Python GUIs: PySide2 vs PySide6: What are the differences, and is it time to upgrade? — Is it time to upgrade?

If you are already developing Python GUI apps with PySide2, you might be asking yourself whether it's time to upgrade to PySide6 and use the latest version of the Qt library. In this article we'll look at the main differences between PySide2 and PySide6, benefits of upgrading and problems you might encounter when doing so.

Background

Qt is a GUI framework written in the C++ programming language, now owned by The Qt Company. They also maintain the Qt for Python project, which provides the official Python binding for Qt under the name PySide.

The name PySide was chosen because the word side means binding in the Finnish language.

The development of Qt itself started with Trolltech in 1992, but it wasn't until 2009 that the Python binding PySide became available. Development of PySide lagged behind Qt for many years, and the other Python binding PyQt became more popular. However, in recent years The Qt Company have been putting increased resources into development, and it now tracks Qt releases closely.

The first version of PySide6 was released on December 10, 2020, just two days after the release of Qt6 itself.

Upgrading from PySide2 to PySide6

The upgrade path from PySide2 to PySide6 is very straightforward. For most applications, just renaming the imports from PySide2 to PySide6 will be enough to convert your application to work with the new library.

If you are considering upgrading, I recommend you try this first and see if works -- if not, take a look at the differences below and see if they apply to your project.

Where things might go wrong

Let’s get acquainted with a few differences between the two versions to know how to write code that works seamlessly with both. After reading this, you should be able to take any PySide2 example online and convert it to work with PySide6. These changes reflect underlying differences in Qt6 vs. Qt5 and aren't unique to PySide itself.

If you’re still using Python 2.x, note that PySide6 is available only for Python 3.x versions.

High DPI Scaling

The high DPI (dots per inch) scaling attributes Qt.AA_EnableHighDpiScaling, Qt.AA_DisableHighDpiScaling and Qt.AA_UseHighDpiPixmaps have been deprecated because high DPI is enabled by default in PySide6 and can’t be disabled.

QMouseEvent

QMouseEvent.pos() and QMouseEvent.globalPos() methods returning a QPoint object as well as QMouseEvent.x() and QMouseEvent.y() returning an int object have been deprecated – use QMouseEvent.position() and QMouseEvent.globalPosition() returning a QPointF object instead, so like QMouseEvent.position().x() and QMouseEvent.position().y().

Qt.MidButton has been renamed to Qt.MiddleButton

Platform specific

Finally, platform-specific methods in the QtWin and QtMac modules have been deprecated, in favor of using the native calls instead. In PySide applications the only likely consequence of this will be the setCurrentProcessExplicitAppUserModelID call to set an application ID, for taskbar grouping.

• QtWin
• Native

python

try:
    # Include in try/except block if you're also targeting Mac/Linux
    from PyQt5.QtWinExtras import QtWin
    myappid = 'com.learnpyqt.examples.helloworld'
    QtWin.setCurrentProcessExplicitAppUserModelID(myappid)
except ImportError:
    pass

python

try:
    # Include in try/except block if you're also targeting Mac/Linux
    from ctypes import windll  # Only exists on Windows.
    myappid = 'mycompany.myproduct.subproduct.version'
    windll.shell32.SetCurrentProcessExplicitAppUserModelID(myappid)
except ImportError:
    pass

Miscellaneous

• QDesktopWidget has been removed – use QScreen instead, which can be retrieved using QWidget.screen(), QGuiApplication.primaryScreen(), or QGuiApplication.screens().
• .width() of QFontMetrics has been renamed to .horizontalAdvance().
• .get() of QOpenGLVersionFunctionsFactory() has been recommended to be used instead of .versionFunctions() of QOpenGLContext() when obtaining any functions of the Open GL library.
• QRegularExpression has replaced QRegExp.
• QWidget.mapToGlobal() and QWidget.mapFromGlobal() now accept and return a QPointF object.
• All methods named .exec_() (in classes QCoreApplication, QDialog, and QEventLoop) have been deprecated, and should be replaced with .exec() which became possible since Python 3. However, they are still available under the underscored names for backwards compatibility.

snake_case and the new true_property

PySide2 introduced the snake_case feature to write Qt method names – like .addWidget() – in a Python-friendly snake-case style like .add_widget(). This allows your PySide code to follow the Python standard PEP8 style.

Introduced in PySide6 is a new feature which allows direct access to Qt properties as Python object properties, eliminating the setter and getter methods. This can be enabled explicitly on a per-module basis by importing the true_property feature.

The example below demonstrates the effect on PySide6 code of applying both these features.

• Standard API
• Pythonic API

python

table = QTableWidget()
table.setColumnCount(2)

button = QPushButton("Add")
button.setEnabled(False)

layout = QVBoxLayout()
layout.addWidget(table)
layout.addWidget(button)

python

from __feature__ import snake_case, true_property

table = QTableWidget()
table.column_count = 2

button = QPushButton("Add")
button.enabled = False

layout = QVBoxLayout()
layout.add_widget(table)
layout.add_widget(button)

As you can see, the true_property feature allows you to assign a value to a Qt property directly – rather than using setters.

In the pre-PySide6 code, you could only do .setEnabled(False) to set the enabled property of a widget to the value False, hence disabling the widget. However, with true_property enabled, you can set a property directly with, for example, button.enabled = False. While this may seem like a cosmetic change, following Pythonic style in this way makes code easier for Python developers to understand and maintain.

