# PyQt v4 - Python Bindings for Qt v4

## Reference Guide

Contact: | info@riverbankcomputing.co.uk
Version: | 4.3.1
Copyright: | Copyright (c) 2007 Phil Thompson

Contents

# 1 Introduction

This is the reference guide for PyQt 4.3.1. PyQt v4 is a set of Python bindings for v4 of the Qt application framework from Trolltech.

There is a separate PyQt API Reference.

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. SIP must be installed in order to build and use PyQt.

Earlier versions of Qt are supported by PyQt v3.

## 1.1 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.

## 1.2 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 `QtScript` module. This module contains classes that enable PyQt applications to be scripted using Qt's JavaScript interpreter.
> - 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.
> - The `QtSvg` module. This module contains classes for displaying the contents of SVG files.
> - The `QtTest` module. This module contains functions that enable unit testing of PyQt applications. (PyQt does not implement the complete Qt unit test framework. Instead it assumes that the standard Python unit test framework will be used and implements those functions that simulate a user interacting with a GUI.)
> - 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 `QtDesigner` module. This module contains classes that allow Qt Designer to be extended using PyQt. See Writing Qt Designer Plugins for a full description of how to do this.
> - The `QAxContainer` module. This module contains classes that allow access to ActiveX controls and COM objects. It is only available in the commercial version of PyQt for Windows.
> - 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 DBus support module is installed as `dbus.mainloop.qt`. PyQt does not support Qt's native DBus classes (which are very C++ orientated). Instead the dbus.mainloop.qt` module provides support for the Qt event loop in the same way that the dbus.mainloop.glib` included with the standard `dbus-python` bindings package provides support for the GLib event loop. The API is described in The DBus Support Module. It is only available for PyQt for X11 and only if the `dbus-python` v0.80 (or later) bindings package is installed.
> - 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.)

When PyQt is configured a file called `PyQt4.api` is generated. This can be used by the QScintilla editor component (at http://www.riverbankcomputing.co.uk/qscintilla/) to enable the use of auto-completion and call tips when editing PyQt code. The API file is installed automatically if QScintilla is already installed.

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.

# 2 Installing PyQt

## 2.1 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.

## 2.2 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.

## 2.3 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:\python25\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:

`--version` | Display the PyQt version number.
`-h, --help` | Display a help message.
`-k, --static` | The PyQt modules will be built as static libraries. This is useful when building a custom interpreter with the PyQt modules built in to the interpreter.
`-r, --trace` | The generated PyQt modules contain additional tracing code that is enabled using SIP's `sip.settracemask()` function.
`-u, --debug` | The PyQt modules will be built with debugging symbols. On Windows this requires that a debug version of Python is installed.
`-w, --verbose` | 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.
`-c, --concatenate`
| 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 `--concatenate-split` option.
`-j N, --concatenate-split=N`
| If the `--concatenate` option is used to concatenate the C++ source files then this option determines how many files are created. The default is 1.
`-g, --consolidate`

Normally each PyQt module (except for the `Qt` module) is linked against the corresponding Qt library. This option creates a module called `_qt` which is linked against all the required Qt libraries and the other modules are stub modules that populate their module dictionaries from this one. This is useful when linking against static Qt libraries to eliminate the need to distribute the Qt libraries while minimising the memory footprint of the PyQt modules.

`-e MODULE, --enable=MODULE`
| Normally checks for all PyQt4 modules are enabled and are built if the corresponding Qt library can be found. Using this option only those modules specifically enabled will be checked for and built. The option may be specified any number of times.
`-t PLUGIN, --plugin=PLUGIN`
| If Qt has been built as static libraries then the static plugin `PLUGIN` will be linked with the appropriate PyQt module. The option may be specified any number of times.
`-q FILE, --qmake=FILE`
| 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. This option is not available on Windows.
`-s DIR, --dbus=DIR`
| The `dbus-python.h` header file of the dbus-python package can be found in the directory `DIR/dbus`.
`-b DIR, --bindir=DIR`
| The `pyuic4`, `pyrcc4` and `pylupdate4` utilities will be installed in the directory `DIR`.
`-d DIR, --destdir=DIR`
| The PyQt Python package will be installed in the directory `DIR`. The default is the Python installation's `site-packages` directory. If you use this option then the `PYTHONPATH` environment variable must include `DIR`.
`-p DIR, --plugin-destdir=DIR`
| The Designer plugin that manages plugins implemented in Python will be installed in the `designer` subdirectory of the directory `DIR`.
`--no-sip-files` | The `.sip` files for the PyQt modules will not be installed.
`-v DIR, --sipdir=DIR`
| The `.sip` files for the PyQt modules will be installed in the directory `DIR`.
`-i, --vendorid` | The checking of signed Python interpreters using the VendorID package is enabled. See also the `--vendorid-incdir` and `--vendorid-libdir` options and Deploying Commercial PyQt Applications.
`-l DIR, --vendorid-incdir=DIR`
| The header file of the VendorID package can be found in the directory `DIR`.
`-m DIR, --vendorid-libdir=DIR`
| The library of the VendorID package can be found in the directory `DIR`.
`-a, --qsci-api` | The `PyQt4.api` QScintilla API file is installed even if QScintilla does not appear to be installed. This option is implied if the `--qsci-api-destdir` option is specified.
`--no-qsci-api` | The `PyQt4.api` QScintilla API file is not installed even if QScintilla does appear to be installed.
`-n DIR, --qsci-api-destdir=DIR`
| The QScintilla API file will be installed in the `python` subdirectory of the `api` subdirectory of the directory ``DIR`.

