QCoreApplication¶

PyQt5.QtCore.QCoreApplication

Inherits from QObject.

Description¶

The QCoreApplication class provides an event loop for Qt applications without UI.

This class is used by non-GUI applications to provide their event loop. For non-GUI application that uses Qt, there should be exactly one QCoreApplication object. For GUI applications, see QGuiApplication. For applications that use the Qt Widgets module, see QApplication.

QCoreApplication contains the main event loop, where all events from the operating system (e.g., timer and network events) and other sources are processed and dispatched. It also handles the application’s initialization and finalization, as well as system-wide and application-wide settings.

The Event Loop and Event Handling¶

The event loop is started with a call to exec(). Long-running operations can call processEvents() to keep the application responsive.

In general, we recommend that you create a QCoreApplication, QGuiApplication or a QApplication object in your main() function as early as possible. exec() will not return until the event loop exits; e.g., when quit() is called.

Several static convenience functions are also provided. The QCoreApplication object is available from instance(). Events can be sent with sendEvent() or posted to an event queue with postEvent(). Pending events can be removed with removePostedEvents() or dispatched with sendPostedEvents().

The class provides a quit() slot and an aboutToQuit signal.

Application and Library Paths¶

An application has an applicationDirPath() and an applicationFilePath(). Library paths (see QLibrary) can be retrieved with libraryPaths() and manipulated by setLibraryPaths(), addLibraryPath(), and removeLibraryPath().

Internationalization and Translations¶

Translation files can be added or removed using installTranslator() and removeTranslator(). Application strings can be translated using translate(). The tr() and QObject::trUtf8() functions are implemented in terms of translate().

Accessing Command Line Arguments¶

The command line arguments which are passed to QCoreApplication’s constructor should be accessed using the arguments() function.

Note: QCoreApplication removes option -qmljsdebugger="...". It parses the argument of qmljsdebugger, and then removes this option plus its argument.

For more advanced command line option handling, create a QCommandLineParser.

Locale Settings¶

On Unix/Linux Qt is configured to use the system locale settings by default. This can cause a conflict when using POSIX functions, for instance, when converting between data types such as floats and strings, since the notation may differ between locales. To get around this problem, call the POSIX function setlocale(LC_NUMERIC,"C") right after initializing QApplication, QGuiApplication or QCoreApplication to reset the locale that is used for number formatting to “C”-locale.

Methods¶

__init__(List[str]): TODO

@staticmethod addLibraryPath(str): TODO

@staticmethod applicationDirPath() → str

Returns the directory that contains the application executable.

For example, if you have installed Qt in the C:\Qt directory, and you run the regexp example, this function will return “C:/Qt/examples/tools/regexp”.

On macOS and iOS this will point to the directory actually containing the executable, which may be inside an application bundle (if the application is bundled).

Warning: On Linux, this function will try to get the path from the /proc file system. If that fails, it assumes that argv[0] contains the absolute file name of the executable. The function also assumes that the current directory has not been changed by the application.

See also

applicationFilePath().

@staticmethod applicationFilePath() → str

Returns the file path of the application executable.

For example, if you have installed Qt in the /usr/local/qt directory, and you run the regexp example, this function will return “/usr/local/qt/examples/tools/regexp/regexp”.

Warning: On Linux, this function will try to get the path from the /proc file system. If that fails, it assumes that argv[0] contains the absolute file name of the executable. The function also assumes that the current directory has not been changed by the application.

See also

applicationDirPath().

@staticmethod applicationName() → str: See also

setApplicationName().

@staticmethod applicationPid() → int: Returns the current process ID for the application.

@staticmethod applicationVersion() → str: See also

setApplicationVersion().

@staticmethod arguments() → List[str]

Returns the list of command-line arguments.

Usually .at(0) is the program name, .at(1) is the first argument, and .last() is the last argument. See the note below about Windows.

Calling this function is slow - you should store the result in a variable when parsing the command line.

Warning: On Unix, this list is built from the argc and argv parameters passed to the constructor in the main() function. The string-data in argv is interpreted using QString::fromLocal8Bit(); hence it is not possible to pass, for example, Japanese command line arguments on a system that runs in a Latin1 locale. Most modern Unix systems do not have this limitation, as they are Unicode-based.

On Windows, the list is built from the argc and argv parameters only if modified argv/argc parameters are passed to the constructor. In that case, encoding problems might occur.

