=====================================
 PyQt v4 - Python Bindings for Qt v4
=====================================

-----------------
 Reference Guide
-----------------

:Contact:   info@riverbankcomputing.co.uk
:Version:   4.0.1
:Copyright: Copyright (c) 2006 Riverbank Computing Limited

.. contents::
.. section-numbering::


Introduction
============

This is the reference guide for PyQt 4.0.1.  PyQt v4 is a set of
`Python <http://www.python.org>`__ bindings for v4 of the Qt application
framework from `Trolltech <http://www.trolltech.com>`__.

There is a separate `PyQt API Reference <html/classes.html>`__.

Qt is a set of C++ libraries and development tools that includes platform
independent abstractions for graphical user interfaces, networking, threads,
Unicode, regular expressions, SQL databases, SVG, OpenGL, XML, and user and
application settings.  PyQt implements 440 of these classes as a set of
Python modules.

PyQt supports the Windows, Linux, UNIX and MacOS/X platforms.

PyQt does not include Qt itself - you must obtain it separately.

The homepage for PyQt is http://www.riverbankcomputing.co.uk/pyqt/.  Here you
will always find the latest stable version, current development snapshots, and
the latest version of this documentation.

PyQt is built using the `SIP bindings generator
<http://www.riverbankcomputing.co.uk/sip/>`__.  SIP must be installed in order
to build and use PyQt.

Earlier versions of Qt are supported by PyQt v3.


License
-------

Like Qt v4, PyQt is licensed on all platforms under a commercial license and
the GPL v2.  Your PyQt license must be the same as your Qt license, ie. use
either the commercial versions of both or the GPL versions of both.  If you
use the GPL versions then your own code must also be licensed under the GPL.

You can purchase a commercial PyQt license `here
<http://www.riverbankcomputing.co.uk/pyqt/buy.php>`__.


PyQt Components
---------------

PyQt comprises a number of different components.  First of all there are a
number of Python extension modules.  These are all installed in the ``PyQt4``
Python package.

    - The ``QtCore`` module.  This contains the core non-GUI classes, including
      the event loop and Qt's signal and slot mechanism.  It also includes
      platform independent abstractions for Unicode, threads, regular
      expressions, and user and application settings.

    - The ``QtGui`` module.  This contains the majority of the GUI classes.

    - The ``QtNetwork`` module.  This module contains classes for writing UDP
      and TCP clients and servers.  It includes classes that implement FTP and
      HTTP clients and support DNS lookups.

    - The ``QtOpenGL`` module.  This module contains classes that enable the
      use of OpenGL in rendering 3D graphics in PyQt applications.

    - The ``QtSql`` module.  This module contains classes that integrate with
      SQL databases.  It includes editable data models for database tables that
      can be used with GUI classes.  It also includes an implementation of
      `SQLite <http://www.sqlite.org>`__.

    - The ``QtSvg`` module.  This module contains classes for displaying the
      contents of SVG files.

    - The ``QtXml`` module.  This module contains classes that implement SAX
      and DOM interfaces to Qt's XML parser.

    - The ``QtAssistant`` module.  This module contains classes that allow Qt
      Assistant to be integrated with a PyQt application to provide online
      help.

    - The ``Qt`` module.  This module consolidates the classes contained in all
      of the modules described above into a single module.  This has the
      advantage that you don't have to worry about which underlying module
      contains a particular class.  It has the disadvantage that it loads the
      whole of the Qt framework, thereby increasing the memory footprint of an
      application.  Whether you use this consolidated module, or the individual
      component modules is down to personal taste.

    - The ``uic`` module.  This module contains classes for handling the
      ``.ui`` files created by Qt Designer that describe the whole or part of a
      graphical user interface.  It includes classes that load a ``.ui`` file
      and render it directly, and classes that generate Python code from a
      ``.ui`` file for later execution.  It is covered in detail in `The uic
      Module`_.

