QApplication¶

PyQt6.QtWidgets.QApplication: Inherits from QGuiApplication.

Description¶

The QApplication class manages the GUI application’s control flow and main settings.

QApplication specializes QGuiApplication with some functionality needed for QWidget-based applications. It handles widget specific initialization, finalization.

For any GUI application using Qt, there is precisely one QApplication object, no matter whether the application has 0, 1, 2 or more windows at any given time. For non-QWidget based Qt applications, use QGuiApplication instead, as it does not depend on the QtWidgets library.

Some GUI applications provide a special batch mode ie. provide command line arguments for executing tasks without manual intervention. In such non-GUI mode, it is often sufficient to instantiate a plain QCoreApplication to avoid unnecessarily initializing resources needed for a graphical user interface. The following example shows how to dynamically create an appropriate type of application instance:

# QCoreApplication* createApplication(int &argc, char *argv[])
# {
#     for (int i = 1; i < argc; ++i) {
#         if (!qstrcmp(argv[i], "-no-gui"))
#             return new QCoreApplication(argc, argv);
#     }
#     return new QApplication(argc, argv);
# }

# int main(int argc, char* argv[])
# {
#     QScopedPointer<QCoreApplication> app(createApplication(argc, argv));

#     if (qobject_cast<QApplication *>(app.data())) {
#        // start GUI version...
#     } else {
#        // start non-GUI version...
#     }

#     return app->exec();
# }

The QApplication object is accessible through the instance() function that returns a pointer equivalent to the global qApp pointer.

QApplication’s main areas of responsibility are:

It initializes the application with the user’s desktop settings such as palette(), font() and doubleClickInterval(). It keeps track of these properties in case the user changes the desktop globally, for example through some kind of control panel.
It performs event handling, meaning that it receives events from the underlying window system and dispatches them to the relevant widgets. By using sendEvent() and postEvent() you can send your own events to widgets.
It parses common command line arguments and sets its internal state accordingly. See the __init__() below for more details.
It defines the application’s look and feel, which is encapsulated in a QStyle object. This can be changed at runtime with setStyle().
It provides localization of strings that are visible to the user via translate().
It provides some magical objects like the clipboard().
It knows about the application’s windows. You can ask which widget is at a certain position using widgetAt(), get a list of topLevelWidgets() and closeAllWindows(), etc.
It manages the application’s mouse cursor handling, see setOverrideCursor()

Since the QApplication object does so much initialization, it must be created before any other objects related to the user interface are created. QApplication also deals with common command line arguments. Hence, it is usually a good idea to create it before any interpretation or modification of argv is done in the application itself.

Groups of functions
System settings	desktopSettingsAware(), setDesktopSettingsAware(), cursorFlashTime(), setCursorFlashTime(), doubleClickInterval(), setDoubleClickInterval(), setKeyboardInputInterval(), wheelScrollLines(), setWheelScrollLines(), palette(), setPalette(), font(), setFont(), fontMetrics().
Event handling	exec(), processEvents(), exit(), quit(). sendEvent(), postEvent(), sendPostedEvents(), removePostedEvents(), notify().
GUI Styles	style(), setStyle().
Text handling	installTranslator(), removeTranslator() translate().
Widgets	allWidgets(), topLevelWidgets(), activePopupWidget(), activeModalWidget(), clipboard(), focusWidget(), activeWindow(), widgetAt().
Advanced cursor handling	overrideCursor(), setOverrideCursor(), restoreOverrideCursor().
Miscellaneous	closeAllWindows(), startingUp(), closingDown().

Methods¶

__init__(list[str]): TODO

@staticmethod aboutQt()

Displays a simple message box about Qt. The message includes the version number of Qt being used by the application.

This is useful for inclusion in the Help menu of an application, as shown in the Menus example.

This function is a convenience slot for aboutQt().

@staticmethod activeModalWidget() → QWidget

Returns the active modal widget.

