If you start building Python application with Qt5 you'll soon discover that there are in fact two packages which you can use to do this — PyQt5 and PySide2. In this short guide I'll run through why exactly this is, whether you need to care (spoiler: you really don't), what the few differences are and how to work around them. By the end you should be comfortable re-using code examples from both PyQt5 and PySide2 tutorials to build your apps, regardless of which package you're using yourself. Background Why are there two packages? PyQt has been developed by Phil Thompson of Riverbank Computing Ltd. for a very long time — supporting versions of Qt going back to 2.x. Back in 2009 Nokia, who owned the Qt toolkit at the time, wanted to have Python bindings for Qt available under the LGPL license (like Qt itself). Unable to come to agreement with Riverbank (who would lose money from this, so fair enough) they then released their own bindings as PySide (also, fair enough). Edit: it's called PySide because "side" is Finnish for "binder" — thanks to Renato Araujo Oliveira Filho in the comments.

The two interfaces were comparable at first but PySide ultimately development lagged behind PyQt. This was particularly noticeable following the release of Qt 5 — the Qt5 version of PyQt (PyQt5) was available from mid-2016, while the first stable release of PySide2 was 2 years later. It is this delay which explains why many Qt 5 on Python examples use PyQt5 rather than PySide2 — it's not necessarily better, but it existed. However, the Qt project has recently adopted PySide as the official Qt for Python release which should ensure its viability and increase it's popularity going forward. PyQt5 PySide2 Current stable version (2019-06-23) 5.12 5.12 First stable release Apr 2016 Jul 2018 Developed by Riverbank Computing Ltd. Qt License GPL or commercial LGPL Platforms Python 3 Python 3 and Python 2.7 (Linux and MacOS only) Which should you use? Well, honestly, it doesn't really matter. Both packages are wrapping the same library — Qt5 — and so have 99.9% identical APIs (see below for the few differences). Code that is written for one can often be used as-is with other, simply changing the imports from PyQt5 to PySide2 . Anything you learn for one library will be easily applied to a project using the other. Also, no matter with one you choose to use, it's worth familiarising yourself with the other so you can make the best use of all available online resources — using PyQt5 tutorials to build your PySide2 applications for example, and vice versa. In this short chapter I'll run through the few notable differences between the two packages and explain how to write code which works seamlessly with both. After reading this you should be able to take any PyQt5 example online and convert it to work with PySide2. Licensing The key difference in the two versions — in fact the entire reason PySide2 exists — is licensing. PyQt5 is available under a GPL or commercial license, and PySide2 under a LGPL license. If you are planning to release your software itself under the GPL, or you are developing software which will not be distributed, the GPL requirement of PyQt5 is unlikely to be an issue. However, if you plan to distribute your software without distributing the source you will either need to purchase a commercial license from Riverbank for PyQt5 or use PySide2. The LGPL license does not require you to share the source code of your own applications, even if they are bundled with PySide2. You only need to ensure that the source code covered by the LGPL is made available, including modifications you have made to it, if any. In normal use there will not be any modifications and the standard distributions of PySide2/Qt5 source code will already cover this for you.

Qt itself is available under a Qt Commercial License, GPL 2.0, GPL 3.0 and LGPL 3.0 licenses.

