Event handling is an important mechanism in every application. The application should not only recognize the event, but must take the respective action to serve the event, too. The action taken on any event determines the course of the application. Each programming language has a different technique for handling or listening to events. Let's see how Python handles its events.

In PyQt, the event handling mechanism is also known as signals and slots. An event can be in the form of clicking or double-clicking on a widget, or pressing the Enter key, or selecting an option from a radio button, checkbox, and so on. Every widget emits a signal when any event is applied on it and, that signal needs to be connected to a method, also known as a slot. A slot refers to the method containing the code that you want to be executed on the occurrence of a signal. Most widgets have predefined slots; you don't have to write code to connect a predefined signal to a predefined slot.

You can even edit a signal/slot by navigating to the Edit | Edit Signals/Slots tool in the toolbar.

This recipe will make you understand how an event performed on one widget invokes a predefined action on the associated widget. Because we want to copy content from one Line Edit widget on clicking the push button, we need to invoke the selectAll() method on the occurrence of the pressed() event on push button. Also, we need to invoke the copy() method on occurrence of the released() event on the push button. To paste the content in the clipboard into another Line Edit widget on clicking of another push button, we need to invoke the paste() method on the occurrence of the clicked() event on another push button.

Let's create an application...

The most commonly used widget for accepting one-line data is the Line Edit widget, and the default data type in a Line Edit widget is string. In order to do any computation on two integer values, you need to convert the string data entered in the Line Edit widget to the integer data type and then convert the result of computation, which will be a numeric data type, back to string type before being displaying through a Label widget. This recipe does exactly that.

To understand how data is accepted by the user and how type casting is done, let's create an application based on the Dialog without Buttons template by performing the following steps:

1. Add...

The Spin Box widget is used for displaying integer values, floating-point values, and text. It applies a constraint on the user: the user cannot enter any random data, but can select only from the available options displayed through Spin Box. A Spin Box widget displays an initial value by default that can be increased or decreased by selecting the up/down button or up/down arrow key on the keyboard. You can choose a value that is displayed by either clicking on it or typing it in manually.

A Spin Box widget can be created using two classes, QSpinBox and QDoubleSpinBox, where QSpinBox displays only integer values, and the QDoubleSpinBox class displays floating-point values....

Scrollbars are useful while looking at large documents or images that cannot appear in a limited visible area. Scrollbars appear horizontally or vertically, indicating your current position in the document or image and the size of the region that is not visible. Using the slider handle provided with these bars, you can access the hidden part of the document or image.

Sliders are a way of selecting an integer value between two values. That is, a slider can represent a minimum and maximum range of values, and the user can select a value within this range by moving the slider handle to the desired location in the slider.

Scrollbars are used for viewing documents or images that are larger...

To display several values in an easier and expandable format, you can use List Widget, which is an instance of the QListWidget class. List Widget displays several items that can not only be viewed, but can be edited and deleted, too. You can add or remove list items one at a time from the List Widget item, or collectively you can set list items by using its internal model.

Items in the list are instances of the QListWidgetItem class. The methods provided by QListWidget are shown in the following list:

• insertItem(): This method inserts a new item with the supplied text into List Widget at the specified location.
• insertItems(): This method inserts multiple items from the supplied list, starting...

In the preceding application, you were selecting only a single diagnosis test from the List Widget item. What if I want to do multiple selections from the List Widget item? In the case of multiple selections, instead of a Line Edit widget, you need another List Widget to store the selected diagnosis test.

Let's create an application that displays certain diagnosis tests through List Widget and when user selects any test from List Widget, the selected test will be displayed in another List Widget:

1. So, create a new application of the Dialog without Buttons template and drag and drop two Label widgets and two...