## 2.4 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.

# 3 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.

## 3.1 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.

## 3.2 The `PyQt_PyObject` Signal Argument Type

It is possible to pass any Python object as a signal argument by specifying `PyQt_PyObject` as the type of the argument in the signature. For example:

    QtCore.SIGNAL("finished(PyQt_PyObject)")

While this would normally used for passing objects like lists and dictionaries as signal arguments, it can be used for any Python type. Its advantage when passing, for example, an integer is that the normal conversions from a Python object to a C++ integer and back again are not required.

The reference count of the object being passed is maintained automatically. There is no need for the emitter of a signal to keep a reference to the object after the call to `QtCore.QObject.emit()`, even if a connection is queued.

## 3.3 Short-circuit Signals

There is also a special form of a PyQt v4 signal known as a short-circuit signal. Short-circut signals implicitly declare each argument as being of type `PyQt_PyObject`.

Short-circuit signals do not have a list of arguments or the surrounding parentheses.

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.

## 3.4 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 not usually necessary to use `QtCore.SLOT()` for Qt slots. However, doing so is more efficient as it 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, if a slot is a lambda function or a partial function then its reference count is automatically incremented to prevent it from being immediately garbage collected.

## 3.5 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()"), b, QtCore.SLOT("QtSlot()"))
    QtCore.QObject.connect(a, QtCore.SIGNAL("PySig()"), b, 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.

## 3.6 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")

## 3.7 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 C++ signature. If the signature does not include the slot name and parenthesis then the name is taken from the method.

For example, 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

If you wanted to handle both variants of the signal, but with different Python methods, then your slot definitions might look like the following:

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

    @QtCore.pyqtSignature("on_spinbox_valueChanged(const QString &)")
    def spinbox_qstring_value(self, qs):
        # qs will be a QString.
        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

When a method is decorated in this way, it is also defined as a Qt slot. See Writing Qt Designer Plugins see how to use it in this way.

# 4 Support for Pickling

The following PyQt classes may be pickled.

> - QByteArray
> - QChar
> - QColor
> - QDate
> - QDateTime
> - QKeySequence
> - QLatin1Char
> - QLatin1String
> - QLine
> - QLineF
> - QMatrix
> - QPoint
> - QPointF
> - QPolygon
> - QRect
> - QRectF
> - QSize
> - QSizeF
> - QString
> - QTime

Also all named enums (`QtCore.Qt.Key` for example) may be pickled.

# 5 Using PyQt from the Python Shell

PyQt installs an input hook (using `PyOS_InputHook`) that processes events when an interactive interpreter is waiting for user input. This means that you can, for example, create widgets from the Python shell prompt, interact with them, and still being able to enter other Python commands.

For example, if you enter the following in the Python shell:

    >>> from PyQt4 import QtGui
    >>> a = QtGui.QApplication([])
    >>> w = QtGui.QWidget()
    >>> w.show()
    >>> w.hide()
    >>>

The widget would be displayed when `w.show()` was entered amd hidden as soon as `w.hide()` was entered.

The installation of an input hook can cause problems for certain applications (particularly those that implement a similar feature using different means). The `QtCore` module contains the `pyqtRemoveInputHook()` and `pyqtRestoreInputHook()` functions that remove and restore the input hook respectively.

# 6 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.

## 6.1 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.

## 6.2 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.

## 6.3 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.

## 6.4 Writing Qt Designer Plugins

Qt Designer can be extended by writing plugins. Normally this is done using C++ but PyQt also allows you to write plugins in Python. Most of the time a plugin is used to expose a custom widget to Designer so that it appears in Designer's widget box just like any other widget. It is possibe to change the widget's properties and to connect its signals and slots.

It is also possible to add new functionality to Designer. See the Qt documentation for the full details. Here we will concentrate on describing how to write custom widgets in Python.

The process of integrating Python custom widgets with Designer is very similar to that used with widget written using C++. However, there are particular issues that have to be addressed.

> - Designer needs to have a C++ plugin that conforms to the interface defined by the `QDesignerCustomWidgetInterface` class. (If the plugin exposes more than one custom widget then it must conform to the interface defined by the `QDesignerCustomWidgetCollectionInterface` class.) In addition the plugin class must sub-class `QObject` as well as the interface class. PyQt does not allow Python classes to be sub-classed from more than one Qt class.
> - Designer can only connect Qt signals and slots. It has no understanding of Python signals or callables.
> - Designer can only edit Qt properties that represent C++ types. It has no understanding of Python attributes or Python types.

