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The application and the event loop ​

From a console program to a GUI application ​

A console program often determines the order of actions itself: it reads data, computes a result, and exits. In a GUI application, the user can edit any field, click buttons, and resize the window. Therefore, the program reacts to events rather than waiting for the next input. The computational functions from previous topics remain useful: now event handlers call them.

Qt is a set of libraries for GUI and other applications. Qt for Python provides the official Python bindings, PySide6. In this topic, we use Qt Widgets: ordinary desktop buttons, fields, tables, and windows. Qt Quick/QML is a different way of building interfaces and is not needed in this lab. https://doc.qt.io/qtforpython-6/.

The QtCore module contains the Qt object model, signals, timers, and basic types; QtGui contains actions, fonts, images, and validators; QtWidgets contains widgets and layouts. QAction is imported from QtGui, not from QtWidgets. Do not mix PySide6 and PyQt6 imports in one process. They are different Qt bindings with their own usage rules.

tkinter ships with the Python standard library and is suitable for simple forms. PySide6 provides a mature system of models and delegates, Designer, and many ready-made components. Cross-platform support does not remove the need to check fonts, paths, keyboard shortcuts, and sizes on the target operating system.

Setting up the environment ​

As of September 17, 2026, PySide6 6.11.2 has been verified: the package metadata specifies Python from 3.10 up to (but not including) 3.15, so Python 3.14 is supported. Open a project with its own .venv and install the package; record the dependency version in the project file. https://pypi.org/project/PySide6/.

powershell
uv add pyside6==6.11.2
uv run python -c "import PySide6; print(PySide6.__version__)"

The expected result of the check is 6.11.2. PyCharm must run the interpreter from exactly this environment. A ModuleNotFoundError after installation most often means that the package was installed in one environment and the program runs in another. The official getting-started guide: https://doc.qt.io/qtforpython-6/gettingstarted.html.

PySide6 has open-source licensing options, including LGPLv3, as well as a commercial path. This does not mean the same terms apply to every Qt module and every method of distribution. Before publishing a product, check the terms of the chosen components; for course code, keep information about dependencies and the authorship of resources. https://doc.qt.io/qtforpython-6/licenses.html.

QApplication and the event loop ​

A process creates one QApplication before any widgets. It manages events, the style, and the services of a GUI application. The window is created separately; show() makes it visible, but it is app.exec() that starts processing events. Without an event loop, the window will not work as an interactive application.

py
import sys
from PySide6.QtWidgets import (
    QApplication, QLabel, QPushButton, QVBoxLayout, QWidget,
)

if __name__ == "__main__":
    app = QApplication(sys.argv)
    window = QWidget()
    window.setWindowTitle("Hello, Qt")
    layout = QVBoxLayout(window)
    layout.addWidget(QLabel("My first application"))
    close = QPushButton("Close")
    close.clicked.connect(window.close)
    layout.addWidget(close)
    window.show()
    raise SystemExit(app.exec())

The program shows a label and a button. Clicking "Close" closes the last window; with the default settings, the loop then ends. raise SystemExit(...) passes the process exit code. The window and the application object are kept in variables for the whole run.

Figure 14.1. Handling an event and returning to the Qt loop

The event loop receives clicks, mouse movements, timer signals, and repaint requests. It delivers each event to the appropriate object. If a handler executes time.sleep(5) or a long loop, subsequent events wait, and the window stops responding (Fig. 14.1). Calling processEvents() in random places is not a substitute for properly organizing a long-running operation.

Screenshot

Run the minimal program on Windows 11, light theme; show title, label and Close button.

Figure 14.2. The first PySide6 window