• Python Signal Example
• Use Signals And Slots Python
• Python Signal Windows

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One of the key features of Qt is its use of signals and slots to communicatebetween objects. Their use encourages the development of reusable components.

1. They are used in various places inside Marble such as the signal MarbleWidget.mouseMoveGeoPosition(string). Finally we connect the signals and slots that MarbleWidget offers to the signals and slots of the slider and the label (and the label, through a custom method that prefixes the string 'Zoom Level:').
2. Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks. Signals and slots are made possible by Qt’s meta-object system.
3. An overview of Qt’s signals and slots inter-object communication mechanism. Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks.
4. This signal does nothing, by itself; it must be connected to a slot, which is an object that acts as a recipient for a signal and, given one, acts on it. Connecting Built-In PySide/PyQt Signals. Qt widgets have a number of signals built in. For example, when a QPushButton is clicked, it emits its clicked signal.

A signal is emitted when something of potential interest happens. A slot is aPython callable. If a signal is connected to a slot then the slot is calledwhen the signal is emitted. If a signal isn't connected then nothing happens.The code (or component) that emits the signal does not know or care if thesignal is being used.

The signal/slot mechanism has the following features.

• A signal may be connected to many slots.
• A signal may also be connected to another signal.
• Signal arguments may be any Python type.
• A slot may be connected to many signals.
• Connections may be direct (ie. synchronous) or queued (ie. asynchronous).
• Connections may be made across threads.
• Signals may be disconnected.

Unbound and Bound Signals¶

A signal (specifically an unbound signal) is a class attribute. When a signalis referenced as an attribute of an instance of the class then PyQt5automatically binds the instance to the signal in order to create a boundsignal. This is the same mechanism that Python itself uses to create boundmethods from class functions.

A bound signal has connect(), disconnect() and emit() methods thatimplement the associated functionality. It also has a signal attributethat is the signature of the signal that would be returned by Qt's SIGNAL()macro.

A signal may be overloaded, ie. a signal with a particular name may supportmore than one signature. A signal may be indexed with a signature in order toselect the one required. A signature is a sequence of types. A type is eithera Python type object or a string that is the name of a C++ type. The name of aC++ type is automatically normalised so that, for example, QVariant can beused instead of the non-normalised constQVariant&.

If a signal is overloaded then it will have a default that will be used if noindex is given.

When a signal is emitted then any arguments are converted to C++ types ifpossible. If an argument doesn't have a corresponding C++ type then it iswrapped in a special C++ type that allows it to be passed around Qt's meta-typesystem while ensuring that its reference count is properly maintained.

Defining New Signals with pyqtSignal()¶

PyQt5 automatically defines signals for all Qt's built-in signals. New signalscan be defined as class attributes using the pyqtSignal()factory.

Create one or more overloaded unbound signals as a class attribute.

The following example shows the definition of a number of new signals:

New signals should only be defined in sub-classes ofQObject. They must be part of the class definition andcannot be dynamically added as class attributes after the class has beendefined.

New signals defined in this way will be automatically added to the class'sQMetaObject. This means that they will appear in QtDesigner and can be introspected using the QMetaObjectAPI.

Overloaded signals should be used with care when an argument has a Python typethat has no corresponding C++ type. PyQt5 uses the same internal C++ class torepresent such objects and so it is possible to have overloaded signals withdifferent Python signatures that are implemented with identical C++ signatureswith unexpected results. The following is an example of this:

Connecting, Disconnecting and Emitting Signals¶

Signals are connected to slots using the connect() method of a boundsignal.

Connect a signal to a slot. An exception will be raised if the connectionfailed.

Signals are disconnected from slots using the disconnect() method of abound signal.

Disconnect one or more slots from a signal. An exception will be raised ifthe slot is not connected to the signal or if the signal has no connectionsat all.

Signals are emitted from using the emit() method of a bound signal.

Emit a signal.

The following code demonstrates the definition, connection and emit of asignal without arguments:

The following code demonstrates the connection of overloaded signals:

Python Signal Example

Connecting Signals Using Keyword Arguments¶

It is also possible to connect signals by passing a slot as a keyword argumentcorresponding to the name of the signal when creating an object, or using thepyqtConfigure() method. For example the followingthree fragments are equivalent:

The pyqtSlot() Decorator¶

Although PyQt5 allows any Python callable to be used as a slot when connectingsignals, it is sometimes necessary to explicitly mark a Python method as beinga Qt slot and to provide a C++ signature for it. PyQt5 provides thepyqtSlot() function decorator to do this.

Decorate a Python method to create a Qt slot.

Connecting a signal to a decorated Python method also has the advantage ofreducing the amount of memory used and is slightly faster.

For example:

It is also possible to chain the decorators in order to define a Python methodseveral times with different signatures. For example:

The PyQt_PyObject Signal Argument Type¶

It is possible to pass any Python object as a signal argument by specifyingPyQt_PyObject as the type of the argument in the signature. For example:

This would normally be used for passing objects where the actual Python typeisn't known. It can also be used to pass an integer, for example, so that thenormal conversions from a Python object to a C++ integer and back again are notrequired.