    - The ``pyqtconfig`` module is an extention of the SIP build system and is
      created when PyQt is configured.  It encapsulates all the necessary
      information about your Qt installation and makes it easier to write
      installation scripts for bindings built on top of PyQt.  It is covered
      in detail in `The PyQt Build System`_.

PyQt also contains a number of utility programs.

    - `pyuic4`_ corresponds to the Qt ``uic`` utility.  It converts GUIs
      created using Qt Designer to Python code.  It is covered in detail in
      `pyuic4`_.

    - `pyrcc4`_ corresponds to the Qt ``rcc`` utility.  It embeds arbitrary
      resources (eg. icons, images, translation files) described by a resource
      collection file in a Python module.  It is covered in detail in
      `pyrcc4`_.  (*Note* It will only be included if your copy of Qt includes
      the XML module.)

    - `pylupdate4`_ corresponds to the Qt ``lupdate`` utility.  It extracts
      all of the translatable strings from Python code and creates or updates
      ``.ts`` translation files.  These are then used by Qt Linguist to manage
      the translation of those strings.  It is covered in detail in
      `pylupdate4`_.  (*Note* It will only be included if your copy of Qt
      includes the XML module.)

PyQt includes a large number of examples.  These are ports to Python of many
of the C++ examples provided with Qt.  They can be found in the ``examples``
directory.

Finally, PyQt contains the ``.sip`` files used by SIP to generate PyQt
itself.  These can be used by developers of bindings of other Qt based class
libraries - for example `PyQwt and PyQwt3D <http://pyqwt.sourceforge.net/>`__.


Installing PyQt
===============

Downloading SIP
---------------

SIP must be installed before building and using PyQt.  You can get the latest
release of the SIP source code from
http://www.riverbankcomputing.co.uk/sip/download.php.

The SIP documentation can be found at
http://www.riverbankcomputing.com/Docs/sip4/sipref.html.


Downloading PyQt
----------------

You can get the latest release of the GPL version of the PyQt source code from
http://www.riverbankcomputing.co.uk/pyqt/download.php.

If you are using the commercial version of PyQt then you should use the
download instructions which were sent to you when you made your purchase.  You
must also download your license file.


Configuring PyQt
----------------

After unpacking the source package (either a ``.tar.gz`` or a ``.zip`` file
depending on your platform) you should then check for any ``README`` files
that relate to your platform.

If you are using the commercial version of PyQt then you must copy your
license file to the ``sip`` directory.

You need to make sure your environment variables are set properly for your
development environment.  For example, if you are using a binary distribution
of Qt on Windows then make sure you have run the ``qtvars.bat`` file.  For
other platforms it is normally enough to ensure that Qt's ``bin`` directory is
on your ``PATH``.

Next you need to configure SIP by executing the ``configure.py`` script.  For
example::

    python configure.py

This assumes that the Python interpreter is on your path.  Something like the
following may be appropriate on Windows::

    c:\python24\python configure.py

If you have multiple versions of Python installed then make sure you use the
interpreter for which you wish to build PyQt for.

The full set of command line options is:

-h       Display a help message.
-b dir   The ``pyuic4``, ``pyrcc4`` and ``pylupdate4`` utilities will be
         installed in the directory ``dir``.
-c       The C++ source files for a Python module will be concatenated.  This
         results in significantly reduced compilation times.  Most, but not
         all, C++ compilers can handle the large files that result.  It is
         recommended that you use this option if you are using GCC v3.x or
         MSVC v7.x.  See also the ``-j`` option.
-d dir   The PyQt modules will be installed in the directory ``dir``.
-i       The checking of signed Python interpreters using the `VendorID
         package <http://www.riverbankcomputing.co.uk/vendorid/>`__ is
         enabled.  See also the ``-l`` and ``-m`` options and `Deploying
         Commercial PyQt Applications`_.
-j n     If the ``-c`` option is used to concatenate the C++ source files then
         this option determines how many files are created.  The default is 1.
-k       The PyQt modules will be built as static libraries.  This is useful
         when building a custom interpreter with the PyQt modules built in to
         interpreter.
-l dir   The header file of the VendorID package can be found in the directory
         ``dir``.
-m dir   The library of the VendorID package can be found in the directory
         ``dir``.
-q exe   Qt's ``qmake`` program is used to determine how your Qt installation
         is laid out.  Normally ``qmake`` is found on your ``PATH``.  This
         option can be used to specify a particular instance of ``qmake`` to
         use.
-r       The generated PyQt modules contain additional tracing code that is
         enabled using SIP's ``sip.settracemask()`` function.
-u       The PyQt modules will be built with debugging symbols.  On Windows
         this requires that a debug version of Python is installed.
-v dir   The ``.sip`` files for the PyQt modules will be installed in the
         directory ``dir``.
-w       Compiler commands and any output issued during configuration is
         displayed instead of being suppressed.  Use this if ``configure.py``
         is having problems to see what exactly is going wrong.


