>. More concretely,
<a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-mod docutils literal"><span class="pre">gettext</span></code></a> will look for binary <code class="file docutils literal"><span class="pre">.mo</span></code> files for the given domain using
the path (on Unix): <code class="file docutils literal"><span class="pre">localedir/language/LC_MESSAGES/domain.mo</span></code>, where
<em>languages</em> is searched for in the environment variables <span class="target" id="index-0"></span><code class="xref std std-envvar docutils literal"><span class="pre">LANGUAGE</span></code>,
<span class="target" id="index-1"></span><code class="xref std std-envvar docutils literal"><span class="pre">LC_ALL</span></code>, <span class="target" id="index-2"></span><code class="xref std std-envvar docutils literal"><span class="pre">LC_MESSAGES</span></code>, and <span class="target" id="index-3"></span><code class="xref std std-envvar docutils literal"><span class="pre">LANG</span></code> respectively.</p>
<p>If <em>localedir</em> is omitted or <code class="docutils literal"><span class="pre">None</span></code>, then the current binding for <em>domain</em> is
returned. <a class="footnote-reference" href="#id3" id="id1">[1]</a></p>
</dd></dl>
<dl class="function">
<dt id="gettext.bind_textdomain_codeset">
<code class="descclassname">gettext.</code><code class="descname">bind_textdomain_codeset</code><span class="sig-paren">(</span><em>domain</em>, <em>codeset=None</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.bind_textdomain_codeset" title="Permalink to this definition">¶</a></dt>
<dd><p>Bind the <em>domain</em> to <em>codeset</em>, changing the encoding of strings returned by the
<a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-func docutils literal"><span class="pre">gettext()</span></code></a> family of functions. If <em>codeset</em> is omitted, then the current
binding is returned.</p>
</dd></dl>
<dl class="function">
<dt id="gettext.textdomain">
<code class="descclassname">gettext.</code><code class="descname">textdomain</code><span class="sig-paren">(</span><em>domain=None</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.textdomain" title="Permalink to this definition">¶</a></dt>
<dd><p>Change or query the current global domain. If <em>domain</em> is <code class="docutils literal"><span class="pre">None</span></code>, then the
current global domain is returned, otherwise the global domain is set to
<em>domain</em>, which is returned.</p>
</dd></dl>
<dl class="function">
<dt id="gettext.gettext">
<code class="descclassname">gettext.</code><code class="descname">gettext</code><span class="sig-paren">(</span><em>message</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.gettext" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the localized translation of <em>message</em>, based on the current global
domain, language, and locale directory. This function is usually aliased as
<code class="xref py py-func docutils literal"><span class="pre">_()</span></code> in the local namespace (see examples below).</p>
</dd></dl>
<dl class="function">
<dt id="gettext.lgettext">
<code class="descclassname">gettext.</code><code class="descname">lgettext</code><span class="sig-paren">(</span><em>message</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.lgettext" title="Permalink to this definition">¶</a></dt>
<dd><p>Equivalent to <a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-func docutils literal"><span class="pre">gettext()</span></code></a>, but the translation is returned in the
preferred system encoding, if no other encoding was explicitly set with
<a class="reference internal" href="#gettext.bind_textdomain_codeset" title="gettext.bind_textdomain_codeset"><code class="xref py py-func docutils literal"><span class="pre">bind_textdomain_codeset()</span></code></a>.</p>
</dd></dl>
<dl class="function">
<dt id="gettext.dgettext">
<code class="descclassname">gettext.</code><code class="descname">dgettext</code><span class="sig-paren">(</span><em>domain</em>, <em>message</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.dgettext" title="Permalink to this definition">¶</a></dt>
<dd><p>Like <a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-func docutils literal"><span class="pre">gettext()</span></code></a>, but look the message up in the specified <em>domain</em>.</p>
</dd></dl>
<dl class="function">
<dt id="gettext.ldgettext">
<code class="descclassname">gettext.</code><code class="descname">ldgettext</code><span class="sig-paren">(</span><em>domain</em>, <em>message</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.ldgettext" title="Permalink to this definition">¶</a></dt>
<dd><p>Equivalent to <a class="reference internal" href="#gettext.dgettext" title="gettext.dgettext"><code class="xref py py-func docutils literal"><span class="pre">dgettext()</span></code></a>, but the translation is returned in the
preferred system encoding, if no other encoding was explicitly set with
<a class="reference internal" href="#gettext.bind_textdomain_codeset" title="gettext.bind_textdomain_codeset"><code class="xref py py-func docutils literal"><span class="pre">bind_textdomain_codeset()</span></code></a>.</p>
</dd></dl>
<dl class="function">
<dt id="gettext.ngettext">
<code class="descclassname">gettext.</code><code class="descname">ngettext</code><span class="sig-paren">(</span><em>singular</em>, <em>plural</em>, <em>n</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.ngettext" title="Permalink to this definition">¶</a></dt>
<dd><p>Like <a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-func docutils literal"><span class="pre">gettext()</span></code></a>, but consider plural forms. If a translation is found,
apply the plural formula to <em>n</em>, and return the resulting message (some
languages have more than two plural forms). If no translation is found, return
<em>singular</em> if <em>n</em> is 1; return <em>plural</em> otherwise.</p>
<p>The Plural formula is taken from the catalog header. It is a C or Python
expression that has a free variable <em>n</em>; the expression evaluates to the index
of the plural in the catalog. See
<a class="reference external" href="https://www.gnu.org/software/gettext/manual/gettext.html">the GNU gettext documentation</a>
for the precise syntax to be used in <code class="file docutils literal"><span class="pre">.po</span></code> files and the
formulas for a variety of languages.</p>
</dd></dl>
<dl class="function">
<dt id="gettext.lngettext">
<code class="descclassname">gettext.</code><code class="descname">lngettext</code><span class="sig-paren">(</span><em>singular</em>, <em>plural</em>, <em>n</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.lngettext" title="Permalink to this definition">¶</a></dt>
<dd><p>Equivalent to <a class="reference internal" href="#gettext.ngettext" title="gettext.ngettext"><code class="xref py py-func docutils literal"><span class="pre">ngettext()</span></code></a>, but the translation is returned in the
preferred system encoding, if no other encoding was explicitly set with
<a class="reference internal" href="#gettext.bind_textdomain_codeset" title="gettext.bind_textdomain_codeset"><code class="xref py py-func docutils literal"><span class="pre">bind_textdomain_codeset()</span></code></a>.</p>
</dd></dl>
<dl class="function">
<dt id="gettext.dngettext">
<code class="descclassname">gettext.</code><code class="descname">dngettext</code><span class="sig-paren">(</span><em>domain</em>, <em>singular</em>, <em>plural</em>, <em>n</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.dngettext" title="Permalink to this definition">¶</a></dt>
<dd><p>Like <a class="reference internal" href="#gettext.ngettext" title="gettext.ngettext"><code class="xref py py-func docutils literal"><span class="pre">ngettext()</span></code></a>, but look the message up in the specified <em>domain</em>.</p>
</dd></dl>