Python versions PyQt5 is Python 3 only

PySide2 is available for Python3 and Python 2.7, but Python 2.7 builds are only available for 64 bit versions of MacOS and Linux. Windows 32 bit is supported on Python 2 only. UI files Both packages use slightly different approaches for loading .ui files exported from Qt Creator/Designer. PyQt5 provides the uic submodule which can be used to load UI files directly, to produce an object. This feels pretty Pythonic (if you ignore the camelCase). python import sys from PyQt5 import QtWidgets, uic app = QtWidgets.QApplication(sys.argv) window = uic.loadUi("mainwindow.ui") window.show() app.exec() The equivalent with PySide2 is one line longer, since you need to create a QUILoader object first. Unfortunately the api of these two interfaces is different too ( .load vs .loadUI ) and take different parameters. python import sys from PySide2 import QtCore, QtGui, QtWidgets from PySide2.QtUiTools import QUiLoader loader = QUiLoader() app = QtWidgets.QApplication(sys.argv) window = loader.load("mainwindow.ui", None) window.show() app.exec_() To load a UI onto an object in PyQt5, for example in your QMainWindow.__init__ , you can call uic.loadUI passing in self (the target widget) as the second parameter. python import sys from PyQt5 import QtCore, QtGui, QtWidgets from PyQt5 import uic class MainWindow(QtWidgets.QMainWindow): def __init__(self, *args, **kwargs): super().__init__(*args, **kwargs) uic.loadUi("mainwindow.ui", self) app = QtWidgets.QApplication(sys.argv) window = MainWindow() window.show() app.exec_() The PySide2 loader does not support this — the second parameter to .load is the parent widget of the widget you're creating. This prevents you adding custom code to the __init__ block of the widget, but you can work around this with a separate function. python import sys from PySide2 import QtWidgets from PySide2.QtUiTools import QUiLoader loader = QUiLoader() def mainwindow_setup(w): w.setTitle("MainWindow Title") app = QtWidgets.QApplication(sys.argv) window = loader.load("mainwindow.ui", None) mainwindow_setup(window) window.show() app.exec() Converting UI files to Python Both libraries provide identical scripts to generate Python importable modules from Qt Designer .ui files. For PyQt5 the script is named pyuic5 — bash pyuic5 mainwindow.ui -o MainWindow.py You can then import the UI_MainWindow object, subclass using multiple inheritance from the base class you're using (e.g. QMainWIndow ) and then call self.setupUI(self) to set the UI up. python import sys from PyQt5 import QtWidgets from MainWindow import Ui_MainWindow class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow): def __init__(self, *args, **kwargs): super().__init__(*args, **kwargs) self.setupUi(self) app = QtWidgets.QApplication(sys.argv) window = MainWindow() window.show() app.exec() For PySide2 it is named pyside2-uic — bash pyside2-uic mainwindow.ui -o MainWindow.py The subsequent setup is identical. python import sys from PySide2 import QtWidgets from MainWindow import Ui_MainWindow class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow): def __init__(self, *args, **kwargs): super().__init__(*args, **kwargs) self.setupUi(self) app = QtWidgets.QApplication(sys.argv) window = MainWindow() window.show() app.exec_()

For more information on using Qt Designer with either PyQt5 or PySide2 see the Qt Creator tutorial.

exec() or exec_() The .exec() method is used in Qt to start the event loop of your QApplication or dialog boxes. In Python 2.7 exec was a keyword, meaning it could not be used for variable, function or method names. The solution used in both PyQt4 and PySide was to rename uses of .exec to .exec_() to avoid this conflict. Python 3 removed the exec keyword, freeing the name up to be used. As PyQt5 targets only Python 3 it could remove the workaround, and .exec() calls are named just as in Qt itself. However, the .exec_() names are maintained for backwards compatibility. PySide2 is available on both Python 3 and Python 2.7 and so still uses .exec_() . It is however only available for 64bit Linux and Mac. If you're targeting both PySide2 and PyQt5 use .exec_() Slots and Signals Defining custom slots and signals uses slightly different syntax between the two libraries. PySide2 provides this interface under the names Signal and Slot while PyQt5 provides these as pyqtSignal and pyqtSlot respectively. The behaviour of them both is identical for defining and slots and signals. The following PyQt5 and PySide2 examples are identical — python my_custom_signal = pyqtSignal() # PyQt5 my_custom_signal = Signal() # PySide2 my_other_signal = pyqtSignal(int) # PyQt5 my_other_signal = Signal(int) # PySide2 Or for a slot — python @pyqtslot def my_custom_slot(): pass @Slot def my_custom_slot(): pass If you want to ensure consistency across PyQt5 and PySide2 you can use the following import pattern for PyQt5 to use the Signal and @Slot style there too. python from PyQt5.QtCore import pyqtSignal as Signal, pyqtSlot as Slot

You could of course do the reverse from PySide2.QtCore import Signal as pyqtSignal, Slot as pyqtSlot although that's a bit confusing.

Supporting both in libraries

You don't need to worry about this if you're writing a standalone app, just use whichever API you prefer.

If you're writing a library, widget or other tool you want to be compatible with both PyQt5 and PySide2 you can do so easily by adding both sets of imports. python import sys if 'PyQt5' in sys.modules: # PyQt5 from PyQt5 import QtGui, QtWidgets, QtCore from PyQt5.QtCore import pyqtSignal as Signal, pyqtSlot as Slot else: # PySide2 from PySide2 import QtGui, QtWidgets, QtCore from PySide2.QtCore import Signal, Slot This is the approach used in our custom widgets library, where we support for PyQt5 and PySide2 with a single library import. The only caveat is that you must ensure PyQt5 is imported before (as in on the line above or earlier) when importing this library, to ensure it is in sys.modules .

An alternative would be to use an environment variable to switch between them — see QtPy later.