Building PyQt
-------------

The next step is to build PyQt by running your platform's ``make`` command.
For example::

    make

The final step is to install PyQt by running the following command::

    make install

(Depending on your system you may require root or administrator privileges.)

This will install the various PyQt components.


Signal and Slot Support
=======================

One of the key features of Qt is its use of signals and slots to communicate
between objects.  Their use encourages the development of reusable components.

A signal is emitted when a particular event occurs.  A slot is a function (in
PyQt a slot is any Python callable).  If a signal is connected to a slot
(using the ``QtCore.QObject.connect()`` method) then the slot is called when
the signal is emitted.  If a signal isn't connected then nothing happens.  The
code (or component) that emits the signal does not know or care if the signal
is being used.

A signal may be connected to many slots.

A signal may also be connected to another signal.

A slot may be connected to many signals.

In PyQt signals are emitted using the ``QtCore.QObject.emit()`` method.

Connections may be direct (ie. synchronous) or queued (ie. asynchronous).

Connections may be made across threads.

Signals are disconnected using the ``QtCore.QObject.disconnect()`` method.


PyQt Signals and Qt Signals
---------------------------

Qt signals are statically defined as part of a C++ class.  They are referenced
using the ``QtCore.SIGNAL()`` function.  This method takes a single string
argument that is the name of the signal and its C++ signature.  For example::

    QtCore.SIGNAL("finished(int)")

The returned value is normally passed to the ``QtCore.QObject.connect()``
method.

PyQt allows new signals to be defined dynamically.  The act of emitting a
PyQt signal implicitly defines it.  PyQt v4 signals are also referenced using
the ``QtCore.SIGNAL()`` function.


Short-circuit Signals
---------------------

There is also a special form of PyQt v4 signal known as a short-circuit signal.
Short-circuit signals may only be connected to slots that have been
implemented in Python.  They cannot be connected to Qt slots or the Python
callables that wrap Qt slots.

Short-circuit signals do not have a list of arguments or the surrounding
parentheses.  Their advantage is that they are very efficient as the arguments
are passed as Python arguments without any conversions to C++ data types and
back again.


PyQt Slots and Qt Slots
-----------------------

Qt slots are statically defined as part of a C++ class.  They are referenced
using the ``QtCore.SLOT()`` function.  This method takes a single string
argument that is the name of the slot and its C++ signature.  For example::

    QtCore.SLOT("done(int)")

The returned value is normally passed to the ``QtCore.QObject.connect()``
method.

PyQt allows any Python callable to be used as a slot, not just Qt slots.  This
is done by simply referencing the callable.  Because Qt slots are implemented
as class methods they are also available as Python callables.  Therefore it is
never actually necessary to use ``QtCore.SLOT()`` for Qt slots.  However, doing
so is more efficient as signal delivery happens at the C++ level and avoids a
conversion to Python and back to C++.

Qt allows a signal to be connected to a slot that requires fewer arguments than
the signal passes.  The extra arguments are quietly discarded.  PyQt slots can
be used in the same way.

