This usually works very well and it's probably one of the features that make Python way easier to work with than for example C++.Ĭ++, on the other hand, allows more freedom when it comes to memory management. Your Python object lives longer than the QObjectĪnother "Shoot yourself in the foot" experience that you will potentially have using Python and Qt is related to memory management.Īs we all know, Python supports automatic memory management, meaning a garbage collector looks for variables that aren't referenced anymore and frees memory. NOTE: Don't forget to store the variable containing the timer instance somewhere or your timer instance will be garbage collected. This way we can safely terminate our application with Ctrl-C from the command line. We just need to set up a timer kicking off our event loop every few milliseconds.Īll we added to the hello world application is the code to start to create and start a timer every 100 milliseconds. Therefore, our Python process never sees the signal until we hit the exit button of our Qt application window. So our event loop never churns the Python interpreter and so our signal delivered to the Python process is never processed. The problem here is that we don't have any Python events set up yet. When we execute app.exec_() we start the Qt/C++ event loop, which loops forever until it is stopped. However, the pitfall here is that Qt is implemented in C++ and not in Python. Therefore, you often will find event loops being used in GUI or web frameworks. The concept of event loops is especially useful for applications where a long living process needs to handle interactions from a user or client. Such an event loop enables you to write parallel applications without multithreading. Qt strongly builds on a concept called event loop. Only when hitting the exit button in the application window, the application finally stops as we the expected KeyboardInterruptError: File "./main.py", line 13, in However, the first time you try to stop the application from the console you will see that Ctrl-C has no effect and the application just keeps running. This code snippet works perfectly fine and when you execute it you will see the hello world window as expected. Let's take a look at the following PyQt QML hello world application:įrom PyQt5.QtQml import QQmlApplicationEngine The first thing you will notice when writing your first hello world application with PyQt is that you can't stop it anymore from the command-line with Ctrl-C. The Application can't be stopped with Ctrl-C In this article, you will learn which things you should watch out for when working on GUIs with Qt and Python and how to avoid the resulting problems. In the last few weeks, I have been using Python and Qt, especially PyQt extensively.ĭuring these weeks I have discovered a few ways how to shoot yourself in the foot accidentally.
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