In Qt, GUI events are handled using the signals and slots features. A signal is emitted from the GUI when an event occurs. Qt widgets have many predefined signals, and users can add custom signals for GUI events. A slot is a function that is called in response to a particular signal. In this example, we are using the clicked() signal of PushButton and creating a custom slot for this signal.

We can write our own code inside this custom function. Let's see how we can create a button, connect a signal to a slot, and convert the entire GUI to Python. Here are the steps involved in creating the Hello World GUI application:

1. Drag and create a PushButton from Qt Designer to the empty Form.
2. Assign a slot for the button clicked event, which emits a signal called clicked().
3. Save the designed UI file in the .ui extension.
4. Convert UI files to Python.
5. Write the definition of the custom slot.
6. Print the Hello World message inside the defined slot/function.

We have already dragged a button from Qt Designer to an empty Form. Press the F4 key to insert a slot on the button. When we press F4, the PushButton turns red, and we can drag a line from the button and place the ground symbol in the main window. This is shown in the following screenshot:

Select the clicked() signal from the left-hand side and click on the Edit... button to create a new custom slot. When we click on the Edit... button, another window will pop up to create a custom function. You can create a custom function by clicking on the + symbol. We created a custom slot called message(), as shown in the following screenshot:

Click on the OK button, save the UI file as hello_world.ui, and quit Qt Designer. After saving the UI file, let's see how we can convert a Qt UI file into a Python file.

Read more about Qt Signals and slots from the following link

https://doc.qt.io/qt-5/signalsandslots.html