Note that when a slot is a Python callable its reference count is not
increased.  This means that a class instance can be deleted without having to
explicitly disconnect any signals connected to its methods.  However, it also
means that using lambda expressions as slots will not work unless you keep a
separate reference to the expression to prevent it from being immediately
garbage collected.


Connecting Signals and Slots
----------------------------

Connections between signals and slots (and other signals) are made using the
``QtCore.QObject.connect()`` method.  For example::

    QtCore.QObject.connect(a, QtCore.SIGNAL("QtSig()"), pyFunction)
    QtCore.QObject.connect(a, QtCore.SIGNAL("QtSig()"), pyClass.pyMethod)
    QtCore.QObject.connect(a, QtCore.SIGNAL("QtSig()"), QtCore.SLOT("QtSlot()"))
    QtCore.QObject.connect(a, QtCore.SIGNAL("PySig()"), QtCore.SLOT("QtSlot()"))
    QtCore.QObject.connect(a, QtCore.SIGNAL("PySig"), pyFunction)

Disconnecting signals works in exactly the same way using the
``QtCore.QObject.disconnect()`` method.  However, not all the variations of
that method are supported by PyQt.  Signals must be disconnected one at a
time.


Emitting Signals
----------------

Any instance of a class that is derived from the ``QtCore.QObject`` class can
emit a signal using its ``emit()`` method.  This takes a minimum of one
argument which is the signal.  Any other arguments are passed to the connected
slots as the signal arguments.  For example::

    a.emit(QtCore.SIGNAL("clicked()"))
    a.emit(QtCore.SIGNAL("pySig"), "Hello", "World")


The ``QtCore.pyqtSignature()`` Decorator
----------------------------------------

PyQt supports the ``QtCore.QMetaObject.connectSlotsByName()`` function that
is most commonly used by `pyuic4`_ generated Python code to automatically
connect signals to slots that conform to a simple naming convention.  However,
where a class has overloaded Qt signals (ie. with the same name but with
different arguments) PyQt needs additional information in order to
automatically connect the correct signal.

For example the ``QtGui.QSpinBox`` class has the following signals::

    void valueChanged(int i);
    void valueChanged(const QString &text);

When the value of the spin box changes both of these signals will be emitted.
If you have implemented a slot called ``on_spinbox_valueChanged`` (which
assumes that you have given the ``QSpinBox`` instance the name ``spinbox``)
then it will be connected to both variations of the signal.  Therefore, when
the user changes the value, your slot will be called twice - once with an
integer argument, and once with a ``QString`` argument.

This also happens with signals that take optional arguments.  Qt implements
this using multiple signals.  For example, ``QtGui.QAbstractButton`` has the
following signal::

    void clicked(bool checked = false);

Qt implements this as the following::

    void clicked();
    void clicked(bool checked);

PyQt includes a Python function decorator that can be used to specify which of
the signals should be connected to the slot.  The decorator takes a string
containing the required signal's signature, excluding the parentheses.  If you
were only interested in the integer variant of the signal then your slot
definition would look like the following::

    @QtCore.pyqtSignature("int")
    def on_spinbox_valueChanged(self, i):
        # i will be an integer.
        pass

The following shows an example using a button when you are not interested in
the optional argument::

    @QtCore.pyqtSignature("")
    def on_button_clicked(self):
        pass


Using Qt Designer
=================

Qt Designer is the Qt tool for designing and building graphical user
interfaces.  It allows you to design widgets, dialogs or complete main windows
using on-screen forms and a simple drag-and-drop interface.  It has the ability
to preview your designs to ensure they work as you intended, and to allow you
to prototype them with your users, before you have to write any code.

Qt Designer uses XML ``.ui`` files to store designs and does not generate any
code itself.  Qt includes the ``uic`` utility that generates the C++ code that
creates the user interface.  Qt also includes the ``QUiLoader`` class that
allows an application to load a ``.ui`` file and to create the corresponding
user interface dynamically.