PyQt provides the following components and features to resolve these issues as simply as possible.

> - PyQt's QtDesigner module includes additional classes (all of which have a `QPy` prefix) that are already sub-classed from the necessary Qt classes. This avoids the need to sub-class from more than one Qt class in Python. For example, where a C++ custom widget plugin would sub-class from `QObject` and `QDesignerCustomWidgetInterface`, a Python custom widget plugin would instead sub-class from `QPyDesignerCustomWidgetPlugin`.

> - PyQt installs a C++ plugin in Designer's plugin directory. It conforms to the interface defined by the `QDesignerCustomWidgetCollectionInterface` class. It searches a configurable set of directories looking for Python plugins that implement a class sub-classed from `QPyDesignerCustomWidgetPlugin`. Each class that is found is instantiated and the instance created is added to the custom widget collection.

>   The `PYQTDESIGNERPATH` environment variable specifies the set of directories to search for plugins. Directory names are separated by a path separator (a semi-colon on Windows and a colon on other platforms). If a directory name is empty (ie. there are consecutive path separators or a leading or trailing path separator) then a set of default directories is automatically inserted at that point. The default directories are the `python` subdirectory of each directory that Designer searches for its own plugins. If the environment variable is not set then only the default directories are searched. If a file's basename does not end with `plugin` then it is ignored.

> - A Python custom widget may define new Qt signals using the `__pyqtSignals__` class attribute. This should define a sequence of strings each of which is the C++ signature (but excluding the return type) of the signal. For example:

>       __pyqtSignals__ = ("nameChanged(const QString &)", "failed()")

> - A Python class method may be defined as a new Qt slot by using the `QtCore.pyqtSignature` decorator. The decorator takes a single string argument which defines the C++ signature of the slot. The name and parenthesis of the signature may be omitted in which case the name of the method is used instead. For example:

>       # Define a Qt slot that takes a C++ integer argument.
>       @QtCore.pyqtSignature("addToTotal(int)")
>       def add_int_to_total(self, value):
>           pass

>       # Define a similar slot that takes its name from the method.
>       @QtCore.pyqtSignature("int")
>       def addToTotal(self, value):
>           pass

> - A new Qt property may be defined using the `QtCore.pyqtProperty()` function. It is used in the same way as the standard Python `property()` function. In fact, Qt properties defined in this way also behave as Python properties. The full signature of the function is as follows:

>       pyqtProperty(type, fget=None, fset=None, freset=None, fdel=None, doc=None)

>   `type` is a string that defines the C++ type of the property. `freset` is a function used to reset the value of the property to its default value. The remaining arguments are the same as those used by the standard `property()` function. Qt makes no use of the `fdel` function and Python makes no use of the `freset` function.

Note that the ability to define new Qt signals, slots and properties from Python is potentially useful to plugins conforming to any plugin interface and not just that used by Designer.

For a simple but complete and fully documented example of a custom widget that defines new Qt signals, slots and properties, and its plugin, look in the `examples/designer/plugins` directory of the PyQt source package. The `widgets` subdirectory contains the `pydemo.py` custom widget and the `python` subdirectory contains its `pydemoplugin.py` plugin.

# 7 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.

## 7.1 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.

# 8 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.

## 8.1 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.

## 8.2 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")

# 9 The DBus Support Module

The DBus support module is installed as `dbus.mainloop.qt` and provides support for the Qt event loop to the standard `dbus-python` language bindings package. The module's API is almost identical to that of the `dbus.mainloop.glib` modules that provides support for the GLib event loop.

The `dbus.mainloop.qt` module contains the following function.

DBusQtMainLoop(set_as_default=False)

  This function returns a `dbus.mainloop.NativeMainLoop` object that uses the the Qt event loop.

  `set_as_default` is set to make the main loop instance the default for all new Connection and Bus instances. It may only be specified as a keyword argument, and not as a positional argument.

The following code fragment is all that is normally needed to set up the standard `dbus-python` language bindings package to be used with PyQt:

    import dbus.mainloop.qt

    dbus.mainloop.qt.DBusQtMainLoop(set_as_default=True)

# 10 Things to be Aware Of

## 10.1 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.

## 10.2 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)

## 10.3 Multiple Inheritance

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

## 10.4 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.

## 10.5 `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.

## 10.6 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.

## 10.7 `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.

# 11 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 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`.

# 12 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.

## 12.1 `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.
QAxContainerModuleMakefile(QtGuiModuleMakefile)
  This class encapsulates a Makefile to build a SIP generated Python extension module that is built on the PyQt `QAxContainer` 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.
QtScriptModuleMakefile(QtCoreModuleMakefile)
  This class encapsulates a Makefile to build a SIP generated Python extension module that is built on the PyQt `QtScript` 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.
QtTestModuleMakefile(QtCoreModuleMakefile)
  This class encapsulates a Makefile to build a SIP generated Python extension module that is built on the PyQt `QtTest` module.
QtXmlModuleMakefile(QtCoreModuleMakefile)
  This class encapsulates a Makefile to build a SIP generated Python extension module that is built on the PyQt `QtXml` module.