QApplication

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

Inheritance diagram of PySide2.QtWidgets.QApplication

Synopsis

Functions

Slots

Signals

Static functions

Detailed Description

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:

def main():
    useGUI = not '-no-gui' in sys.argv
    app = QApplication(sys.argv) if useGUI else QCoreApplication(sys.argv)
    ...
    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 constructor documentation 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 specifies how the application is to allocate colors. See setColorSpec() for details.

  • It provides localization of strings that are visible to the user via translate() .

  • It provides some magical objects like the desktop() and 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() , hasPendingEvents() , notify() .

GUI Styles

style() , setStyle() .

Color usage

colorSpec() , setColorSpec() .

Text handling

installTranslator() , removeTranslator() translate() .

Widgets

allWidgets() , topLevelWidgets() , desktop() , activePopupWidget() , activeModalWidget() , clipboard() , focusWidget() , activeWindow() , widgetAt() .

Advanced cursor handling

overrideCursor() , setOverrideCursor() , restoreOverrideCursor() .

Miscellaneous

closeAllWindows() , startingUp() , closingDown() .

class QApplication

QApplication(arg__1)

Parameters

arg__1 – list of strings

PySide2.QtWidgets.QApplication.ColorSpec

Constant

Description

QApplication.NormalColor

the default color allocation policy

QApplication.CustomColor

the same as for X11; allocates colors to a palette on demand under Windows

QApplication.ManyColor

the right choice for applications that use thousands of colors

See setColorSpec() for full details.

static PySide2.QtWidgets.QApplication.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() .

static PySide2.QtWidgets.QApplication.activeModalWidget()
Return type

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.

static PySide2.QtWidgets.QApplication.activePopupWidget()
Return type

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.

static PySide2.QtWidgets.QApplication.activeWindow()
Return type

QWidget

Returns the application top-level window that has the keyboard input focus, or None if no application window has the focus. There might be an even if there is no focusWidget() , for example if no widget in that window accepts key events.

static PySide2.QtWidgets.QApplication.alert(widget[, duration=0])
Parameters
  • widgetQWidget

  • durationint

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.

static PySide2.QtWidgets.QApplication.allWidgets()
Return type

Returns a list of all the widgets in the application.

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

Note

Some of the widgets may be hidden.

Example:

def updateAllWidgets():
    for widget in QApplication.allWidgets()
        widget.update()
PySide2.QtWidgets.QApplication.autoSipEnabled()
Return type

bool

static PySide2.QtWidgets.QApplication.beep()

Sounds the bell, using the default volume and sound. The function is not available in Qt for Embedded Linux.

static PySide2.QtWidgets.QApplication.closeAllWindows()

Closes all top-level windows.

This function is particularly useful for applications with many top-level windows. It could, for example, be connected to a Exit entry in the File menu:

exitAct = QAction(tr("E&xit"), self)
exitAct.setShortcut(tr("Ctrl+Q")")
exitAct.setStatusTip(tr("Exit the application")")
connect(exitAct, SIGNAL("triggered()"), qApp, SLOT("closeAllWindows()"))

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; this can be turned off by setting quitOnLastWindowClosed to false.

See also

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

static PySide2.QtWidgets.QApplication.colorSpec()
Return type

int

Note

This function is deprecated.

Returns the color specification.

See also

setColorSpec()

static PySide2.QtWidgets.QApplication.cursorFlashTime()
Return type

int

static PySide2.QtWidgets.QApplication.desktop()
Return type

QDesktopWidget

Returns the desktop widget (also called the root window).

The desktop may be composed of multiple screens, so it would be incorrect, for example, to attempt to center some widget in the desktop’s geometry. QDesktopWidget has various functions for obtaining useful geometries upon the desktop, such as screenGeometry() and availableGeometry() .

On X11, it is also possible to draw on the desktop.

static PySide2.QtWidgets.QApplication.doubleClickInterval()
Return type

int

PySide2.QtWidgets.QApplication.focusChanged(old, now)
Parameters
static PySide2.QtWidgets.QApplication.focusWidget()
Return type

QWidget

Returns the application widget that has the keyboard input focus, or None if no widget in this application has the focus.

static PySide2.QtWidgets.QApplication.font(arg__1)
Parameters

arg__1QWidget

Return type

QFont

This is an overloaded function.

Returns the default font for the widget .

static PySide2.QtWidgets.QApplication.font(className)
Parameters

className – str

Return type

QFont

This is an overloaded function.

Returns the font for widgets of the given className .

See also

setFont() font()

static PySide2.QtWidgets.QApplication.fontMetrics()
Return type

QFontMetrics

Returns display (screen) font metrics for the application font.