A modal widget is a special top-level widget which is a subclass of QDialog that specifies the modal parameter of the constructor as true. A modal widget must be closed before the user can continue with other parts of the program.

Modal widgets are organized in a stack. This function returns the active modal widget at the top of the stack.

See also

activePopupWidget(), topLevelWidgets().

@staticmethod activePopupWidget() → QWidget

Returns the active popup widget.

A popup widget is a special top-level widget that sets the Qt::WType_Popup widget flag, e.g. the QMenu widget. When the application opens a popup widget, all events are sent to the popup. Normal widgets and modal widgets cannot be accessed before the popup widget is closed.

Only other popup widgets may be opened when a popup widget is shown. The popup widgets are organized in a stack. This function returns the active popup widget at the top of the stack.

See also

activeModalWidget(), topLevelWidgets().

@staticmethod activeWindow() → QWidget: Returns the application top-level window that has the keyboard input focus, or nullptr if no application window has the focus. There might be an activeWindow() even if there is no focusWidget(), for example if no widget in that window accepts key events.

See also

setActiveWindow(), setFocus(), hasFocus(), focusWidget().

@staticmethod alert(QWidget, msecs: int = 0)

Causes an alert to be shown for widget if the window is not the active window. The alert is shown for msec miliseconds. If msec is zero (the default), then the alert is shown indefinitely until the window becomes active again.

Currently this function does nothing on Qt for Embedded Linux.

On macOS, this works more at the application level and will cause the application icon to bounce in the dock.

On Windows, this causes the window’s taskbar entry to flash for a time. If msec is zero, the flashing will stop and the taskbar entry will turn a different color (currently orange).

On X11, this will cause the window to be marked as “demands attention”, the window must not be hidden (i.e. not have hide() called on it, but be visible in some sort of way) in order for this to work.

@staticmethod allWidgets() → list[QWidget]

Returns a list of all the widgets in the application.

The list is empty (QList::isEmpty()) if there are no widgets.

Note: Some of the widgets may be hidden.

Example:

# void updateAllWidgets()
# {
#     const QWidgetList allWidgets = QApplication::allWidgets();
#     for (QWidget *widget : allWidgets)
#         widget->update();
# }

See also

topLevelWidgets(), isVisible().

autoSipEnabled() → bool: See also

setAutoSipEnabled().

@staticmethod beep(): Sounds the bell, using the default volume and sound. The function is not available in Qt for Embedded Linux.

@staticmethod closeAllWindows()

Closes all top-level windows.

This function is particularly useful for applications with many top-level windows.

The windows are closed in random order, until one window does not accept the close event. The application quits when the last window was successfully closed, unless quitOnLastWindowClosed is set to false. To trigger application termination from e.g. a menu, use quit() instead of this function.

See also

quitOnLastWindowClosed, lastWindowClosed(), close(), closeEvent(), lastWindowClosed(), quit(), topLevelWidgets(), isWindow().

@staticmethod cursorFlashTime() → int: See also

setCursorFlashTime().

@staticmethod doubleClickInterval() → int: See also

setDoubleClickInterval().

event(QEvent) → bool: TODO

@staticmethod exec() → int

Enters the main event loop and waits until exit() is called, then returns the value that was set to exit() (which is 0 if exit() is called via quit()).

It is necessary to call this function to start event handling. The main event loop receives events from the window system and dispatches these to the application widgets.

Generally, no user interaction can take place before calling exec(). As a special case, modal widgets like QMessageBox can be used before calling exec(), because modal widgets call exec() to start a local event loop.

To make your application perform idle processing, i.e., executing a special function whenever there are no pending events, use a QChronoTimer with 0ns timeout. More advanced idle processing schemes can be achieved using processEvents().