PyQt does not wrap the ``QUiLoader`` class but instead includes the ``uic``
Python module.  Like ``QUiLoader`` this module can load ``.ui`` files to create
a user interface dynamically.  Like the ``uic`` utility it can also generate
the Python code that will create the user interface.  PyQt's ``pyuic4``
utility is a command line interface to the ``uic`` module.  Both are described
in detail in the following sections.


Using the Generated Code
------------------------

The code that is generated has an identical structure to that generated by Qt's
``uic`` and can be used in the same way.

The code is structured as a single class that is derived from the Python
``object`` type.  The name of the class is the name of the toplevel object set
in Designer with ``Ui_`` prepended.  (In the C++ version the class is defined
in the ``Ui`` namespace.)  We refer to this class as the *form class*.

The class contains a method called ``setupUi()``.  This takes a single argument
which is the widget in which the user interface is created.  The type of this
argument (typically ``QDialog``, ``QWidget`` or ``QMainWindow``) is set in
Designer.  We refer to this type as the *Qt base class*.

In the following examples we assume that a ``.ui`` file has been created
containing a dialog and the name of the ``QDialog`` object is ``ImageDialog``.
We also assume that the name of the file containing the generated Python code
is ``ui_imagedialog.py``.  The generated code can then be used in a number of
ways.

The first example shows the direct approach where we simply create a simple
application to create the dialog::

    import sys
    from PyQt4 import QtGui
    from ui_imagedialog import Ui_ImageDialog

    app = QtGui.QApplication(sys.argv)
    window = QtGui.QDialog()
    ui = Ui_ImageDialog()
    ui.setupUi(window)

    window.show()
    sys.exit(app.exec_())

The second example shows the single inheritance approach where we sub-class
``QDialog`` and set up the user interface in the ``__init__()`` method::

    from PyQt4 import QtCore, QtGui
    from ui_imagedialog import Ui_ImageDialog

    class ImageDialog(QtGui.QDialog):
        def __init__(self):
            QtGui.QDialog.__init__(self)

            # Set up the user interface from Designer.
            self.ui = Ui_ImageDialog()
            self.ui.setupUi(self)

            # Make some local modifications.
            self.ui.colorDepthCombo.addItem("2 colors (1 bit per pixel)")

            # Connect up the buttons.
            self.connect(self.ui.okButton, QtCore.SIGNAL("clicked()"),
                         self, QtCore.SLOT("accept()"))
            self.connect(self.ui.cancelButton, QtCore.SIGNAL("clicked()"),
                         self, QtCore.SLOT("reject()"))

The third example shows the multiple inheritance approach::

    from PyQt4 import QtCore, QtGui
    from ui_imagedialog import Ui_ImageDialog

    class ImageDialog(QtGui.QDialog, Ui_ImageDialog):
        def __init__(self):
            QtGui.QDialog.__init__(self)

            # Set up the user interface from Designer.
            self.setupUi(self)

            # Make some local modifications.
            self.colorDepthCombo.addItem("2 colors (1 bit per pixel)")

            # Connect up the buttons.
            self.connect(self.okButton, QtCore.SIGNAL("clicked()"),
                         self, QtCore.SLOT("accept()"))
            self.connect(self.cancelButton, QtCore.SIGNAL("clicked()"),
                         self, QtCore.SLOT("reject()"))

For a full description see the Qt Designer Manual in the Qt Documentation.


The ``uic`` Module
------------------

The ``uic`` module contains the following functions.

compileUi(uifile, pyfile, execute=False, indent=4)
    This function generates the Python code that will create a user interface 
    from a Qt Designer ``.ui`` file.

    ``uifile`` is a file name or file-like object containing the ``.ui`` file.

    ``pyfile`` is the file-like object to which the generated Python code will
    be written to.

    ``execute`` is optionally set if a small amount of additional code is to be
    generated that will display the user interface if the code is run as a
    standalone application.

    ``indent`` is the optional number of spaces used for indentation in the
    generated code.  If it is zero then a tab character is used instead.

loadUiType(uifile)
    This function loads a Qt Designer ``.ui`` file and returns a tuple of the
    generated *form class* and the *Qt base class*.  These can then be used to
    create any number of instances of the user interface without having to
    parse the ``.ui`` file more than once.

    ``uifile`` is a file name or file-like object containing the ``.ui`` file.

loadUi(uifile, baseinstance=None)
    This function loads a Qt Designer ``.ui`` file and returns an instance of
    the user interface.

    ``uifile`` is a file name or file-like object containing the ``.ui`` file.

    ``baseinstance`` is an optional instance of the *Qt base class*.  If
    specified then the user interface is created in it.  Otherwise a new
    instance of the base class is automatically created.


pyuic4
------

The ``pyuic4`` utility is a command line interface to the ``uic`` module.  The
command has the following syntax::

    pyuic4 [options] .ui-file

The full set of command line options is:

-h, --help              A help message is written to ``stdout``.
--version               The version number is written to ``stdout``.
-i N, --indent=N        The Python code is generated using an indentation of N
                        spaces.  If N is 0 then a tab is used.  The default is
                        4.
-o FILE, --output=FILE  The Python code generated is written to the file FILE.
-p, --preview           The GUI is created dynamically and displayed.  No
                        Python code is generated.
-x, --execute           The generated Python code includes a small amount of
                        additional code that creates and displays the GUI when
                        it is executes as a standalone application.


The PyQt Resource System
========================

PyQt supports Qt's resource system.  This is a facility for embedding
resources such as icons and translation files in an application.  This makes
the packaging and distribution of those resources much easier.

A ``.qrc`` resource collection file is an XML file used to specify which
resource files are to be embedded.  The application then refers to the resource
files by their original names but preceded by a colon.

For a full description, including the format of the ``.qrc`` files, see the Qt
Resource System in the Qt documentation.


pyrcc4
------

``pyrcc4`` is PyQt's equivalent to Qt's ``rcc`` utility and is used in exactly
the same way.  ``pyrcc4`` reads the ``.qrc`` file, and the resource files, and
generates a Python module that only needs to be ``import`` ed by the
application in order for those resources to be made available just as if they
were the original files.

`pyrcc4`_ will only be included if your copy of Qt includes the XML module.


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 `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.  `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 ``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 `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.


pylupdate4
----------

``pylupdate4`` is PyQt's equivalent to Qt's ``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 ``pylupdate4`` updates (or creates if necessary)
and are subsequently used by Qt Linguist.

`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 to 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 behavour.  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")


Things to be Aware Of
=====================

Python Strings, Qt Strings and Unicode
--------------------------------------

Unicode support was added to Qt in v2.0 and to Python in v1.6.  In Qt, Unicode
support is implemented using the ``QString`` class.  It is important to
understand that ``QString`` instances, Python string objects and Python Unicode
objects are all different but conversions between them are automatic in almost
all cases and easy to achieve manually when needed.

Whenever PyQt expects a ``QString`` as a function argument, a Python string
object or a Python Unicode object can be provided instead, and PyQt will do
the necessary conversion automatically.

You may also manually convert Python string and Unicode objects to ``QString``
instances by using the ``QString`` constructor as demonstrated in the following
code fragment::

    qs1 = QtCore.QString("Converted Python string object")
    qs2 = QtCore.QString(u"Converted Python Unicode object")

In order to convert a ``QString`` to a Python string object use the Python
``str()`` builtin.  Applying ``str()`` to a null ``QString`` and an empty
``QString`` both result in an empty Python string object.

In order to convert a ``QString`` to a Python Unicode object use the Python
``unicode()`` builtin.  Applying ``unicode()`` to a null ``QString`` and an
empty ``QString`` both result in an empty Python Unicode object.

``QString`` also implements Python's buffer protocol which means that a
``QString`` can be used in many places where a Python string or Unicode object
is expected without being explicitly converted.


