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PyQt5 Quick Start (2) PyQt5 Signal Slot Mechanism 1. Introduction of Signal Slot Mechanism 1. Introduction of signal slots. Signal slot is the core mechanism of Qt and also the mechanism of object communication in PyQt programming.In Qt, the QObject object and all controls in PyQt that inherit from QWidget support the slot mechanism.When the signal is transmitted, the connected slot function.

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.

1. The custom output (QRect, QImage) signal is connected to the addImage slot so that we can update the viewer label every time a new star is drawn.
2. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type.

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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:

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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:

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.

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:

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

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Mixing New-style and Old-style Connections¶

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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.

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