We recommend that you connect clean-up code to the aboutToQuit signal, instead of putting it in your application’s main() function. This is because, on some platforms the QApplication::exec() call may not return. For example, on the Windows platform, when the user logs off, the system terminates the process after Qt closes all top-level windows. Hence, there is no guarantee that the application will have time to exit its event loop and execute code at the end of the main() function, after the QApplication::exec() call.

See also

quitOnLastWindowClosed, quit(), exit(), processEvents(), exec().

@staticmethod focusWidget() → QWidget: Returns the application widget that has the keyboard input focus, or nullptr if no widget in this application has the focus.

See also

setFocus(), hasFocus(), activeWindow(), focusChanged.

@staticmethod font() → QFont: Returns the default application font.

See also

setFont(), fontMetrics(), font().

@staticmethod font(QWidget) → QFont

This is an overloaded function.

Returns the default font for the widget. If a default font was not registered for the widget’s class, it returns the default font of its nearest registered superclass.

See also

fontMetrics(), setFont(), setFont().

@staticmethod font(str) → QFont

This is an overloaded function.

Returns the font for widgets of the given className.

See also

setFont(), font().

@staticmethod isEffectEnabled(UIEffect) → bool

Returns true if effect is enabled; otherwise returns false.

By default, Qt will try to use the desktop settings. To prevent this, call setDesktopSettingsAware(false).

Note: All effects are disabled on screens running at less than 16-bit color depth.

See also

setEffectEnabled(), UIEffect.

@staticmethod keyboardInputInterval() → int: See also

setKeyboardInputInterval().

notify(QObject, QEvent) → bool: TODO

@staticmethod palette() → QPalette: TODO

@staticmethod palette(QWidget) → QPalette: If a widget is passed, the default palette for the widget’s class is returned. This may or may not be the application palette. In most cases there is no special palette for certain types of widgets, but one notable exception is the popup menu under Windows, if the user has defined a special background color for menus in the display settings.

See also

setPalette(), palette().

@staticmethod palette(str) → QPalette

This is an overloaded function.

Returns the palette for widgets of the given className.

See also

setPalette(), palette().

@staticmethod setActiveWindow(QWidget)

Use activateWindow() instead.

Sets the active window to the active widget in response to a system event. The function is called from the platform specific event handlers.

Warning: This function does not set the keyboard focus to the active widget. Call activateWindow() instead.

It sets the activeWindow() and focusWidget() attributes and sends proper WindowActivate/WindowDeactivate and FocusIn/FocusOut events to all appropriate widgets. The window will then be painted in active state (e.g. cursors in line edits will blink), and it will have tool tips enabled.

See also

activeWindow(), activateWindow().

setAutoSipEnabled(bool): See also

autoSipEnabled().

@staticmethod setCursorFlashTime(int): See also

cursorFlashTime().

@staticmethod setDoubleClickInterval(int): See also

doubleClickInterval().

@staticmethod setEffectEnabled(UIEffect, enabled: bool = True)

Enables the UI effect effect if enable is true, otherwise the effect will not be used.

Note: All effects are disabled on screens running at less than 16-bit color depth.

See also

isEffectEnabled(), UIEffect.

@staticmethod setFont(QFont, className: str = None)

Changes the default application font to font. If className is passed, the change applies only to classes that inherit className (as reported by inherits()).

On application start-up, the default font depends on the window system. It can vary depending on both the window system version and the locale. This function lets you override the default font; but overriding may be a bad idea because, for example, some locales need extra large fonts to support their special characters.

Warning: Do not use this function in conjunction with Qt Style Sheets. The font of an application can be customized using the “font” style sheet property. To set a bold font for all QPushButtons, set the application styleSheet() as “QPushButton { font: bold }”

See also

font(), fontMetrics(), setFont().

@staticmethod setKeyboardInputInterval(int): See also

keyboardInputInterval().

@staticmethod setPalette(QPalette, className: str = None)

Changes the application palette to palette.

If className is passed, the change applies only to widgets that inherit className (as reported by inherits()). If className is left 0, the change affects all widgets, thus overriding any previously set class specific palettes.

