PyQt5 - Signal and slot function. The signal and slot operation are used to handle events and signals of the objects or widgets at the python app development level. It will also enable communication between some designed objects. For our signal and slot example, we will handle two types of widgets such as LineEdit and Label. I am having some trouble applying the new pyqt5 signals and slots into a script thats purpose is to test/invoke another problem I've been trying to solve, GUI freezing/crashing. The aim is so that once these signals and slots are functioning correctly the GUI will not crash after +/- 30 seconds of runtime, and just continue on counting. Sep 04, 2016  It can be difficult for newcomers to configure signal and slot in PyQt5 who have no prior experience in Qt programming. Signal-Slot is one of the fundamental topics of Qt one should have a. I am having some trouble applying the new pyqt5 signals and slots into a script thats purpose is to test/invoke another problem I've been trying to solve, GUI freezing/crashing. The aim is so that once these signals and slots are functioning correctly the GUI will not crash after +/- 30 seconds of runtime, and just continue on counting numbers until the end of time. May 22, 2016  This video introduces signals and slots so that we can now respond to events. It also wraps up our window in a class. PyQt5 Lesson 5 Signals and Slots Mark Winfield. 7 PyQt5 Signal And.

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.


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.

Current stable version (2019-06-23)5.125.12
First stable releaseApr 2016Jul 2018
Developed byRiverbank Computing Ltd.Qt
LicenseGPL or commercialLGPL
PlatformsPython 3Python 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.


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 commercially you will either need to purchase a commercial license from Riverbank for PyQt5 or use PySide2.

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).

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.

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.

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.

Converting UI files to Python

Pyqt5 Signals And Slots Examples

Both libraries provide identical scripts to generate Python importable modules from Qt Designer .ui files. For PyQt5 the script is named pyuic5

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.

For PySide2 it is named pyside2-uic

The subsequent setup is identical.

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 —

Or for a slot —

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.

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

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.

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.

If you're doing this in multiple files it can get a bit cumbersome. A nice solution to this is to move the import logic to its own file, e.g. named qt.py in your project root. This module imports the Qt modules (QtCore, QtGui, QtWidgets, etc.) from one of the two libraries, and then you import into your application from there.

The contents of the qt.py are the same as we used earlier —

You must remember to add any other PyQt5 modules you use (browser, multimedia, etc.) in both branches of the if block. You can then import Qt5 into your own application with —

SignalsPyqt5 Signals And Slots Example

…and it will work seamlessly across either library.


If you need to target more than just Qt5 support (e.g. including PyQt4 and PySide v1) take a look at QtPy. This provides a standardised PySide2-like API for PyQt4, PySide, PyQt5 and PySide2. Using QtPy you can control which API to load from your application using the QT_API environment variable e.g.

Pyqt5 Signals And Slots Example 3

That's really it

There's not much more to say — the two are really very similar. With the above tips you should feel comfortable taking code examples or documentation from PyQt5 and using it to write an app with PySide2. If you do stumble across any PyQt5 or PySide2 examples which you can't easily convert, drop a note in the comments and I'll update this page with advice.