PySide6 demo

Let’s demonstrate how these two features could be valuable in your PySide6 code.

python

import sys
import random

from PySide6.QtCore import Slot, Qt
from PySide6.QtWidgets import (
    QLabel,
    QWidget,
    QMainWindow,
    QPushButton,
    QVBoxLayout,
    QApplication,
)

# Import snake_case and true_property after PySide6 imports.
from __feature__ import snake_case, true_property


class MainWindow(QMainWindow):

    def __init__(self):
        super().__init__()

        # Since QMainWindow does not have a fixedSize property,
        # we use the setFixedSize() method but call it in the
        # snake-case style.
        self.set_fixed_size(300, 100)

        # However, QMainWindow does have a windowTitle property
        # for which we assign a value directly but must write
        # the property's name in snake-case style.
        self.window_title = "PySide6 Translator"

        # And this is our non-Qt Python property to which
        # we must assign a value just like above anyway,
        # so assigning values to properties uniformly
        # throughout our code could be intriguing.
        self.multilingual_greetings = (
            "&Pcy&rcy&icy&vcy&iecy&tcy &mcy&icy&rcy!",    # Russian ("Privet mir!" in Cyrillic)
            "Hallo Welt!",    # German
            "¡Hola Mundo!",   # Spanish
            "Hei maailma!",   # Finnish
            "Helló Világ!",   # Hungarian
            "Hallo Wereld!",  # Dutch
        )

        # We create a label with an English greeting by default.
        self.greeting = QLabel("Hello world!")

        # Instead of self.message.setAlignment(Qt.AlignCenter),
        # we set a value to the alignment property directly...
        self.greeting.alignment = Qt.AlignCenter

        # We now also create a button to translate our
        # English greeting and then connect it with
        # our translate_greeting() slot.
        self.translate_button = QPushButton("Translate")
        self.translate_button.clicked.connect(self.translate_greeting)

        self.vertical_layout = QVBoxLayout()
        self.vertical_layout.add_widget(self.greeting)
        self.vertical_layout.add_widget(self.translate_button)

        self.widget_container = QWidget()
        self.widget_container.set_layout(self.vertical_layout)

        # Instead of calling .setCentralWidget(),
        # we call it by its snake-case name...
        self.set_central_widget(self.widget_container)

    @Slot()
    def translate_greeting(self):
        # Here, instead of using the .setText() method,
        # we set a value to the text property directly...
        self.greeting.text = random.choice(self.multilingual_greetings)


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

    main_window = MainWindow()
    main_window.show()

    app.exec()

.exec() or .exec_()?

The .exec() method in Qt starts the event loop of your QApplication or dialog boxes. In Python 2.7, exec was a keyword, meaning that it could not be used as a variable name, a function name, or a method name. The solution used in PySide was to name the method as .exec_() – adding a trailing underscore – to avoid a conflict.

Python 3.0 removed the exec keyword, freeing up the name to be used. And since PySide6 targets only Python 3.x versions, it currently deprecates the workaround name and will later remove it. The .exec() method is named just as in Qt itself. However, the .exec_() name only exists for short-term backward compatibility with old code.

If your code must target PySide2 and PySide6 libraries, you can use .exec_(), but beware that this method name will be removed.

Missing modules

This isn’t a concern anymore, but when Qt6 was new, not all of the Qt modules had been ported yet, and so were not available in PySide6. If you needed any of these modules, the upgrade to PySide6 was not desirable then. Fast forward to Qt 6.2 and PySide 6.2, the good news is that all of those missing modules are now back. You can upgrade with no hesitation.

Is it time to upgrade?

Whether or not it's time to upgrade depends on your project. If you're starting out learning PySide (or GUI programming in general), you may prefer to stick with PySide2 for the time being as there are more examples available for PySide2 online. While the differences are minor, anything not working is confusing when you learn. Anything you know using PySide2 will carry over when you choose to upgrade to PySide6.

However, if you're starting a new project and are reasonably familiar with PySide/Qt, I'd recommend jumping into PySide6 now.

If you want to get started with PySide6, the PySide6 book is available with all code examples updated for this latest PySide edition.

PySide Backwards compatibility

If you're developing software that's targeting both PySide2 and PySide6 you can use conditional imports to import the classes from whichever module is loaded.

python

try:
    from PySide6 import QtWidgets, QtGui, QtCore # ...
except ImportError:
    from PySide2 import QtWidgets, QtGui, QtCore # ...

If you add these imports to a file in the root of your project named qt.py. You can then, in your own code files import use from qt import QtWidgets and the available library will be imported automatically.

Note however, that importing in this way won't work around any of the other differences between PySide2 and PySide6 mentioned above. For that, we recommend using the QtPy library.

Universal compatibility

If you need to support all Python Qt libraries (PySide2, PySide6, PyQt5, PyQt6) or are dependent on features which have changed between versions of Qt, then you should consider using QtPy. This package is a small abstraction layer around all versions of the Qt libraries, which allows you to use them interchangeably (as far as possible) in your applications.

If you're developing Python libraries or applications that need to be portable across different versions it is definitely worth a look.

Conclusion

As we've discovered, there are no major differences between PySide2 and PySide6. The changes that are there can be easily worked around. If you are new to Python GUI programming with Qt you may find it easier to start with PySide2 still, but for any new project I'd suggest starting with PySide6. It is an LTS (Long Term Support) version. If you’re not upgrading for the benefits, which are not significant, do it for the long-term bug fixes.

For more, see the complete PyQt6 tutorial.

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