3. Connecting widgets to a function

To make a widget interactive, you need to connect it to a function. In PySide6 this is done using a signal-slot mechanism. A signal is emitted when a particular event occurs, and a slot is a function that is called in response to the signal. In PySide6, you can connect a signal to a slot using the connect method of the signal object.

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Creating a slot function

The slot function doesn't take any arguments. To handle this, we can group all the widgets together with all slot functions in a single class. This way, we can access the widgets inside the slot function through the self reference. Assuming that the following code is inside a class in which a QLabel widget is defined as self.label, we can define a slot function as follows:

@Slot()
def change_label_text(self):
    self.label.setText("Hello, PySide6!")

In the code above, we use the @Slot() decorator to indicate that the function is a slot. So, if we want to connect this slot function to the signal of the QPushButton in our simple_gui.py file, we first need to change up our code quite a bit. We can put all the code in a class that inherits from QMainWindow and let's call it UserInterface:

Without classWith class

from PySide6 import QtWidgets
from PySide6.QtCore import Slot


app = QtWidgets.QApplication()
window = QtWidgets.QMainWindow()

# Setting the central widget 
central_widget = QtWidgets.QWidget()
window.setCentralWidget(central_widget)

# Adding a layout to the central widget
vbox = QtWidgets.QVBoxLayout()
central_widget.setLayout(vbox)

# Creating a button and a label
label = QtWidgets.QLabel()
label.setText("Hello, User! Please click the button.")
button = QtWidgets.QPushButton()
button.setText("Click me!")

# Adding the button and label to the layout
vbox.addWidget(label)
vbox.addWidget(button)

window.show()
app.exec()

from PySide6 import QtWidgets
from PySide6.QtCore import Slot


class UserInterface(QtWidgets.QMainWindow):
    def __init__(self):
        super().__init__() # (1)

        # Setting the central widget 
        central_widget = QtWidgets.QWidget()
        self.setCentralWidget(central_widget) # (2)

        # Adding a layout to the central widget
        vbox = QtWidgets.QVBoxLayout()
        central_widget.setLayout(vbox)

        # Creating a button and a label
        self.label = QtWidgets.QLabel() 
        self.label.setText("Hello, User! Please click the button.") # (3)
        button = QtWidgets.QPushButton()
        button.setText("Click me!")

        # Adding the button and label to the layout
        vbox.addWidget(self.label)
        vbox.addWidget(button)


if __name__ == "__main__": # (4)
    app = QtWidgets.QApplication() 
    window = UserInterface() # (5)
    window.show()
    app.exec()

1. With this line of code, we call the __init__ method of the parent class QMainWindow. In the without class case, we do this implicitly when we create the QMainWindow object:

   window = QtWidgets.QMainWindow()
   

2. In this case, we use the self reference to access the QMainWindow object. The equivalent line in the without class case is:

   window.setCentralWidget(central_widget)
   

3. We give the Qlabel to the self reference so that we can access it in slot functions, which we will add later. The equivalent lines in the without class case are:

   label = QtWidgets.QLabel()
   label.setText("Hello, User! Please click the button.")
   

4. We only execute the code if the script is run as the main program, not as an imported module. This is a common Python idiom .

5. Equivalent lines in the without class case are:

   app = QtWidgets.QApplication()
   window = QtWidgets.QMainWindow()
   

In the code above the changes made to simple_gui.py are highlighted. Press the buttons for more information about specific lines of code.

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Connecting the slot function to a widget

Now we can connect a slot function to the signal of a widget. Some relevant signals of QWidgets are mentioned in the API reference. We can for example connect the CurrentTextChanged signal of a QComboBox to a slot function as follows:

combobox = QtWidgets.QComboBox()
combobox.currentTextChanged.connect(do_something)

In the code above is assumed the the do_something slot function is defined in the same scope as the combobox object. In the same way, we can connect the clicked signal of a QPushButton to the change_label_text slot function in simple_gui.py:

from PySide6 import QtWidgets
from PySide6.QtCore import Slot


class UserInterface(QtWidgets.QMainWindow):
    def __init__(self):
        super().__init__()

        # Setting the central widget 
        central_widget = QtWidgets.QWidget()
        self.setCentralWidget(central_widget)

        # Adding a layout to the central widget
        vbox = QtWidgets.QVBoxLayout()
        central_widget.setLayout(vbox)

        # Creating a button and a label
        self.label = QtWidgets.QLabel() 
        self.label.setText("Hello, User! Please click the button.")
        button = QtWidgets.QPushButton()
        button.setText("Click me!")

        # Adding the button and label to the layout
        vbox.addWidget(self.label)
        vbox.addWidget(button)

        # Connecting the button to the slot function
        button.clicked.connect(self.change_label_text)

    @Slot()
    def change_label_text(self):
        self.label.setText("Hello, PySide6!")


if __name__ == "__main__":
    app = QtWidgets.QApplication() 
    window = UserInterface() 
    window.show()
    app.exec()

Final Result

That's it ! You have succesfully created your first interactive GUI application using PySide6 . The final result of running simple_gui.py is shown below:

Before clickAfter click