Garbage Collection
------------------

C++ does not garbage collect unreferenced class instances, whereas Python does.
In the following C++ fragment both colours exist even though the first can no
longer be referenced from within the program::

    col = new QColor();
    col = new QColor();

In the corresponding Python fragment, the first colour is destroyed when the
second is assigned to ``col``::

    col = QtGui.QColor()
    col = QtGui.QColor()

In Python, each colour must be assigned to different names.  Typically this is
done within class definitions, so the code fragment would be something like::

    self.col1 = QtGui.QColor()
    self.col2 = QtGui.QColor()

Sometimes a Qt class instance will maintain a pointer to another instance and
will eventually call the destructor of that second instance.  The most common
example is that a ``QObject`` (and any of its sub-classes) keeps pointers to
its children and will automatically call their destructors.  In these cases,
the corresponding Python object will also keep a reference to the corresponding
child objects.

So, in the following Python fragment, the first ``QLabel`` is not destroyed
when the second is assigned to ``lab`` because the parent ``QWidget`` still has
a reference to it::

    parent = QtGui.QWidget()
    lab = QtGui.QLabel("First label", parent)
    lab = QtGui.QLabel("Second label", parent)


Multiple Inheritance
--------------------

It is not possible to define a new Python class that sub-classes from more than
one Qt class.


Access to Protected Member Functions
------------------------------------

When an instance of a C++ class is not created from Python it is not possible
to access the protected member functions, or emit any signals, of that
instance.  Attempts to do so will raise a Python exception.  Also, any Python
methods corresponding to the instance's virtual member functions will never be
called.


``None`` and ``NULL``
---------------------

Throughout PyQt, the ``None`` value can be specified wherever ``NULL`` is
acceptable to the underlying C++ code.

Equally, ``NULL`` is converted to ``None`` whenever it is returned by the
underlying C++ code.


Support for ``void *``
----------------------

PyQt (actually SIP) represents ``void *`` values as objects of type
``sip.voidptr``.  Such values are often used to pass the addresses of external
objects between different Python modules.  To make this easier, a Python
integer (or anything that Python can convert to an integer) can be used
whenever a ``sip.voidptr`` is expected.

A ``sip.voidptr`` may be converted to a Python integer by using the ``int()``
builtin function.

A ``sip.voidptr`` may be converted to a Python string by using its
``asstring()`` method.  The ``asstring()`` method takes an integer argument
which is the length of the data in bytes.


``super`` and PyQt Classes
--------------------------

Internally PyQt implements a lazy technique for attribute lookup where
attributes are only placed in type and instance dictionaries when they are
first referenced.  This technique is needed to reduce the time taken to import
large modules such as PyQt.

In most circumstances this technique is transparent to an application.  The
exception is when ``super`` is used with a PyQt class.  The way that ``super``
is currently implemented means that the lazy lookup is bypassed resulting in
``AttributeError`` exceptions unless the attribute has been previously
referenced.

Note that this restriction applies to any class wrapped by SIP and not just
PyQt.


Deploying Commercial PyQt Applications
======================================

When deploying commercial PyQt applications it is necessary to discourage
users from accessing the underlying PyQt modules for themselves.  A user that
used the modules shipped with your application to develop new applications
would themselves be considered a developer and would need their own commercial
Qt and PyQt licenses.

One solution to this problem is the `VendorID
<http://www.riverbankcomputing.co.uk/vendorid/>`__ package.  This allows you to
build Python extension modules that can only be imported by a digitally signed
custom interpreter.  The package enables you to create such an interpreter with
your application embedded within it.  The result is an interpreter that can
only run your application, and PyQt modules that can only be imported by that
interpreter.  You can use the package to similarly restrict access to any
extension module.

In order to build PyQt with support for the VendorID package, pass the ``-i``
command line flag to ``configure.py``.


The PyQt Build System
=====================

The PyQt build system is an extension of the SIP build system and is
implemented by the ``pyqtconfig`` module, part of the ``PyQt4`` package.  It
can be used by configuration scripts of other bindings that build on top of
PyQt and takes care of the details of the Qt installation.

