# Differences Between PyQt4 and PyQt5

PyQt5 is not compatibile with PyQt4 (although experience shows that the effort in porting applications from PyQt4 to PyQt5 is not great). This section describes the main differences between the two.

## Supported Python Versions

Versions of Python earlier than v2.6 are not supported.

## Deprecated Features

PyQt5 does not support any parts of the Qt API that are marked as deprecated or obsolete in Qt v5.0. However it is possible that some of these have been included accidentaly. These are considered bugs and will be removed if found.

## Multiple APIs

PyQt4 supports a number of different API versions (`QString`, `QVariant` etc.). With the exception of `QVariant`, PyQt5 only implements v2 of those APIs for all versions of Python. The changed support for `QVariant`, including the removal of `QPyNullVariant`, is described in Support for QVariant.

## Old-style Signals and Slots

PyQt4’s old-style signals and slots are not supported. Therefore the following are not implemented in PyQt5:

- `QObject.connect()`
- `QObject.emit()`
- `SIGNAL()`
- `SLOT()`

All methods that had arguments that are usually the results of calls to `SIGNAL()` or `SLOT()` are no longer supported. There will always be an equivalent that takes a bound signal or callable respectively.

In addition the following methods have differences:

- `disconnect()` takes no arguments and disconnects all connections to the `QObject` instance.

## New-style Signals and Slots

Qt implements signals with an optional argument as two separate signals, one with the argument and one without it. PyQt4 exposed both of these allowing you to connect to each of them. However, when emitting the signal, you had to use the signal appropriate to the number of arguments being emitted.

PyQt5 exposes only the signal where all arguments are specified. However it allows any optional arguments to be omitted when emitting the signal.

Unlike PyQt4, PyQt5 supports the definition of properties, signals and slots in classes not sub-classed from `QObject` (i.e. in mixins).

## `QtDeclarative`, `QtScript` and `QtScriptTools` Modules

PyQt4’s `QtDeclarative`, `QtScript` and `QtScriptTools` modules are not supported. These have been replaced by PyQt5’s `QtQml` and `QtQuick` modules. Unlike PyQt4, PyQt5 supports the creation of Python objects from QML.

## `QtGui` Module

PyQt4’s `QtGui` module has been split into PyQt5’s `QtGui`, `QtPrintSupport` and `QtWidgets` modules.

## `QtOpenGL` Module

Only the `QGLContext`, `QGLFormat` and `QGLWidget` classes are supported by PyQt5.

## `QtWebKit` Module

PyQt4’s `QtWebKit` module has been split into PyQt5’s `QtWebKit` and `QtWebKitWidgets` modules.

## `pyqtconfig` Module

PyQt4’s `pyqtconfig` module is not supported. The section The PyQt5 Extension API describes the support that PyQt5 provides to third-party packages (e.g. QScintilla) that want to build on top of PyQt5.

## `dbus.mainloop.qt` Module

PyQt4’s `dbus.mainloop.qt` module is called `dbus.mainloop.pyqt5` in PyQt5. This allows them to be installed side by side. Their functionality is identical.

## `QDataStream`

The `readUInt8()`, `readInt8()`, `writeUInt8()` and `writeInt8()` methods all interpret the values being read and written as numeric values. In PyQt4 they are interpreted as single character strings.

## `QFileDialog`

The `getOpenFileNameAndFilter()`, `getOpenFileNamesAndFilter()` and `getSaveFileNameAndFilter()` methods of PyQt4’s `QFileDialog` have now been renamed `getOpenFileName()`, `getOpenFileNames()` and `getSaveFileName()` respectively in PyQt5. PyQt4’s implementations of `getOpenFileName()`, `getOpenFileNames()` and `getSaveFileName()` are not supported in PyQt5.

## `QGraphicsItemAnimation`

Support for the deprecated `QGraphicsItemAnimation` class has been removed. If porting an existing PyQt4 application then consider first updating it to use `QPropertyAnimation` instead.

## `QMatrix`

Support for the deprecated `QMatrix` class has been removed. If porting an existing PyQt4 application then consider first updating it to use `QTransform` instead.

## `QPyTextObject`

PyQt4 implements the `QPyTextObject` as a workaround for the inability to define a Python class that is sub-classed from more than one Qt class. PyQt5 does support the ability to define a Python class that is sub-classed from more than one Qt class so long as all but one of the Qt classes are interfaces, i.e. they have been declared in C++ as such using `Q_DECLARE_INTERFACE`. Therefore `QPyTextObject` is not implemented in PyQt5.

## `QSet`

In PyQt4, `QSet` was implemented as a list in Python v2 and a set in Python v3. In PyQt5 `QSet` is always implemented as a set.

## `pyuic5`

pyuic5 does not support the `--pyqt3-wrapper` flag of `pyuic4`.

## `pyrcc5`

pyrcc5 does not support the `-py2` and `-py3` flags of `pyrcc4`. The output of pyrcc5 is compatible with all versions of Python starting with Python v2.6.

## Cooperative Multi-inheritance

Unlike PyQt4, PyQt5 classes implement cooperative multi-inheritance. In other words PyQt5 classes always do the equivalent of the following Python v3 code in their `__init__` methods (where `kwds` is a dictionary of unused keyword arguments):

    super().__init__(**kwds)

This means that those unused keyword arguments are passed to the `__init__` methods of any mixin classes. Those mixin classes must cooperate, i.e. they must make a similar call if they have their own `__init__` implementations.

When using multiple inheritance in PyQt4 it is common to call the `__init__` methods of the super-classes explicitly, for example:

    class MyQObject(QObject, MyMixin):
        def __init__(self, parent, mixin_arg):
            QObject.__init__(self, parent)
            MyMixin.__init__(self, mixin_arg)

            # Other initialisation...

In PyQt5 the above would cause `MyMixin.__init__` to be called twice. Instead it should be implemented as follows:

    class MyQObject(QObject, MyMixin):
        def __init__(self, **kwds):
            super().__init__(**kwds)

            # Other initialisation...

Note that if there is no other initialisation to do then the `__init__` method isn’t actually needed.

The mixin class should be implemented as follows:

    class MyMixin:
        def __init__(self, mixin_arg, **kwds):
            super().__init__(**kwds)

            # Other initialisation...

If a class only inherits from a single class then it can still call the super-class’s `__init__` method explicitly (although it is recommended to use `super()`).

See Support for Cooperative Multi-inheritance.

## Releasing the GIL

The GIL is only released when it is known to be needed. PyQt4 always released the GIL when calling Qt.

## Object Destruction on Exit

When the Python interpreter exits PyQt4 (by default) calls the C++ destructor of all wrapped instances that it owns. This happens in a random order and can therefore cause the interpreter to crash. This behavior can be disabled by calling the `sip.setdestroyonexit()` function.

PyQt5 always calls `sip.setdestroyonexit()` automatically. However if the wrapped instances have not been created at the module level and have instead been created in a function then the problem will still exist. For example, do not do the following:

    def main():
        app = QApplication(sys.argv)

        w = QWidget()
        w.show()

        app.exec()

    if __name__ == '__main__':
        main()

Instead, do the following:

    if __name__ == '__main__':
        app = QApplication(sys.argv)

        w = QWidget()
        w.show()

        app.exec()