============================
Using the flufl.i18n library
============================

Set up
======

There are two basic ways that your application can set up translations using
this library.  The simple initialization will work for most applications,
where there is only one language context for the entire run of the
application.  The more complex initialization works well for applications like
servers that may want to use multiple language contexts during their
execution.


Single language contexts
------------------------

If your application only needs one language context for its entire execution,
you can use the simple API to set things up.

    >>> from flufl.i18n import initialize

The library by default uses the ``$LANG`` and ``$LOCPATH`` environment
variables to set things up::

    >>> # The testing 'xx' language rot13's the source string.  The
    >>> # gettext catalogs are in this package directory.
    >>> import os
    >>> import flufl.i18n.testing.messages

    >>> os.environ['LANG'] = 'xx'
    >>> os.environ['LOCPATH'] = os.path.dirname(
    ...     flufl.i18n.testing.messages.__file__)

Now you just need to call the ``initialize()`` function with the application's
name and you'll get an object back that you can assign to the ``_()`` function
for run-time translations.

    >>> _ = initialize('flufl')
    >>> print(_('A test message'))
    N grfg zrffntr

It's probably best to just share this function through imports, but it does no
harm to call ``initialize()`` again.

    >>> _ = initialize('flufl')
    >>> print(_('A test message'))
    N grfg zrffntr

..
    >>> # Unregister the application domain used earlier.  Also, clear the
    >>> # environment settings from above.
    >>> from flufl.i18n import registry
    >>> registry._registry.clear()
    >>> del os.environ['LANG']
    >>> del os.environ['LOCPATH']


Multiple language contexts
--------------------------

Some applications, such as servers, are more complex; they need multiple
language contexts during their execution.  To support this, there is a global
registry of catalog look up :doc:`strategies <strategies>`.  When a particular
language code is specified, the strategy is used to find the catalog that
provides that language's translations.

``flufl.i18n`` comes with a couple of fairly simple strategies, but you can of
course write your own.  A convenient built-in strategy looks up catalogs from
within the package directory using `GNU gettext`_'s convention, where the base
directory for the catalogs is rooted in a subpackage.

    >>> from flufl.i18n import PackageStrategy
    >>> strategy = PackageStrategy('flufl', flufl.i18n.testing.messages)

The first argument is the application name, which must be unique among all
registered strategies.  The second argument is the package where the
translations can be found.

Once you have the desired strategy, register this with the global registry.
The registration process returns an application object which can be used to
look up language codes.

    >>> from flufl.i18n import registry
    >>> application = registry.register(strategy)

The application object keeps track of a current translation catalog, and
exports a method which you can bind to the *underscore* function in your
module globals for convenient gettext usage.  By doing so, at run time,
``_()`` will always translate the string argument to the current catalog's
language.

    >>> _ = application._

By default the application just translates the source string back into the
source string.  I.e. it is a null translator.

    >>> print(_('A test message'))
    A test message

And it has no language code.

    >>> print(_.code)
    None

You can temporarily push a new language context to the top of the stack, which
automatically rebinds the underscore function to the language's catalog.

    >>> _.push('xx')
    >>> print(_.code)
    xx
    >>> print(_('A test message'))
    N grfg zrffntr

Pop the current language to return to the default.  Once you're at the bottom
of the stack, more pops will just give you the default translation.

    >>> _.pop()
    >>> print(_.code)
    None
    >>> print(_('A test message'))
    A test message
    >>> _.pop()
    >>> print(_.code)
    None
    >>> print(_('A test message'))
    A test message

The underscore method has a context manager called ``using`` which can be used
to temporarily set a new language inside a ``with`` statement::

    >>> with _.using('xx'):
    ...     print(_('A test message'))
    N grfg zrffntr

    >>> print(_('A test message'))
    A test message

These ``with`` statements are nestable::

    >>> with _.using('xx'):
    ...     print(_('A test message'))
    ...     with _.using('yy'):
    ...         print(_('A test message'))
    ...     print(_('A test message'))
    N grfg zrffntr
    egassem tset A
    N grfg zrffntr

    >>> print(_('A test message'))
    A test message

You can set the bottom language context, which replaces the default null
translation::

    >>> _.default = 'xx'
    >>> print(_('A test message'))
    N grfg zrffntr

    >>> _.pop()
    >>> print(_.code)
    xx

    >>> print(_('A test message'))
    N grfg zrffntr

    >>> with _.using('yy'):
    ...     print(_('A test message'))
    egassem tset A

    >>> print(_('A test message'))
    N grfg zrffntr


Substitutions and placeholders
==============================

As you can see from the example above, using the library is very simple.  You
just put the string to translate inside the underscore function.  What if your
source strings need placeholders for other runtime information?

In that case, you use `PEP 292`_ style substitution strings as arguments to
the underscore function.  Substitutions are taken from the locals and globals
of the function doing the translation, so that you don't have to repeat
yourself.

    >>> ordinal = 'first'
    >>> def print_it(name):
    ...     print(_('The $ordinal test message $name'))

In this example, when ``print_it()`` is called, the ``$ordinal`` placeholder
is taken from globals, while the ``$name`` placeholder is taken from the
function locals (i.e. the arguments).

..
    >>> # Reset the language context.
    >>> del _.default
    >>> print(_.code)
    None

With no language context in place, the source string is printed unchanged,
except that the substitutions are made.

    >>> print_it('Anne')
    The first test message Anne

When a substitution is missing, rather than raise an exception, the
``$variable`` is used unchanged.

    >>> del ordinal
    >>> print_it('Bart')
    The $ordinal test message Bart

When there is a language context in effect, the substitutions happen after
translation.

    >>> ordinal = 'second'
    >>> with _.using('xx'):
    ...     print_it('Cris')
    second si n grfg zrffntr Cris

Some languages change the order of the substitution variables, but of course
there is no problem with that.

    >>> ordinal = 'third'
    >>> with _.using('yy'):
    ...     print_it('Dave')
    Dave egassem tset third eht

Locals always take precedence over globals::

    >>> def print_it(name, ordinal):
    ...     print(_('The $ordinal test message $name'))

    >>> with _.using('yy'):
    ...     print_it('Elle', 'fourth')
    Elle egassem tset fourth eht


Deferred translations
=====================

Sometimes you have a bunch of strings you want to mark for translation, but
you want to defer the translation of some of them until later.  The way to do
this is::

    >>> with _.defer_translation():
    ...     print(_('This gets marked but not translated'))
    This gets marked but not translated

Because the string is wrapped in the ``_()`` function, it will get extracted
and added to the catalog, but it will not get translated until later.  This is
true even if there is a translation context in effect.

    >>> with _.using('xx'):
    ...     with _.defer_translation():
    ...         print(_('A test message'))
    ...     print(_('A test message'))
    A test message
    N grfg zrffntr


.. _`GNU gettext`: http://www.gnu.org/software/gettext/manual/gettext.html
.. _`PEP 292`: http://www.python.org/dev/peps/pep-0292/