<dl class="function">
<dt id="gettext.ldngettext">
<code class="descclassname">gettext.</code><code class="descname">ldngettext</code><span class="sig-paren">(</span><em>domain</em>, <em>singular</em>, <em>plural</em>, <em>n</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.ldngettext" title="Permalink to this definition">¶</a></dt>
<dd><p>Equivalent to <a class="reference internal" href="#gettext.dngettext" title="gettext.dngettext"><code class="xref py py-func docutils literal"><span class="pre">dngettext()</span></code></a>, but the translation is returned in the
preferred system encoding, if no other encoding was explicitly set with
<a class="reference internal" href="#gettext.bind_textdomain_codeset" title="gettext.bind_textdomain_codeset"><code class="xref py py-func docutils literal"><span class="pre">bind_textdomain_codeset()</span></code></a>.</p>
</dd></dl>
<p>Note that GNU <strong class="program">gettext</strong> also defines a <code class="xref py py-func docutils literal"><span class="pre">dcgettext()</span></code> method, but
this was deemed not useful and so it is currently unimplemented.</p>
<p>Here’s an example of typical usage for this API:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">gettext</span>
<span class="n">gettext</span><span class="o">.</span><span class="n">bindtextdomain</span><span class="p">(</span><span class="s1">'myapplication'</span><span class="p">,</span> <span class="s1">'/path/to/my/language/directory'</span><span class="p">)</span>
<span class="n">gettext</span><span class="o">.</span><span class="n">textdomain</span><span class="p">(</span><span class="s1">'myapplication'</span><span class="p">)</span>
<span class="n">_</span> <span class="o">=</span> <span class="n">gettext</span><span class="o">.</span><span class="n">gettext</span>
<span class="c1"># ...</span>
<span class="nb">print</span><span class="p">(</span><span class="n">_</span><span class="p">(</span><span class="s1">'This is a translatable string.'</span><span class="p">))</span>
</pre></div>
</div>
</div>
<div class="section" id="class-based-api">
<h2>23.1.2. Class-based API<a class="headerlink" href="#class-based-api" title="Permalink to this headline">¶</a></h2>
<p>The class-based API of the <a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-mod docutils literal"><span class="pre">gettext</span></code></a> module gives you more flexibility and
greater convenience than the GNU <strong class="program">gettext</strong> API. It is the recommended
way of localizing your Python applications and modules. <a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-mod docutils literal"><span class="pre">gettext</span></code></a> defines
a “translations” class which implements the parsing of GNU <code class="file docutils literal"><span class="pre">.mo</span></code> format
files, and has methods for returning strings. Instances of this “translations”
class can also install themselves in the built-in namespace as the function
<code class="xref py py-func docutils literal"><span class="pre">_()</span></code>.</p>
<dl class="function">
<dt id="gettext.find">
<code class="descclassname">gettext.</code><code class="descname">find</code><span class="sig-paren">(</span><em>domain</em>, <em>localedir=None</em>, <em>languages=None</em>, <em>all=False</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.find" title="Permalink to this definition">¶</a></dt>
<dd><p>This function implements the standard <code class="file docutils literal"><span class="pre">.mo</span></code> file search algorithm. It
takes a <em>domain</em>, identical to what <a class="reference internal" href="#gettext.textdomain" title="gettext.textdomain"><code class="xref py py-func docutils literal"><span class="pre">textdomain()</span></code></a> takes. Optional
<em>localedir</em> is as in <a class="reference internal" href="#gettext.bindtextdomain" title="gettext.bindtextdomain"><code class="xref py py-func docutils literal"><span class="pre">bindtextdomain()</span></code></a> Optional <em>languages</em> is a list of
strings, where each string is a language code.</p>
<p>If <em>localedir</em> is not given, then the default system locale directory is used.
<a class="footnote-reference" href="#id4" id="id2">[2]</a> If <em>languages</em> is not given, then the following environment variables are
searched: <span class="target" id="index-4"></span><code class="xref std std-envvar docutils literal"><span class="pre">LANGUAGE</span></code>, <span class="target" id="index-5"></span><code class="xref std std-envvar docutils literal"><span class="pre">LC_ALL</span></code>, <span class="target" id="index-6"></span><code class="xref std std-envvar docutils literal"><span class="pre">LC_MESSAGES</span></code>, and
<span class="target" id="index-7"></span><code class="xref std std-envvar docutils literal"><span class="pre">LANG</span></code>. The first one returning a non-empty value is used for the
<em>languages</em> 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.</p>
<p><a class="reference internal" href="#gettext.find" title="gettext.find"><code class="xref py py-func docutils literal"><span class="pre">find()</span></code></a> then expands and normalizes the languages, and then iterates
through them, searching for an existing file built of these components:</p>
<p><code class="file docutils literal"><em><span class="pre">localedir</span></em><span class="pre">/</span><em><span class="pre">language</span></em><span class="pre">/LC_MESSAGES/</span><em><span class="pre">domain</span></em><span class="pre">.mo</span></code></p>
<p>The first such file name that exists is returned by <a class="reference internal" href="#gettext.find" title="gettext.find"><code class="xref py py-func docutils literal"><span class="pre">find()</span></code></a>. If no such
file is found, then <code class="docutils literal"><span class="pre">None</span></code> is returned. If <em>all</em> 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.</p>
</dd></dl>
<dl class="function">
<dt id="gettext.translation">
<code class="descclassname">gettext.</code><code class="descname">translation</code><span class="sig-paren">(</span><em>domain</em>, <em>localedir=None</em>, <em>languages=None</em>, <em>class_=None</em>, <em>fallback=False</em>, <em>codeset=None</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.translation" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a <code class="xref py py-class docutils literal"><span class="pre">Translations</span></code> instance based on the <em>domain</em>, <em>localedir</em>,
and <em>languages</em>, which are first passed to <a class="reference internal" href="#gettext.find" title="gettext.find"><code class="xref py py-func docutils literal"><span class="pre">find()</span></code></a> to get a list of the
associated <code class="file docutils literal"><span class="pre">.mo</span></code> file paths. Instances with identical <code class="file docutils literal"><span class="pre">.mo</span></code> file
names are cached. The actual class instantiated is either <em>class_</em> if
provided, otherwise <code class="xref py py-class docutils literal"><span class="pre">GNUTranslations</span></code>. The class’s constructor must
take a single <a class="reference internal" href="../glossary.html#term-file-object"><span class="xref std std-term">file object</span></a> argument. If provided, <em>codeset</em> will change
the charset used to encode translated strings in the <a class="reference internal" href="#gettext.lgettext" title="gettext.lgettext"><code class="xref py py-meth docutils literal"><span class="pre">lgettext()</span></code></a> and
<a class="reference internal" href="#gettext.lngettext" title="gettext.lngettext"><code class="xref py py-meth docutils literal"><span class="pre">lngettext()</span></code></a> methods.</p>
<p>If multiple files are found, later files are used as fallbacks for earlier ones.