The reference count of the object being passed is maintained automatically.There is no need for the emitter of a signal to keep a reference to the objectafter the call to finished.emit(), even if a connection is queued.

Connecting Slots By Name¶

PyQt5 supports the connectSlotsByName() functionthat is most commonly used by pyuic5 generated Python code toautomatically connect signals to slots that conform to a simple namingconvention. However, where a class has overloaded Qt signals (ie. with thesame name but with different arguments) PyQt5 needs additional information inorder to automatically connect the correct signal.

For example the QSpinBox class has the followingsignals:

When the value of the spin box changes both of these signals will be emitted.If you have implemented a slot called on_spinbox_valueChanged (whichassumes that you have given the QSpinBox instance thename spinbox) then it will be connected to both variations of the signal.Therefore, when the user changes the value, your slot will be called twice -once with an integer argument, and once with a string argument.

The pyqtSlot() decorator can be used to specify which ofthe signals should be connected to the slot.

For example, if you were only interested in the integer variant of the signalthen your slot definition would look like the following:

If you wanted to handle both variants of the signal, but with different Pythonmethods, then your slot definitions might look like the following:

Build complex application behaviours using signals and slots, and override widget event handling with custom events.

As already described, every interaction the user has with a Qt application causes an Event. There are multiple types of event, each representing a difference type of interaction — e.g. mouse or keyboard events.

Events that occur are passed to the event-specific handler on the widget where the interaction occurred. For example, clicking on a widget will cause a QMouseEvent to be sent to the .mousePressEvent event handler on the widget. This handler can interrogate the event to find out information, such as what triggered the event and where specifically it occurred.

You can intercept events by subclassing and overriding the handler function on the class, as you would for any other function. You can choose to filter, modify, or ignore events, passing them through to the normal handler for the event by calling the parent class function with super().

However, imagine you want to catch an event on 20 different buttons. Subclassing like this now becomes an incredibly tedious way of catching, interpreting and handling these events.

Thankfully Qt offers a neater approach to receiving notification of things happening in your application: Signals.

Signals

Instead of intercepting raw events, signals allow you to 'listen' for notifications of specific occurrences within your application. While these can be similar to events — a click on a button — they can also be more nuanced — updated text in a box. Data can also be sent alongside a signal - so as well as being notified of the updated text you can also receive it.

The receivers of signals are called Slots in Qt terminology. A number of standard slots are provided on Qt classes to allow you to wire together different parts of your application. However, you can also use any Python function as a slot, and therefore receive the message yourself.

Load up a fresh copy of `MyApp_window.py` and save it under a new name for this section. The code is copied below if you don't have it yet.

Basic signals

First, let's look at the signals available for our QMainWindow. You can find this information in the Qt documentation. Scroll down to the Signals section to see the signals implemented for this class.

Qt 5 Documentation — QMainWindow Signals

As you can see, alongside the two QMainWindow signals, there are 4 signals inherited from QWidget and 2 signals inherited from Object. If you click through to the QWidget signal documentation you can see a .windowTitleChanged signal implemented here. Next we'll demonstrate that signal within our application.

Qt 5 Documentation — Widget Signals

The code below gives a few examples of using the windowTitleChanged signal.

Use Signals And Slots Python

Try commenting out the different signals and seeing the effect on what the slot prints.

We start by creating a function that will behave as a ‘slot’ for our signals.

Then we use .connect on the .windowTitleChanged signal. We pass the function that we want to be called with the signal data. In this case the signal sends a string, containing the new window title.

If we run that, we see that we receive the notification that the window title has changed.

Events

Next, let’s take a quick look at events. Thanks to signals, for most purposes you can happily avoid using events in Qt, but it’s important to understand how they work for when they are necessary.

Python Signal Windows

As an example, we're going to intercept the .contextMenuEvent on QMainWindow. This event is fired whenever a context menu is about to be shown, and is passed a single value event of type QContextMenuEvent.

To intercept the event, we simply override the object method with our new method of the same name. So in this case we can create a method on our MainWindow subclass with the name contextMenuEvent and it will receive all events of this type.

If you add the above method to your MainWindow class and run your program you will discover that right-clicking in your window now displays the message in the print statement.

Sometimes you may wish to intercept an event, yet still trigger the default (parent) event handler. You can do this by calling the event handler on the parent class using super as normal for Python class methods.

This allows you to propagate events up the object hierarchy, handling only those parts of an event handler that you wish.

However, in Qt there is another type of event hierarchy, constructed around the UI relationships. Widgets that are added to a layout, within another widget, may opt to pass their events to their UI parent. In complex widgets with multiple sub-elements this can allow for delegation of event handling to the containing widget for certain events.

However, if you have dealt with an event and do not want it to propagate in this way you can flag this by calling .accept() on the event.

Alternatively, if you do want it to propagate calling .ignore() will achieve this.

In this section we've covered signals, slots and events. We've demonstratedsome simple signals, including how to pass less and more data using lambdas.We've created custom signals, and shown how to intercept events, pass onevent handling and use .accept() and .ignore() to hide/show eventsto the UI-parent widget. In the next section we will go on to takea look at two common features of the GUI — toolbars and menus.