See also

font() setFont() fontMetrics() fontMetrics()

static PySide2.QtWidgets.QApplication.globalStrut()
Return type

QSize

See also

setGlobalStrut()

static PySide2.QtWidgets.QApplication.isEffectEnabled(arg__1)
Parameters

arg__1UIEffect

Return type

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

static PySide2.QtWidgets.QApplication.keyboardInputInterval()
Return type

int

static PySide2.QtWidgets.QApplication.palette(arg__1)
Parameters

arg__1QWidget

Return type

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.

static PySide2.QtWidgets.QApplication.palette(className)
Parameters

className – str

Return type

QPalette

This is an overloaded function.

Returns the palette for widgets of the given className .

static PySide2.QtWidgets.QApplication.setActiveWindow(act)
Parameters

actQWidget

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.

PySide2.QtWidgets.QApplication.setAutoSipEnabled(enabled)
Parameters

enabledbool

See also

autoSipEnabled()

static PySide2.QtWidgets.QApplication.setColorSpec(arg__1)
Parameters

arg__1int

Note

This function is deprecated.

Sets the color specification for the application to spec .

This call has no effect.

The color specification controls how the application allocates colors when run on a display with a limited amount of colors, e.g. 8 bit / 256 color displays.

The color specification must be set before you create the QApplication object.

The options are:

Be aware that the CustomColor and ManyColor choices may lead to colormap flashing: The foreground application gets (most) of the available colors, while the background windows will look less attractive.

Example:

def main():
    QApplication.setColorSpec(QApplication.ManyColor)
    QApplication app(sys.argv)
    ...
    return app.exec_()

See also

colorSpec()

static PySide2.QtWidgets.QApplication.setCursorFlashTime(arg__1)
Parameters

arg__1int

static PySide2.QtWidgets.QApplication.setDoubleClickInterval(arg__1)
Parameters

arg__1int

static PySide2.QtWidgets.QApplication.setEffectEnabled(arg__1[, enable=true])
Parameters
  • arg__1UIEffect

  • enablebool

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 setDesktopSettingsAware()

static PySide2.QtWidgets.QApplication.setFont(arg__1[, className=None])
Parameters
  • arg__1QFont

  • className – str

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 }”

static PySide2.QtWidgets.QApplication.setGlobalStrut(arg__1)
Parameters

arg__1QSize

See also

globalStrut()

static PySide2.QtWidgets.QApplication.setKeyboardInputInterval(arg__1)
Parameters

arg__1int

static PySide2.QtWidgets.QApplication.setPalette(arg__1[, className=None])
Parameters
  • arg__1QPalette

  • className – str

Changes the default 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.

static PySide2.QtWidgets.QApplication.setStartDragDistance(l)
Parameters

lint

static PySide2.QtWidgets.QApplication.setStartDragTime(ms)
Parameters

msint

See also

startDragTime()

static PySide2.QtWidgets.QApplication.setStyle(arg__1)
Parameters

arg__1QStyle

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(QWindowsStyle())

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() desktopSettingsAware()

static PySide2.QtWidgets.QApplication.setStyle(arg__1)
Parameters

arg__1 – unicode

Return type

QStyle

PySide2.QtWidgets.QApplication.setStyleSheet(sheet)
Parameters

sheet – unicode

See also

styleSheet()

static PySide2.QtWidgets.QApplication.setWheelScrollLines(arg__1)
Parameters

arg__1int

static PySide2.QtWidgets.QApplication.startDragDistance()
Return type

int

static PySide2.QtWidgets.QApplication.startDragTime()
Return type

int

static PySide2.QtWidgets.QApplication.style()
Return type

QStyle

Returns the application’s style object.

See also

setStyle() QStyle

PySide2.QtWidgets.QApplication.styleSheet()
Return type

unicode

See also

setStyleSheet()

static PySide2.QtWidgets.QApplication.topLevelAt(x, y)
Parameters
  • xint

  • yint

Return type

QWidget

This is an overloaded function.

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

static PySide2.QtWidgets.QApplication.topLevelWidgets()
Return type

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:

def showAllHiddenTopLevelWidgets():
    for widget in QApplication.topLevelWidgets():
        if widget.isHidden():
            widget.show()
static PySide2.QtWidgets.QApplication.wheelScrollLines()
Return type

int

static PySide2.QtWidgets.QApplication.widgetAt(p)
Parameters

pQPoint

Return type

QWidget

Returns the widget at global screen position point , or None if there is no Qt widget there.

This function can be slow.

See also

pos() grabMouse() grabKeyboard()

static PySide2.QtWidgets.QApplication.widgetAt(x, y)
Parameters
  • xint

  • yint

Return type

QWidget

This is an overloaded function.

Returns the widget at global screen position (x , y ), or None if there is no Qt widget there.