To allow setting the fallback, <a class="reference internal" href="copy.html#copy.copy" title="copy.copy"><code class="xref py py-func docutils literal"><span class="pre">copy.copy()</span></code></a> is used to clone each
translation object from the cache; the actual instance data is still shared with
the cache.</p>
<p>If no <code class="file docutils literal"><span class="pre">.mo</span></code> file is found, this function raises <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal"><span class="pre">OSError</span></code></a> if
<em>fallback</em> is false (which is the default), and returns a
<a class="reference internal" href="#gettext.NullTranslations" title="gettext.NullTranslations"><code class="xref py py-class docutils literal"><span class="pre">NullTranslations</span></code></a> instance if <em>fallback</em> is true.</p>
<div class="versionchanged">
<p><span class="versionmodified">Changed in version 3.3: </span><a class="reference internal" href="exceptions.html#IOError" title="IOError"><code class="xref py py-exc docutils literal"><span class="pre">IOError</span></code></a> used to be raised instead of <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal"><span class="pre">OSError</span></code></a>.</p>
</div>
</dd></dl>
<dl class="function">
<dt id="gettext.install">
<code class="descclassname">gettext.</code><code class="descname">install</code><span class="sig-paren">(</span><em>domain</em>, <em>localedir=None</em>, <em>codeset=None</em>, <em>names=None</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.install" title="Permalink to this definition">¶</a></dt>
<dd><p>This installs the function <code class="xref py py-func docutils literal"><span class="pre">_()</span></code> in Python’s builtins namespace, based on
<em>domain</em>, <em>localedir</em>, and <em>codeset</em> which are passed to the function
<a class="reference internal" href="#gettext.translation" title="gettext.translation"><code class="xref py py-func docutils literal"><span class="pre">translation()</span></code></a>.</p>
<p>For the <em>names</em> parameter, please see the description of the translation
object’s <a class="reference internal" href="#gettext.NullTranslations.install" title="gettext.NullTranslations.install"><code class="xref py py-meth docutils literal"><span class="pre">install()</span></code></a> method.</p>
<p>As seen below, you usually mark the strings in your application that are
candidates for translation, by wrapping them in a call to the <code class="xref py py-func docutils literal"><span class="pre">_()</span></code>
function, like this:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="nb">print</span><span class="p">(</span><span class="n">_</span><span class="p">(</span><span class="s1">'This string will be translated.'</span><span class="p">))</span>
</pre></div>
</div>
<p>For convenience, you want the <code class="xref py py-func docutils literal"><span class="pre">_()</span></code> function to be installed in Python’s
builtins namespace, so it is easily accessible in all modules of your
application.</p>
</dd></dl>
<div class="section" id="the-nulltranslations-class">
<h3>23.1.2.1. The <a class="reference internal" href="#gettext.NullTranslations" title="gettext.NullTranslations"><code class="xref py py-class docutils literal"><span class="pre">NullTranslations</span></code></a> class<a class="headerlink" href="#the-nulltranslations-class" title="Permalink to this headline">¶</a></h3>
<p>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 <a class="reference internal" href="#gettext.NullTranslations" title="gettext.NullTranslations"><code class="xref py py-class docutils literal"><span class="pre">NullTranslations</span></code></a>; this provides the basic
interface you can use to write your own specialized translation classes. Here
are the methods of <a class="reference internal" href="#gettext.NullTranslations" title="gettext.NullTranslations"><code class="xref py py-class docutils literal"><span class="pre">NullTranslations</span></code></a>:</p>
<dl class="class">
<dt id="gettext.NullTranslations">
<em class="property">class </em><code class="descclassname">gettext.</code><code class="descname">NullTranslations</code><span class="sig-paren">(</span><em>fp=None</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.NullTranslations" title="Permalink to this definition">¶</a></dt>
<dd><p>Takes an optional <a class="reference internal" href="../glossary.html#term-file-object"><span class="xref std std-term">file object</span></a> <em>fp</em>, which is ignored by the base class.
Initializes “protected” instance variables <em>_info</em> and <em>_charset</em> which are set
by derived classes, as well as <em>_fallback</em>, which is set through
<a class="reference internal" href="#gettext.NullTranslations.add_fallback" title="gettext.NullTranslations.add_fallback"><code class="xref py py-meth docutils literal"><span class="pre">add_fallback()</span></code></a>. It then calls <code class="docutils literal"><span class="pre">self._parse(fp)</span></code> if <em>fp</em> is not
<code class="docutils literal"><span class="pre">None</span></code>.</p>
<dl class="method">
<dt id="gettext.NullTranslations._parse">
<code class="descname">_parse</code><span class="sig-paren">(</span><em>fp</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.NullTranslations._parse" title="Permalink to this definition">¶</a></dt>
<dd><p>No-op’d in the base class, this method takes file object <em>fp</em>, 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.</p>
</dd></dl>
<dl class="method">
<dt id="gettext.NullTranslations.add_fallback">
<code class="descname">add_fallback</code><span class="sig-paren">(</span><em>fallback</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.NullTranslations.add_fallback" title="Permalink to this definition">¶</a></dt>
<dd><p>Add <em>fallback</em> 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.</p>
</dd></dl>
<dl class="method">
<dt id="gettext.NullTranslations.gettext">
<code class="descname">gettext</code><span class="sig-paren">(</span><em>message</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.NullTranslations.gettext" title="Permalink to this definition">¶</a></dt>
<dd><p>If a fallback has been set, forward <a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-meth docutils literal"><span class="pre">gettext()</span></code></a> to the fallback.
