The Label widget is an instance of theQLabelclass and is used for displaying messages and images. Because the Label widgets simply display results of computations and don't take any input, they are simply used for supplying information on the screen. 

The Line Edit widget is that is popularly used for entering single-line data. The Line Edit widget is an instance of the QLineEdit class, and you can not only enter, but also edit the data too. Besides entering data, you can undo, redo, cut, and paste data in the Line Edit widget. 

To display a push button in an application, you need to create an instance of the QPushButton class. When assigning text to buttons, you can create shortcut keys by preceding any character in the text with an ampersand. For example, if the text assigned to a push button is Click Me, the character C will be underlined to indicate that it is a shortcut key, and the user can select the button by pressing Alt + C. The button emits the clicked() signal if it is activated. Besides text, an icon can also be displayed in the push button. The methods for displaying text and an icon in a push button are as follows:

Let's create a new application based on the Dialog without Buttons template. As said earlier, this application will prompt the user to enter a name and, on clicking the push button after entering a name, the application with display a hello message along with the entered name. Here are the steps to create this application:

The user interface that you create with Qt Designer is stored in a .ui file that includes all the form's information: its widgets, layout, and so on. The .ui file is an XML file, and you need to convert it to Python code. That way, you can maintain a clear separation between the visual interface and the behavior implemented in code.

C:\Pythonbook\PyQt5>pyuic5 demoLineEdit.ui -o demoLineEdit.py

Let's assume that we saved the form at this location: C:\Pythonbook\PyQt5>. The preceding command shows the conversion of the demoLineEdit.ui file into a Python script, demoLineEdit.py.

The code of the generated Python script file, demoLineEdit.py, can be seen in the source code bundle of this book.

import sys
from PyQt5.QtWidgets import QDialog, QApplication
from demoLineEdit import *
class MyForm(QDialog):
    def __init__(self):
        super().__init__()
        self.ui = Ui_Dialog()
        self.ui.setupUi(self)
        self.ui.ButtonClickMe.clicked.connect(self.dispmessage)
        self.show()
    def dispmessage(self):
        self.ui.labelResponse.setText("Hello "
        +self.ui.lineEditName.text())
if __name__=="__main__":
    app = QApplication(sys.argv)
    w = MyForm()
    w.show()
    sys.exit(app.exec_())

The demoLineEdit.py file is very easy to understand. A class with the name of the top-level object is created, with Ui_ prepended. Since the top-level object used in our application is Dialog, the Ui_Dialog class is created and stores the interface elements of our widget. That class has two methods, setupUi() and retranslateUi(). The setupUi() method sets up the widgets; it creates the widgets that you use while defining the user interface in Qt Designer. The method creates the widgets one by one and also sets their properties. The setupUi() method takes a single argument, which is the top-level widget in which the user interface (child widgets) is created. In our application, it is an instance of QDialog. The retranslateUi() method translates the interface.

Let's understand what callLineEdit.py does statement-wise:

On executing the preceding program, you get a window with the Line Edit and Push Button widgets, as shown in the following screenshot. When the push button is selected, the displmessage() method will be executed, displaying the Hello message along with the user's name that is entered in the Line Edit widget: