Internationalisation of PyQt Applications
=========================================

PyQt and Qt include a comprehensive set of tools for translating applications
into local languages.  For a full description, see the Qt Linguist Manual in
the Qt documentation.

The process of internationalising an application comprises the following
steps.

- The programmer uses :program:`pylupdate4` to create or update a ``.ts``
  translation file for each language that the application is to be translated
  into.  A ``.ts`` file is an XML file that contains the strings to be
  translated and the corresponding translations that have already been made.
  :program:`pylupdate4` can be run any number of times during development to
  update the ``.ts`` files with the latest strings for translation.

- The translator uses Qt Linguist to update the ``.ts`` files with translations
  of the strings.

- The release manager then uses Qt's :program:`lrelease` utility to convert the
  ``.ts`` files to ``.qm`` files which are compact binary equivalents used by
  the application.  If an application cannot find an appropriate ``.qm`` file,
  or a particular string hasn't been translated, then the strings used in the
  original source code are used instead.

- The release manage may optionally use :program:`pyrcc4` to embed the ``.qm``
  files, along with other application resources such as icons, in a Python
  module.  This may make packaging and distribution of the application easier.


:program:`pylupdate4`
---------------------

.. program:: pylupdate4

:program:`pylupdate4` is PyQt's equivalent to Qt's :program:`lupdate` utility
and is used in exactly the same way.  A Qt ``.pro`` project file is read that
specifies the Python source files and Qt Designer interface files from which
the text that needs to be translated is extracted.  The ``.pro`` file also
specifies the ``.ts`` translation files that :program:`pylupdate4` updates (or
creates if necessary) and are subsequently used by Qt Linguist.

:program:`pylupdate4` will only be included if your copy of Qt includes the XML
module.


Differences Between PyQt and Qt
-------------------------------

Qt implements internationalisation support through the ``QTranslator`` class,
and the ``QCoreApplication::translate()``, ``QObject::tr()`` and
``QObject::trUtf8()`` methods.  Usually the ``tr()`` method is used to obtain
the correct translation of a message.  The translation process uses a message
context to allow the same message to be translated differently.  ``tr()`` is
actually generated by ``moc`` and uses the hardcoded class name as the context.
On the other hand, ``QApplication::translate()`` allows the context to be
explicitly stated.

Unfortunately, because of the way Qt implements ``tr()`` (and ``trUtf8()``) it
is not possible for PyQt to exactly reproduce its behaviour.  The PyQt
implementation of ``tr()`` (and ``trUtf8()``) uses the class name of the
instance as the context.  The key difference, and the source of potential
problems, is that the context is determined dynamically in PyQt, but is
hardcoded in Qt.  In other words, the context of a translation may change
depending on an instance's class hierarchy.  For example::

    class A(QtCore.QObject):
        def hello(self):
            return self.tr("Hello")

    class B(A):
        pass

    a = A()
    a.hello()

    b = B()
    b.hello()

In the above the message is translated by ``a.hello()`` using a context of
``A``, and by ``b.hello()`` using a context of ``B``.  In the equivalent C++
version the context would be ``A`` in both cases.

The PyQt behaviour is unsatisfactory and may be changed in the future.  It is
recommended that ``QCoreApplication.translate()`` be used in preference to
``tr()`` (and ``trUtf8()``).  This is guaranteed to work with current and
future versions of PyQt and makes it much easier to share message files
between Python and C++ code.  Below is the alternative implementation of ``A``
that uses ``QCoreApplication.translate()``::

    class A(QtCore.QObject):
        def hello(self):
            return QtCore.QCoreApplication.translate("A", "Hello")