Otherwise, return the translated message. Overridden in derived classes.</p>
</dd></dl>
<dl class="method">
<dt id="gettext.NullTranslations.lgettext">
<code class="descname">lgettext</code><span class="sig-paren">(</span><em>message</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.NullTranslations.lgettext" title="Permalink to this definition">¶</a></dt>
<dd><p>If a fallback has been set, forward <a class="reference internal" href="#gettext.lgettext" title="gettext.lgettext"><code class="xref py py-meth docutils literal"><span class="pre">lgettext()</span></code></a> to the fallback.
Otherwise, return the translated message. Overridden in derived classes.</p>
</dd></dl>
<dl class="method">
<dt id="gettext.NullTranslations.ngettext">
<code class="descname">ngettext</code><span class="sig-paren">(</span><em>singular</em>, <em>plural</em>, <em>n</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.NullTranslations.ngettext" title="Permalink to this definition">¶</a></dt>
<dd><p>If a fallback has been set, forward <a class="reference internal" href="#gettext.ngettext" title="gettext.ngettext"><code class="xref py py-meth docutils literal"><span class="pre">ngettext()</span></code></a> to the fallback.
Otherwise, return the translated message. Overridden in derived classes.</p>
</dd></dl>
<dl class="method">
<dt id="gettext.NullTranslations.lngettext">
<code class="descname">lngettext</code><span class="sig-paren">(</span><em>singular</em>, <em>plural</em>, <em>n</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.NullTranslations.lngettext" title="Permalink to this definition">¶</a></dt>
<dd><p>If a fallback has been set, forward <a class="reference internal" href="#gettext.lngettext" title="gettext.lngettext"><code class="xref py py-meth docutils literal"><span class="pre">lngettext()</span></code></a> to the fallback.
Otherwise, return the translated message. Overridden in derived classes.</p>
</dd></dl>
<dl class="method">
<dt id="gettext.NullTranslations.info">
<code class="descname">info</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#gettext.NullTranslations.info" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the “protected” <code class="xref py py-attr docutils literal"><span class="pre">_info</span></code> variable.</p>
</dd></dl>
<dl class="method">
<dt id="gettext.NullTranslations.charset">
<code class="descname">charset</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#gettext.NullTranslations.charset" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the “protected” <code class="xref py py-attr docutils literal"><span class="pre">_charset</span></code> variable, which is the encoding of
the message catalog file.</p>
</dd></dl>
<dl class="method">
<dt id="gettext.NullTranslations.output_charset">
<code class="descname">output_charset</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#gettext.NullTranslations.output_charset" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the “protected” <code class="xref py py-attr docutils literal"><span class="pre">_output_charset</span></code> variable, which defines the
encoding used to return translated messages in <a class="reference internal" href="#gettext.lgettext" title="gettext.lgettext"><code class="xref py py-meth docutils literal"><span class="pre">lgettext()</span></code></a> and
<a class="reference internal" href="#gettext.lngettext" title="gettext.lngettext"><code class="xref py py-meth docutils literal"><span class="pre">lngettext()</span></code></a>.</p>
</dd></dl>
<dl class="method">
<dt id="gettext.NullTranslations.set_output_charset">
<code class="descname">set_output_charset</code><span class="sig-paren">(</span><em>charset</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.NullTranslations.set_output_charset" title="Permalink to this definition">¶</a></dt>
<dd><p>Change the “protected” <code class="xref py py-attr docutils literal"><span class="pre">_output_charset</span></code> variable, which defines the
encoding used to return translated messages.</p>
</dd></dl>
<dl class="method">
<dt id="gettext.NullTranslations.install">
<code class="descname">install</code><span class="sig-paren">(</span><em>names=None</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.NullTranslations.install" title="Permalink to this definition">¶</a></dt>
<dd><p>This method installs <code class="xref py py-meth docutils literal"><span class="pre">self.gettext()</span></code> into the built-in namespace,
binding it to <code class="docutils literal"><span class="pre">_</span></code>.</p>
<p>If the <em>names</em> parameter is given, it must be a sequence containing the
names of functions you want to install in the builtins namespace in
addition to <code class="xref py py-func docutils literal"><span class="pre">_()</span></code>. Supported names are <code class="docutils literal"><span class="pre">'gettext'</span></code> (bound to
<code class="xref py py-meth docutils literal"><span class="pre">self.gettext()</span></code>), <code class="docutils literal"><span class="pre">'ngettext'</span></code> (bound to <code class="xref py py-meth docutils literal"><span class="pre">self.ngettext()</span></code>),
<code class="docutils literal"><span class="pre">'lgettext'</span></code> and <code class="docutils literal"><span class="pre">'lngettext'</span></code>.</p>
<p>Note that this is only one way, albeit the most convenient way, to make
the <code class="xref py py-func docutils literal"><span class="pre">_()</span></code> function available to your application. Because it affects
the entire application globally, and specifically the built-in namespace,
localized modules should never install <code class="xref py py-func docutils literal"><span class="pre">_()</span></code>. Instead, they should use
this code to make <code class="xref py py-func docutils literal"><span class="pre">_()</span></code> available to their module:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">gettext</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">gettext</span><span class="o">.</span><span class="n">translation</span><span class="p">(</span><span class="s1">'mymodule'</span><span class="p">,</span> <span class="o">...</span><span class="p">)</span>
<span class="n">_</span> <span class="o">=</span> <span class="n">t</span><span class="o">.</span><span class="n">gettext</span>
</pre></div>
</div>
<p>This puts <code class="xref py py-func docutils literal"><span class="pre">_()</span></code> only in the module’s global namespace and so only
affects calls within this module.</p>
