In PyQt, interactivity is implemented using signals and slots. An event is an action performed by a user in the GUI (like clicking a button). A signal is raised by the corresponding widget when an event occurs on it. A slot is a function that is connected to the signal and executes when the signal is raised. PyQt is more versatile than C/Qt in this regard, because we can connect not just to slots, but also to any callable, and from PyQt 4.2, it is possible to dynamically add 'predefined' signals and slots to QObjects. Let's see how signals and slots works in practice with the Signals and Slots program shown in Figure 4.6.

As part of our PyQt Tutorial series, we’ve gone through some basic layout management in addition to a conversation about some interface design… but now when I click buttons I want things to happen!

In order to achieve that goal, we’re going to have to learn about signals and slots.

Let me let you in on a little secret. Signals and slots? They’re magical. Seriously, they are pretty cool.

Let’s go back to our face recognition example. If you’re jumping around, you can catch up to the source code that we’re starting at here. This time, since we know layouts due to the layout management post, we’re going to build our own widget so that we can better hook up our signals and slots.

This is going to track closely to the face detection post where I originally created this widget.

You’ll notice that in the code above, I didn’t put the QPushButton (instance member name of record_button), as a instance member. Since I added the push button to our layout, the layout will actually keep a reference to the instance, preventing garbage collection.

So all of that code should be review. Create a layout, add some widgets to the layout, and then set the layout on our widget.

Now let’s go ahead and wire our creation up using signals and slots.

As the documentation states, signals and slots are used for communication between objects. In this case, we want to communicate between our push button object and our record video object. Specially, when we push the “Run” button, we want our video recording object to start recording.

So looking at the push button documentation, we can see that we have several signals available to us. The one we’re interested in is clicked. Now the function that we want called after our button is clicked is the start_recording method on the VideoRecord instance. To do this, we’ll call the connect method on the clicked class instance and pass our start_recording method in as the argument. We also need to wire our image_data signal to our image_data_slot. That’s a lot of words. Let’s see it in action.

In PyQt, we can connect signals to any method call as long as the signatures match. In the case of our clicked method, no arguments are transmitted when the signal is emitted. However, if we look at the QComboBox signal documentation, we’ll see that some of the signals (activated for example) emit arguments that we need to catch in our method call.

Let’s go ahead and define our own custom signal. For example, maybe we want to transmit a signal whenever a face is detected in our widget. Let’s go ahead and subclass our FaceDetectionWidget. We’ll create a face_detected signal and override our image_data_slot method to emit the face detected signal whenever we find a face.

Notice that we call the emit method on the face_detected signal.

But how do we emit arguments? Well we’ll need to define the arguments that we want to pass in our signal. So let’s say that we not only want to emit the fact that we detected a face, but we want to emit the coordinates of the face as well.

Note that signals are always defined as class variables instead of instance variables. If you’re confused about the difference, this stack overflow post does a good job of differentiating the two.

That should be enough to get you started. Be sure to check out the PyQt documentation on signals and slots for a more in depth treatment.

This section describes the new style of connecting signals and slotsintroduced in PyQt v4.5.

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.

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 an attribute of a class that is asub-class of QObject. When a signal is referenced as an attribute of aninstance of the class then PyQt automatically binds the instance to the signalin order to create a bound signal. This is the same mechanism that Pythonitself uses to create bound methods 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.

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()¶

PyQt 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 of QObject.

New signals defined in this way will be automatically added to the class’sQMetaObject. This means that they will appear in Qt Designer and can beintrospected using the QMetaObject API.

Overloaded signals should be used with care when an argument has a Python typethat has no corresponding C++ type. PyQt 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:

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 of QObject. For example the following threefragments are equivalent:

The pyqtSlot() Decorator¶

Although PyQt 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. PyQt 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:

Connecting Slots By Name¶

PyQt supports the QtCore.QMetaObject.connectSlotsByName() function thatis most commonly used by pyuic4 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) PyQt needs additional information inorder to automatically connect the correct signal.

For example the QtGui.QSpinBox class has the following signals:

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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 the name spinbox)then it will be connected to both variations of the signal. Therefore, whenthe user changes the value, your slot will be called twice - once with aninteger argument, and once with a unicode or QString argument.

This also happens with signals that take optional arguments. Qt implementsthis using multiple signals. For example, QtGui.QAbstractButton has thefollowing signal:

Qt implements this as the following:

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

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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:

The following shows an example using a button when you are not interested inthe optional argument:

Mixing New-style and Old-style Connections¶

The implementation of new-style connections is slightly different to theimplementation of old-style connections. An application can freely use bothstyles subject to the restriction that any individual new-style connectionshould only be disconnected using the new style. Similarly any individualold-style connection should only be disconnected using the old style.

You should also be aware that pyuic4 generates code that usesold-style connections.