Otherwise, the are constructed from the return value of GetCommandLine(). As a result of this, the string given by .at(0) might not be the program name on Windows, depending on how the application was started.

@staticmethod closingDown() → bool: Returns true if the application objects are being destroyed; otherwise returns false.

See also

startingUp().

__enter__() → object: TODO

event(QEvent) → bool: TODO

@staticmethod eventDispatcher() → QAbstractEventDispatcher: TODO

@staticmethod exec() → int

Enters the main event loop and waits until exit() is called. Returns the value that was passed 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.

To make your application perform idle processing (by executing a special function whenever there are no pending events), use a QTimer with 0 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 because on some platforms the call may not return. For example, on Windows 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 call.

See also

quit(), exit(), processEvents().

@staticmethod exec_() → int

Enters the main event loop and waits until exit() is called. Returns the value that was passed 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.

To make your application perform idle processing (by executing a special function whenever there are no pending events), use a QTimer with 0 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 because on some platforms the call may not return. For example, on Windows 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 call.

See also

quit(), exit(), processEvents().

__exit__(object, object, object): TODO

@staticmethod exit(returnCode: int = 0)

Tells the application to exit with a return code.

After this function has been called, the application leaves the main event loop and returns from the call to exec(). The exec() function returns returnCode. If the event loop is not running, this function does nothing.

By convention, a returnCode of 0 means success, and any non-zero value indicates an error.

It’s good practice to always connect signals to this slot using a QueuedConnection. If a signal connected (non-queued) to this slot is emitted before control enters the main event loop (such as before “int main” calls exec()), the slot has no effect and the application never exits. Using a queued connection ensures that the slot will not be invoked until after control enters the main event loop.

Note that unlike the C library function of the same name, this function does return to the caller – it is event processing that stops.

See also

quit(), exec().

@staticmethod flush(): TODO

@staticmethod hasPendingEvents() → bool: TODO

installNativeEventFilter(QAbstractNativeEventFilter): TODO

@staticmethod installTranslator(QTranslator) → bool

Adds the translation file translationFile to the list of translation files to be used for translations.

Multiple translation files can be installed. Translations are searched for in the reverse order in which they were installed, so the most recently installed translation file is searched first and the first translation file installed is searched last. The search stops as soon as a translation containing a matching string is found.

Installing or removing a QTranslator, or changing an installed QTranslator generates a LanguageChange event for the QCoreApplication instance. A QApplication instance will propagate the event to all toplevel widgets, where a reimplementation of changeEvent can re-translate the user interface by passing user-visible strings via the tr() function to the respective property setters. User-interface classes generated by Qt Designer provide a retranslateUi() function that can be called.

The function returns true on success and false on failure.

@staticmethod instance() → QCoreApplication: TODO

@staticmethod isQuitLockEnabled() → bool: TODO

@staticmethod isSetuidAllowed() → bool: TODO

@staticmethod libraryPaths() → List[str]: See also

setLibraryPaths().

notify(QObject, QEvent) → bool

Sends event to receiver: receiver->event(event). Returns the value that is returned from the receiver’s event handler. Note that this function is called for all events sent to any object in any thread.

For certain types of events (e.g. mouse and key events), the event will be propagated to the receiver’s parent and so on up to the top-level object if the receiver is not interested in the event (i.e., it returns false).

There are five different ways that events can be processed; reimplementing this virtual function is just one of them. All five approaches are listed below:

Reimplementing paintEvent(), mousePressEvent() and so on. This is the most common, easiest, and least powerful way.
Reimplementing this function. This is very powerful, providing complete control; but only one subclass can be active at a time.
Installing an event filter on instance(). Such an event filter is able to process all events for all widgets, so it’s just as powerful as reimplementing ; furthermore, it’s possible to have more than one application-global event filter. Global event filters even see mouse events for isEnabled(). Note that application event filters are only called for objects that live in the main thread.
Reimplementing event() (as QWidget does). If you do this you get Tab key presses, and you get to see the events before any widget-specific event filters.
Installing an event filter on the object. Such an event filter gets all the events, including Tab and Shift+Tab key press events, as long as they do not change the focus widget.

Future direction: This function will not be called for objects that live outside the main thread in Qt 6. Applications that need that functionality should find other solutions for their event inspection needs in the meantime. The change may be extended to the main thread, causing this function to be deprecated.