</dd></dl>
</dd></dl>
</div>
<div class="section" id="the-gnutranslations-class">
<h3>23.1.2.2. The <code class="xref py py-class docutils literal"><span class="pre">GNUTranslations</span></code> class<a class="headerlink" href="#the-gnutranslations-class" title="Permalink to this headline">¶</a></h3>
<p>The <a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-mod docutils literal"><span class="pre">gettext</span></code></a> module provides one additional class derived from
<a class="reference internal" href="#gettext.NullTranslations" title="gettext.NullTranslations"><code class="xref py py-class docutils literal"><span class="pre">NullTranslations</span></code></a>: <code class="xref py py-class docutils literal"><span class="pre">GNUTranslations</span></code>. This class overrides
<code class="xref py py-meth docutils literal"><span class="pre">_parse()</span></code> to enable reading GNU <strong class="program">gettext</strong> format <code class="file docutils literal"><span class="pre">.mo</span></code> files
in both big-endian and little-endian format.</p>
<p><code class="xref py py-class docutils literal"><span class="pre">GNUTranslations</span></code> parses optional meta-data out of the translation
catalog. It is convention with GNU <strong class="program">gettext</strong> to include meta-data as
the translation for the empty string. This meta-data is in <span class="target" id="index-8"></span><a class="rfc reference external" href="https://tools.ietf.org/html/rfc822.html"><strong>RFC 822</strong></a>-style
<code class="docutils literal"><span class="pre">key:</span> <span class="pre">value</span></code> pairs, and should contain the <code class="docutils literal"><span class="pre">Project-Id-Version</span></code> key. If the
key <code class="docutils literal"><span class="pre">Content-Type</span></code> is found, then the <code class="docutils literal"><span class="pre">charset</span></code> property is used to
initialize the “protected” <code class="xref py py-attr docutils literal"><span class="pre">_charset</span></code> instance variable, defaulting to
<code class="docutils literal"><span class="pre">None</span></code> 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 encoding is assumed.</p>
<p>Since message ids are read as Unicode strings too, all <code class="xref py py-meth docutils literal"><span class="pre">*gettext()</span></code> methods
will assume message ids as Unicode strings, not byte strings.</p>
<p>The entire set of key/value pairs are placed into a dictionary and set as the
“protected” <code class="xref py py-attr docutils literal"><span class="pre">_info</span></code> instance variable.</p>
<p>If the <code class="file docutils literal"><span class="pre">.mo</span></code> file’s magic number is invalid, the major version number is
unexpected, or if other problems occur while reading the file, instantiating a
<code class="xref py py-class docutils literal"><span class="pre">GNUTranslations</span></code> class can raise <a class="reference internal" href="exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal"><span class="pre">OSError</span></code></a>.</p>
<p>The following methods are overridden from the base class implementation:</p>
<dl class="method">
<dt id="gettext.GNUTranslations.gettext">
<code class="descclassname">GNUTranslations.</code><code class="descname">gettext</code><span class="sig-paren">(</span><em>message</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.GNUTranslations.gettext" title="Permalink to this definition">¶</a></dt>
<dd><p>Look up the <em>message</em> id in the catalog and return the corresponding message
string, as a Unicode string. If there is no entry in the catalog for the
<em>message</em> id, and a fallback has been set, the look up is forwarded to the
fallback’s <a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-meth docutils literal"><span class="pre">gettext()</span></code></a> method. Otherwise, the <em>message</em> id is returned.</p>
</dd></dl>
<dl class="method">
<dt id="gettext.GNUTranslations.lgettext">
<code class="descclassname">GNUTranslations.</code><code class="descname">lgettext</code><span class="sig-paren">(</span><em>message</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.GNUTranslations.lgettext" title="Permalink to this definition">¶</a></dt>
<dd><p>Equivalent to <a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-meth docutils literal"><span class="pre">gettext()</span></code></a>, but the translation is returned as a
bytestring encoded in the selected output charset, or in the preferred system
encoding if no encoding was explicitly set with <code class="xref py py-meth docutils literal"><span class="pre">set_output_charset()</span></code>.</p>
</dd></dl>
<dl class="method">
<dt id="gettext.GNUTranslations.ngettext">
<code class="descclassname">GNUTranslations.</code><code class="descname">ngettext</code><span class="sig-paren">(</span><em>singular</em>, <em>plural</em>, <em>n</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.GNUTranslations.ngettext" title="Permalink to this definition">¶</a></dt>
<dd><p>Do a plural-forms lookup of a message id. <em>singular</em> is used as the message id
for purposes of lookup in the catalog, while <em>n</em> is used to determine which
plural form to use. The returned message string is a Unicode string.</p>
<p>If the message id is not found in the catalog, and a fallback is specified, the
request is forwarded to the fallback’s <a class="reference internal" href="#gettext.ngettext" title="gettext.ngettext"><code class="xref py py-meth docutils literal"><span class="pre">ngettext()</span></code></a> method. Otherwise, when
<em>n</em> is 1 <em>singular</em> is returned, and <em>plural</em> is returned in all other cases.</p>
<p>Here is an example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">n</span> <span class="o">=</span> <span class="nb">len</span><span class="p">(</span><span class="n">os</span><span class="o">.</span><span class="n">listdir</span><span class="p">(</span><span class="s1">'.'</span><span class="p">))</span>
<span class="n">cat</span> <span class="o">=</span> <span class="n">GNUTranslations</span><span class="p">(</span><span class="n">somefile</span><span class="p">)</span>
<span class="n">message</span> <span class="o">=</span> <span class="n">cat</span><span class="o">.</span><span class="n">ngettext</span><span class="p">(</span>
<span class="s1">'There is </span><span class="si">%(num)d</span><span class="s1"> file in this directory'</span><span class="p">,</span>