The palette may be changed according to the current GUI style in polish().

Warning: Do not use this function in conjunction with Qt Style Sheets. When using style sheets, the palette of a widget can be customized using the “color”, “background-color”, “selection-color”, “selection-background-color” and “alternate-background-color”.

Note: Some styles do not use the palette for all drawing, for instance, if they make use of native theme engines. This is the case for the Windows Vista and macOS styles.

See also

setPalette(), palette(), polish().

@staticmethod setStartDragDistance(int): See also

startDragDistance().

@staticmethod setStartDragTime(int): See also

startDragTime().

@staticmethod setStyle(QStyle)

Sets the application’s GUI style to style. Ownership of the style object is transferred to QApplication, so QApplication will delete the style object on application exit or when a new style is set and the old style is still the parent of the application object.

Example usage:

# QApplication::setStyle(QStyleFactory::create("Fusion"));

When switching application styles, the color palette is set back to the initial colors or the system defaults. This is necessary since certain styles have to adapt the color palette to be fully style-guide compliant.

Setting the style before a palette has been set, i.e., before creating QApplication, will cause the application to use standardPalette() for the palette.

Warning: Qt style sheets are currently not supported for custom QStyle subclasses. We plan to address this in some future release.

See also

style(), QStyle, setPalette().

@staticmethod setStyle(Optional[str]) → QStyle

This is an overloaded function.

Requests a QStyle object for style from the QStyleFactory.

The string must be one of the keys(), typically one of “windows”, “windowsvista”, “fusion”, or “macos”. Style names are case insensitive.

Returns nullptr if an unknown style is passed, otherwise the QStyle object returned is set as the application’s GUI style.

Warning: To ensure that the application’s style is set correctly, it is best to call this function before the QApplication constructor, if possible.

setStyleSheet(Optional[str]): See also

styleSheet().

@staticmethod setWheelScrollLines(int): See also

wheelScrollLines().

@staticmethod startDragDistance() → int: See also

setStartDragDistance().

@staticmethod startDragTime() → int: See also

setStartDragTime().

@staticmethod style() → QStyle: Returns the application’s style object.

See also

setStyle(), QStyle.

styleSheet() → str: See also

setStyleSheet().

@staticmethod topLevelAt(QPoint) → QWidget: Returns the top-level widget at the given point; returns nullptr if there is no such widget.

@staticmethod topLevelAt(int, int) → QWidget

This is an overloaded function.

Returns the top-level widget at the point (x, y); returns 0 if there is no such widget.

@staticmethod topLevelWidgets() → list[QWidget]

Returns a list of the top-level widgets (windows) in the application.

Note: Some of the top-level widgets may be hidden, for example a tooltip if no tooltip is currently shown.

Example:

# void showAllHiddenTopLevelWidgets()
# {
#     const QWidgetList topLevelWidgets = QApplication::topLevelWidgets();
#     for (QWidget *widget : topLevelWidgets) {
#         if (widget->isHidden())
#             widget->show();
#     }
# }

See also

allWidgets(), isWindow(), isHidden().

@staticmethod wheelScrollLines() → int: See also

setWheelScrollLines().

@staticmethod widgetAt(QPoint) → QWidget

Returns the widget at global screen position point, or nullptr if there is no Qt widget there.

This function can be slow.

See also

pos(), grabMouse(), grabKeyboard().

@staticmethod widgetAt(int, int) → QWidget

This is an overloaded function.

Returns the widget at global screen position (x, y), or nullptr if there is no Qt widget there.

Signals¶

focusChanged(QWidget, QWidget)

This signal is emitted when the widget that has keyboard focus changed from old to now, i.e., because the user pressed the tab-key, clicked into a widget or changed the active window. Both old and now can be nullptr.

The signal is emitted after both widget have been notified about the change through QFocusEvent.

See also

setFocus(), clearFocus(), FocusReason.