Warning: If you override this function, you must ensure all threads that process events stop doing so before your application object begins destruction. This includes threads started by other libraries that you may be using, but does not apply to Qt’s own threads.

See also

event().

@staticmethod organizationDomain() → str: See also

setOrganizationDomain().

@staticmethod organizationName() → str: See also

setOrganizationName().

@staticmethod postEvent(QObject, QEvent, priority: int = Qt.EventPriority.NormalEventPriority): TODO

@staticmethod processEvents(flags: Union[ProcessEventsFlags, ProcessEventsFlag] = QEventLoop.ProcessEventsFlag.AllEvents): TODO

@staticmethod processEvents(Union[ProcessEventsFlags, ProcessEventsFlag], int)

This function overloads processEvents().

Processes pending events for the calling thread for maxtime milliseconds or until there are no more events to process, whichever is shorter.

You can call this function occasionally when your program is busy doing a long operation (e.g. copying a file).

Calling this function processes events only for the calling thread.

See also

exec(), QTimer, processEvents().

@staticmethod quit()

Tells the application to exit with return code 0 (success). Equivalent to calling exit()(0).

It’s common to connect the lastWindowClosed signal to , and you also often connect e.g. QAbstractButton::clicked() or signals in QAction, QMenu, or QMenuBar to it.

It’s good practice to always connect signals to this slot using a QueuedConnection. If a signal connected (non-queued) to this slot is emitted before control enters the main event loop (such as before “int main” calls exec()), the slot has no effect and the application never exits. Using a queued connection ensures that the slot will not be invoked until after control enters the main event loop.

Example:

# QPushButton *quitButton = new QPushButton("Quit");
# connect(quitButton, SIGNAL(clicked()), &app, SLOT(quit()), Qt::QueuedConnection);

See also

exit(), aboutToQuit, lastWindowClosed.

@staticmethod removeLibraryPath(str): TODO

removeNativeEventFilter(QAbstractNativeEventFilter): TODO

@staticmethod removePostedEvents(QObject, eventType: int = 0)

Removes all events of the given eventType that were posted using postEvent() for receiver.

The events are not dispatched, instead they are removed from the queue. You should never need to call this function. If you do call it, be aware that killing events may cause receiver to break one or more invariants.

If receiver is null, the events of eventType are removed for all objects. If eventType is 0, all the events are removed for receiver. You should never call this function with eventType of 0.

@staticmethod removeTranslator(QTranslator) → bool

Removes the translation file translationFile from the list of translation files used by this application. (It does not delete the translation file from the file system.)

The function returns true on success and false on failure.

See also

installTranslator(), translate(), tr().

@staticmethod sendEvent(QObject, QEvent) → bool: TODO

@staticmethod sendPostedEvents(receiver: QObject = None, eventType: int = 0)

Immediately dispatches all events which have been previously queued with postEvent() and which are for the object receiver and have the event type event_type.

Events from the window system are not dispatched by this function, but by processEvents().

If receiver is null, the events of event_type are sent for all objects. If event_type is 0, all the events are sent for receiver.

Note: This method must be called from the thread in which its QObject parameter, receiver, lives.

See also

flush(), postEvent().

@staticmethod setApplicationName(str): See also

applicationName().

@staticmethod setApplicationVersion(str): See also

applicationVersion().

@staticmethod setAttribute(ApplicationAttribute, on: bool = True): Sets the attribute attribute if on is true; otherwise clears the attribute.

See also

testAttribute().

@staticmethod setEventDispatcher(QAbstractEventDispatcher): TODO

@staticmethod setLibraryPaths(Iterable[str]): See also

libraryPaths().

@staticmethod setOrganizationDomain(str): See also

organizationDomain().

@staticmethod setOrganizationName(str): See also

organizationName().

@staticmethod setQuitLockEnabled(bool): TODO

@staticmethod setSetuidAllowed(bool): TODO

@staticmethod startingUp() → bool: Returns true if an application object has not been created yet; otherwise returns false.

See also

closingDown().

@staticmethod testAttribute(ApplicationAttribute) → bool: Returns true if attribute attribute is set; otherwise returns false.

See also

setAttribute().

@staticmethod translate(str, str, disambiguation: str = None, n: int = -1) → str: TODO

Signals¶

aboutToQuit(): TODO