<span class="s1">'There are </span><span class="si">%(num)d</span><span class="s1"> files in this directory'</span><span class="p">,</span>
<span class="n">n</span><span class="p">)</span> <span class="o">%</span> <span class="p">{</span><span class="s1">'num'</span><span class="p">:</span> <span class="n">n</span><span class="p">}</span>
</pre></div>
</div>
</dd></dl>
<dl class="method">
<dt id="gettext.GNUTranslations.lngettext">
<code class="descclassname">GNUTranslations.</code><code class="descname">lngettext</code><span class="sig-paren">(</span><em>singular</em>, <em>plural</em>, <em>n</em><span class="sig-paren">)</span><a class="headerlink" href="#gettext.GNUTranslations.lngettext" title="Permalink to this definition">¶</a></dt>
<dd><p>Equivalent to <a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-meth docutils literal"><span class="pre">gettext()</span></code></a>, but the translation is returned as a
bytestring encoded in the selected output charset, or in the preferred system
encoding if no encoding was explicitly set with <code class="xref py py-meth docutils literal"><span class="pre">set_output_charset()</span></code>.</p>
</dd></dl>
</div>
<div class="section" id="solaris-message-catalog-support">
<h3>23.1.2.3. Solaris message catalog support<a class="headerlink" href="#solaris-message-catalog-support" title="Permalink to this headline">¶</a></h3>
<p>The Solaris operating system defines its own binary <code class="file docutils literal"><span class="pre">.mo</span></code> file format, but
since no documentation can be found on this format, it is not supported at this
time.</p>
</div>
<div class="section" id="the-catalog-constructor">
<h3>23.1.2.4. The Catalog constructor<a class="headerlink" href="#the-catalog-constructor" title="Permalink to this headline">¶</a></h3>
<p id="index-9">GNOME uses a version of the <a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-mod docutils literal"><span class="pre">gettext</span></code></a> module by James Henstridge, but this
version has a slightly different API. Its documented usage was:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">gettext</span>
<span class="n">cat</span> <span class="o">=</span> <span class="n">gettext</span><span class="o">.</span><span class="n">Catalog</span><span class="p">(</span><span class="n">domain</span><span class="p">,</span> <span class="n">localedir</span><span class="p">)</span>
<span class="n">_</span> <span class="o">=</span> <span class="n">cat</span><span class="o">.</span><span class="n">gettext</span>
<span class="nb">print</span><span class="p">(</span><span class="n">_</span><span class="p">(</span><span class="s1">'hello world'</span><span class="p">))</span>
</pre></div>
</div>
<p>For compatibility with this older module, the function <code class="xref py py-func docutils literal"><span class="pre">Catalog()</span></code> is an
alias for the <a class="reference internal" href="#gettext.translation" title="gettext.translation"><code class="xref py py-func docutils literal"><span class="pre">translation()</span></code></a> function described above.</p>
<p>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.</p>
</div>
</div>
<div class="section" id="internationalizing-your-programs-and-modules">
<h2>23.1.3. Internationalizing your programs and modules<a class="headerlink" href="#internationalizing-your-programs-and-modules" title="Permalink to this headline">¶</a></h2>
<p>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:</p>
<ol class="arabic simple">
<li>prepare your program or module by specially marking translatable strings</li>
<li>run a suite of tools over your marked files to generate raw messages catalogs</li>
<li>create language specific translations of the message catalogs</li>
<li>use the <a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-mod docutils literal"><span class="pre">gettext</span></code></a> module so that message strings are properly translated</li>
</ol>
<p>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 <code class="docutils literal"><span class="pre">_('...')</span></code> — that is, a call to the function <code class="xref py py-func docutils literal"><span class="pre">_()</span></code>. For example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">filename</span> <span class="o">=</span> <span class="s1">'mylog.txt'</span>
<span class="n">message</span> <span class="o">=</span> <span class="n">_</span><span class="p">(</span><span class="s1">'writing a log message'</span><span class="p">)</span>
<span class="n">fp</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">filename</span><span class="p">,</span> <span class="s1">'w'</span><span class="p">)</span>
<span class="n">fp</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">message</span><span class="p">)</span>
<span class="n">fp</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</pre></div>
</div>
<p>In this example, the string <code class="docutils literal"><span class="pre">'writing</span> <span class="pre">a</span> <span class="pre">log</span> <span class="pre">message'</span></code> is marked as a candidate
for translation, while the strings <code class="docutils literal"><span class="pre">'mylog.txt'</span></code> and <code class="docutils literal"><span class="pre">'w'</span></code> are not.</p>
<p>There are a few tools to extract the strings meant for translation.
The original GNU <strong class="program">gettext</strong> only supported C or C++ source
code but its extended version <strong class="program">xgettext</strong> scans code written
in a number of languages, including Python, to find strings marked as
translatable. <a class="reference external" href="http://babel.pocoo.org/">Babel</a> is a Python
internationalization library that includes a <code class="file docutils literal"><span class="pre">pybabel</span></code> script to
extract and compile message catalogs. François Pinard’s program
called <strong class="program">xpot</strong> does a similar job and is available as part of
his <a class="reference external" href="https://github.com/pinard/po-utils">po-utils package</a>.</p>
<p>(Python also includes pure-Python versions of these programs, called
<strong class="program">pygettext.py</strong> and <strong class="program">msgfmt.py</strong>; some Python distributions
will install them for you. <strong class="program">pygettext.py</strong> is similar to
<strong class="program">xgettext</strong>, but only understands Python source code and
cannot handle other programming languages such as C or C++.
<strong class="program">pygettext.py</strong> supports a command-line interface similar to
<strong class="program">xgettext</strong>; for details on its use, run <code class="docutils literal"><span class="pre">pygettext.py</span>
<span class="pre">--help</span></code>. <strong class="program">msgfmt.py</strong> is binary compatible with GNU
<strong class="program">msgfmt</strong>. With these two programs, you may not need the GNU
<strong class="program">gettext</strong> package to internationalize your Python
applications.)</p>
<p><strong class="program">xgettext</strong>, <strong class="program">pygettext</strong>, and similar tools generate
<code class="file docutils literal"><span class="pre">.po</span></code> 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.</p>
<p>Copies of these <code class="file docutils literal"><span class="pre">.po</span></code> 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 <code class="file docutils literal"><span class="pre"><language-name>.po</span></code> file that’s
compiled into a machine-readable <code class="file docutils literal"><span class="pre">.mo</span></code> binary catalog file using
the <strong class="program">msgfmt</strong> program. The <code class="file docutils literal"><span class="pre">.mo</span></code> files are used by the
<a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-mod docutils literal"><span class="pre">gettext</span></code></a> module for the actual translation processing at
run-time.</p>
<p>How you use the <a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-mod docutils literal"><span class="pre">gettext</span></code></a> 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.</p>
<div class="section" id="localizing-your-module">
<h3>23.1.3.1. Localizing your module<a class="headerlink" href="#localizing-your-module" title="Permalink to this headline">¶</a></h3>
<p>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 <code class="docutils literal"><span class="pre">gettext</span></code>
API but instead the class-based API.</p>
<p>Let’s say your module is called “spam” and the module’s various natural language
translation <code class="file docutils literal"><span class="pre">.mo</span></code> files reside in <code class="file docutils literal"><span class="pre">/usr/share/locale</span></code> in GNU
<strong class="program">gettext</strong> format. Here’s what you would put at the top of your
module:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">gettext</span>
<span class="n">t</span> <span class="o">=</span> <span class="n">gettext</span><span class="o">.</span><span class="n">translation</span><span class="p">(</span><span class="s1">'spam'</span><span class="p">,</span> <span class="s1">'/usr/share/locale'</span><span class="p">)</span>
<span class="n">_</span> <span class="o">=</span> <span class="n">t</span><span class="o">.</span><span class="n">lgettext</span>
</pre></div>
</div>
</div>
<div class="section" id="localizing-your-application">
<h3>23.1.3.2. Localizing your application<a class="headerlink" href="#localizing-your-application" title="Permalink to this headline">¶</a></h3>
<p>If you are localizing your application, you can install the <code class="xref py py-func docutils literal"><span class="pre">_()</span></code> 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
<code class="docutils literal"><span class="pre">_('...')</span></code> without having to explicitly install it in each file.</p>
<p>In the simple case then, you need only add the following bit of code to the main
driver file of your application:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">gettext</span>
<span class="n">gettext</span><span class="o">.</span><span class="n">install</span><span class="p">(</span><span class="s1">'myapplication'</span><span class="p">)</span>
</pre></div>
</div>
<p>If you need to set the locale directory, you can pass it into the
<a class="reference internal" href="#gettext.install" title="gettext.install"><code class="xref py py-func docutils literal"><span class="pre">install()</span></code></a> function:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">gettext</span>
<span class="n">gettext</span><span class="o">.</span><span class="n">install</span><span class="p">(</span><span class="s1">'myapplication'</span><span class="p">,</span> <span class="s1">'/usr/share/locale'</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="changing-languages-on-the-fly">
<h3>23.1.3.3. Changing languages on the fly<a class="headerlink" href="#changing-languages-on-the-fly" title="Permalink to this headline">¶</a></h3>
<p>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:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">gettext</span>
<span class="n">lang1</span> <span class="o">=</span> <span class="n">gettext</span><span class="o">.</span><span class="n">translation</span><span class="p">(</span><span class="s1">'myapplication'</span><span class="p">,</span> <span class="n">languages</span><span class="o">=</span><span class="p">[</span><span class="s1">'en'</span><span class="p">])</span>
<span class="n">lang2</span> <span class="o">=</span> <span class="n">gettext</span><span class="o">.</span><span class="n">translation</span><span class="p">(</span><span class="s1">'myapplication'</span><span class="p">,</span> <span class="n">languages</span><span class="o">=</span><span class="p">[</span><span class="s1">'fr'</span><span class="p">])</span>
<span class="n">lang3</span> <span class="o">=</span> <span class="n">gettext</span><span class="o">.</span><span class="n">translation</span><span class="p">(</span><span class="s1">'myapplication'</span><span class="p">,</span> <span class="n">languages</span><span class="o">=</span><span class="p">[</span><span class="s1">'de'</span><span class="p">])</span>
<span class="c1"># start by using language1</span>
<span class="n">lang1</span><span class="o">.</span><span class="n">install</span><span class="p">()</span>
<span class="c1"># ... time goes by, user selects language 2</span>
<span class="n">lang2</span><span class="o">.</span><span class="n">install</span><span class="p">()</span>
<span class="c1"># ... more time goes by, user selects language 3</span>
<span class="n">lang3</span><span class="o">.</span><span class="n">install</span><span class="p">()</span>
</pre></div>
</div>
</div>
<div class="section" id="deferred-translations">
<h3>23.1.3.4. Deferred translations<a class="headerlink" href="#deferred-translations" title="Permalink to this headline">¶</a></h3>
<p>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:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="n">animals</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'mollusk'</span><span class="p">,</span>
<span class="s1">'albatross'</span><span class="p">,</span>
<span class="s1">'rat'</span><span class="p">,</span>
<span class="s1">'penguin'</span><span class="p">,</span>
<span class="s1">'python'</span><span class="p">,</span> <span class="p">]</span>
<span class="c1"># ...</span>
<span class="k">for</span> <span class="n">a</span> <span class="ow">in</span> <span class="n">animals</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="n">a</span><span class="p">)</span>
</pre></div>
</div>
<p>Here, you want to mark the strings in the <code class="docutils literal"><span class="pre">animals</span></code> list as being
translatable, but you don’t actually want to translate them until they are
printed.</p>
<p>Here is one way you can handle this situation:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">_</span><span class="p">(</span><span class="n">message</span><span class="p">):</span> <span class="k">return</span> <span class="n">message</span>
<span class="n">animals</span> <span class="o">=</span> <span class="p">[</span><span class="n">_</span><span class="p">(</span><span class="s1">'mollusk'</span><span class="p">),</span>
<span class="n">_</span><span class="p">(</span><span class="s1">'albatross'</span><span class="p">),</span>
<span class="n">_</span><span class="p">(</span><span class="s1">'rat'</span><span class="p">),</span>
<span class="n">_</span><span class="p">(</span><span class="s1">'penguin'</span><span class="p">),</span>
<span class="n">_</span><span class="p">(</span><span class="s1">'python'</span><span class="p">),</span> <span class="p">]</span>
<span class="k">del</span> <span class="n">_</span>
<span class="c1"># ...</span>
<span class="k">for</span> <span class="n">a</span> <span class="ow">in</span> <span class="n">animals</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="n">_</span><span class="p">(</span><span class="n">a</span><span class="p">))</span>
</pre></div>
</div>
<p>This works because the dummy definition of <code class="xref py py-func docutils literal"><span class="pre">_()</span></code> simply returns the string
unchanged. And this dummy definition will temporarily override any definition
of <code class="xref py py-func docutils literal"><span class="pre">_()</span></code> in the built-in namespace (until the <a class="reference internal" href="../reference/simple_stmts.html#del"><code class="xref std std-keyword docutils literal"><span class="pre">del</span></code></a> command). Take
care, though if you have a previous definition of <code class="xref py py-func docutils literal"><span class="pre">_()</span></code> in the local
namespace.</p>
<p>Note that the second use of <code class="xref py py-func docutils literal"><span class="pre">_()</span></code> will not identify “a” as being
translatable to the <strong class="program">gettext</strong> program, because the parameter
is not a string literal.</p>
<p>Another way to handle this is with the following example:</p>
<div class="highlight-python3"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">N_</span><span class="p">(</span><span class="n">message</span><span class="p">):</span> <span class="k">return</span> <span class="n">message</span>
<span class="n">animals</span> <span class="o">=</span> <span class="p">[</span><span class="n">N_</span><span class="p">(</span><span class="s1">'mollusk'</span><span class="p">),</span>
<span class="n">N_</span><span class="p">(</span><span class="s1">'albatross'</span><span class="p">),</span>
<span class="n">N_</span><span class="p">(</span><span class="s1">'rat'</span><span class="p">),</span>
<span class="n">N_</span><span class="p">(</span><span class="s1">'penguin'</span><span class="p">),</span>
<span class="n">N_</span><span class="p">(</span><span class="s1">'python'</span><span class="p">),</span> <span class="p">]</span>
<span class="c1"># ...</span>
<span class="k">for</span> <span class="n">a</span> <span class="ow">in</span> <span class="n">animals</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="n">_</span><span class="p">(</span><span class="n">a</span><span class="p">))</span>
</pre></div>
</div>
<p>In this case, you are marking translatable strings with the function
<code class="xref py py-func docutils literal"><span class="pre">N_()</span></code>, which won’t conflict with any definition of <code class="xref py py-func docutils literal"><span class="pre">_()</span></code>.
