Creating your app

# listing 1: creating_a_window_1.py
# Only needed for access to command line arguments
import sys
from PyQt6.QtWidgets import QApplication, QWidget


# You need one (and only one) QApplication instance per application.
# Pass in sys.argv to allow command line arguments for your app.
# If you know you won't use command line arguments QApplication([]) works too.
app = QApplication(sys.argv)

# Create a Qt widget, which will be our window.
window = QWidget()
window.show()   # IMPORTANT!!!!! Windows are hidden by default.

# Start the event loop.
app.exec()

# Your application won't reach here until you exit and the event loop has stopped.

$ python creating_a_window_1.py

Fig 1. Creating a window

Stepping through the code

QApplication, the application handler
QWidget, a basic empty GUI widget
The main modules for Qt are QtWidgets, QtGui and QtCore
sys.argv, which is Python list containing the command line arguments passed to the application
In Qt all top level widgets are windows
Widgets without a parent are invisible by default
app.exec_() to start up the event loop

What's the event loop

The core of every Qt Applications is the QApplication class.
Every application needs one — and only one — QApplication object to function.
This object holds the event loop of your application — the core loop which governs all user interaction with the GUI.
There is only one running event loop per application.

Fig 2. Event loop

The QApplication class

QApplication holds the Qt event loop

One QApplication instance required

You application sits waiting in the event loop until an action is taken

There is only one event loop at any time

QMainWindow

# listing 2: creating_a_window_2.py
import sys
from PyQt6.QtWidgets import QApplication, QPushButton

app = QApplication(sys.argv)

window = QPushButton("Push Me")
window.show()

app.exec()

$ python creating_a_window_2.py

Fig 3. A window with a single push-able button

the ability to nest widgets within other widgets using layouts means you can construct complex UIs inside an empty QWidget
the QMainWindow, a pre-made widget which provides a lot of standard window features you’ll make use of in your apps, including toolbars, menus, a statusbar, dockable widgets and more

$ python creating_a_window_3.py

Fig 4. MainWindow

If you want to create a custom window, the best approach is to subclass QMainWindow and then include the setup for the window in the __init__ block. This allows the window behavior to be self contained.

# Listing 4: creating_a_window_4.py
import sys
from PyQt6.QtWidgets import QApplication, QMainWindow, QPushButton


# Subclass QMainWindow to customize your application's main window
class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()

        self.setWindowTitle("My App")

        button = QPushButton("Press Me!")

        # Set the central widget of the window.
        self.setCentralWidget(button)


app = QApplication(sys.argv)

window = MainWindow()
window.show()

app.exec()

$ python creating_a_window_4.py

Fig5. QMainWindow with a single QPushButton on Windows

When you subclass a Qt class you must always call the super __init__ function to allow Qt to set up the object.

Sizing windows and widgets

# Listing 5: creating_a_window_end.py
import sys
from PyQt6.QtWidgets import QApplication, QMainWindow, QPushButton
from PyQt6.QtCore import QSize


class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()

        self.setWindowTitle("My App")
        button = QPushButton("Press Me!")
        self.setFixedSize(QSize(400, 300))
        self.setCentralWidget(button)


app = QApplication(sys.argv)

window = MainWindow()
window.show()

app.exec()

$ python creating_a_window_end.py

Fig 6. A fixed-size (400x300 pixels) window

01. My First Application