The module contains a number of classes.


``pyqtconfig`` Classes
----------------------

Configuration(sipconfig.Configuration)
    This class encapsulates configuration values that can be accessed as
    instance objects.

    The following configuration values are provided in addition to those
    provided by the super-class:

        pyqt_bin_dir
            The name of the directory where the PyQt utilities are installed.

        pyqt_config_args
            The command line passed to ``configure.py`` when PyQt was
            configured.

        pyqt_mod_dir
            The name of the directory where the ``PyQt4`` Python package is
            installed.

        pyqt_modules
            A space separated string of installed PyQt modules.  The ``Qt``
            module is not included.

        pyqt_sip_dir
            The name of the base directory where PyQt's ``.sip`` files are
            installed.  Each module's ``.sip`` files are installed in a
            sub-directory with the same name as the module.

        pyqt_sip_flags
            A space separated string of the ``sip`` command line arguments used
            to build the PyQt modules.  These should also be used when
            building bindings that ``%Import`` any PyQt modules.

        pyqt_version
            The PyQt version as a 3 part hexadecimal number (e.g. v4.0.1 is
            represented as ``0x040001``).

        pyqt_version_str
            The PyQt version as a string.  For development snapshots it will
            start with ``snapshot-``.

        qt_data_dir
            The value of ``QLibraryInfo::location(DataPath)`` for the Qt
            installation.

        qt_dir
            The root directory of the Qt installation (normally the directory
            that contains the ``bin`` directory).

        qt_edition
            The Qt edition.

        qt_framework
            Set if Qt is built as a MacOS/X framework.

        qt_inc_dir
            The value of ``QLibraryInfo::location(HeadersPath)`` for the Qt
            installation.

        qt_lib_dir
            The value of ``QLibraryInfo::location(LibrariesPath)`` for the Qt
            installation.

        qt_threaded
            Set if Qt is built with thread support (always set for PyQt).

        qt_version
            The Qt version as a 3 part hexadecimal number (e.g. v4.1.2 is
            represented as ``0x040102``).

        qt_winconfig
            Additional Windows specific configuration.

    __init__(self, sub_cfg=None)
        Initialise the instance.

        ``sub_cfg`` is an optional list of sub-class configurations.  It should
        only be used by the ``__init__()`` method of a sub-class to append its
        own dictionary of configuration values before passing the list to its
        super-class.

QtAssistantModuleMakefile(QtNetworkModuleMakefile)
    This class encapsulates a Makefile to build a SIP generated Python
    extension module that is built on the PyQt ``QtAssistant`` module.

QtCoreModuleMakefile(sipconfig.SIPModuleMakefile)
    This class encapsulates a Makefile to build a SIP generated Python
    extension module that is built on the PyQt ``QtCore`` module.

QtGuiModuleMakefile(QtCoreModuleMakefile)
    This class encapsulates a Makefile to build a SIP generated Python
    extension module that is built on the PyQt ``QtGui`` module.

QtNetworkModuleMakefile(QtCoreModuleMakefile)
    This class encapsulates a Makefile to build a SIP generated Python
    extension module that is built on the PyQt ``QtNetwork`` module.

QtOpenGLModuleMakefile(QtGuiModuleMakefile)
    This class encapsulates a Makefile to build a SIP generated Python
    extension module that is built on the PyQt ``QtOpenGL`` module.

QtSqlModuleMakefile(QtGuiModuleMakefile)
    This class encapsulates a Makefile to build a SIP generated Python
    extension module that is built on the PyQt ``QtSql`` module.

QtSvgModuleMakefile(QtGuiModuleMakefile)
    This class encapsulates a Makefile to build a SIP generated Python
    extension module that is built on the PyQt ``QtSvg`` module.

QtXmlModuleMakefile(QtCoreModuleMakefile)
    This class encapsulates a Makefile to build a SIP generated Python
    extension module that is built on the PyQt ``QtXml`` module.