However, you will need to teach your message extraction program to
look for translatable strings marked with <code class="xref py py-func docutils literal"><span class="pre">N_()</span></code>. <strong class="program">xgettext</strong>,
<strong class="program">pygettext</strong>, <code class="docutils literal"><span class="pre">pybabel</span> <span class="pre">extract</span></code>, and <strong class="program">xpot</strong> all
support this through the use of the <code class="xref std std-option docutils literal"><span class="pre">-k</span></code> command-line switch.
The choice of <code class="xref py py-func docutils literal"><span class="pre">N_()</span></code> here is totally arbitrary; it could have just
as easily been <code class="xref py py-func docutils literal"><span class="pre">MarkThisStringForTranslation()</span></code>.</p>
</div>
</div>
<div class="section" id="acknowledgements">
<h2>23.1.4. Acknowledgements<a class="headerlink" href="#acknowledgements" title="Permalink to this headline">¶</a></h2>
<p>The following people contributed code, feedback, design suggestions, previous
implementations, and valuable experience to the creation of this module:</p>
<ul class="simple">
<li>Peter Funk</li>
<li>James Henstridge</li>
<li>Juan David Ibáñez Palomar</li>
<li>Marc-André Lemburg</li>
<li>Martin von Löwis</li>
<li>François Pinard</li>
<li>Barry Warsaw</li>
<li>Gustavo Niemeyer</li>
</ul>
<p class="rubric">Footnotes</p>
<table class="docutils footnote" frame="void" id="id3" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>The default locale directory is system dependent; for example, on RedHat Linux
it is <code class="file docutils literal"><span class="pre">/usr/share/locale</span></code>, but on Solaris it is <code class="file docutils literal"><span class="pre">/usr/lib/locale</span></code>.
The <a class="reference internal" href="#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-mod docutils literal"><span class="pre">gettext</span></code></a> module does not try to support these system dependent
defaults; instead its default is <code class="file docutils literal"><span class="pre">sys.prefix/share/locale</span></code>. For this
reason, it is always best to call <a class="reference internal" href="#gettext.bindtextdomain" title="gettext.bindtextdomain"><code class="xref py py-func docutils literal"><span class="pre">bindtextdomain()</span></code></a> with an explicit
absolute path at the start of your application.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="id4" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id2">[2]</a></td><td>See the footnote for <a class="reference internal" href="#gettext.bindtextdomain" title="gettext.bindtextdomain"><code class="xref py py-func docutils literal"><span class="pre">bindtextdomain()</span></code></a> above.</td></tr>
</tbody>
</table>
</div>
</div>
</div>
</div>
</div>
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<div class="sphinxsidebarwrapper">
<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">23.1. <code class="docutils literal"><span class="pre">gettext</span></code> — Multilingual internationalization services</a><ul>
<li><a class="reference internal" href="#gnu-gettext-api">23.1.1. GNU <strong class="program">gettext</strong> API</a></li>
<li><a class="reference internal" href="#class-based-api">23.1.2. Class-based API</a><ul>
<li><a class="reference internal" href="#the-nulltranslations-class">23.1.2.1. The <code class="docutils literal"><span class="pre">NullTranslations</span></code> class</a></li>
<li><a class="reference internal" href="#the-gnutranslations-class">23.1.2.2. The <code class="docutils literal"><span class="pre">GNUTranslations</span></code> class</a></li>
<li><a class="reference internal" href="#solaris-message-catalog-support">23.1.2.3. Solaris message catalog support</a></li>
<li><a class="reference internal" href="#the-catalog-constructor">23.1.2.4. The Catalog constructor</a></li>
</ul>
</li>
<li><a class="reference internal" href="#internationalizing-your-programs-and-modules">23.1.3. Internationalizing your programs and modules</a><ul>
<li><a class="reference internal" href="#localizing-your-module">23.1.3.1. Localizing your module</a></li>
<li><a class="reference internal" href="#localizing-your-application">23.1.3.2. Localizing your application</a></li>
<li><a class="reference internal" href="#changing-languages-on-the-fly">23.1.3.3. Changing languages on the fly</a></li>
<li><a class="reference internal" href="#deferred-translations">23.1.3.4. Deferred translations</a></li>
</ul>
</li>
<li><a class="reference internal" href="#acknowledgements">23.1.4. Acknowledgements</a></li>
</ul>
</li>
</ul>
<h4>Previous topic</h4>
<p class="topless"><a href="i18n.html"
title="previous chapter">23. Internationalization</a></p>
<h4>Next topic</h4>
<p class="topless"><a href="locale.html"
title="next chapter">23.2. <code class="docutils literal"><span class="pre">locale</span></code> — Internationalization services</a></p>
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