The slot can be any Python call. The connection to the slot is called while the signal is transmitting. Signals & slots. Here’s a simple example to demonstrate PyQt5’s signal and slot. Import sys from PyQt5.QtWidgets import (QApplication, QWidget, QSlider, QLCDNumber, QVBoxLayout) from PyQt5.QtCore import Qt class Example(QWidget): def. QT signal & slot VS Python signal & slot All the predefined signals & slots provided by pyqt are implemented by QT's c code. Whenever you want to have a customized signal & slot in Python, it is a python signal & slot. Hence there are four cases to emits a signal to a slot. Python’s alternative for this is the @property decorator before the getter and setter function definitions. Qt Signals and Slots: Qt offers a unique callback mechanism, where a signal is emitted to notify the occurrence of an event, so that slots connected to this signal can react to it.

This section describes the new style of connecting signals and slotsintroduced in PyQt4 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 PyQt4 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. The name of aC++ type is automatically normalised so that, for example, QString can beused instead of the non-normalised constQString&.

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()¶

PyQt4 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. They must bepart of the class definition and cannot be dynamically added as classattributes after the class has been defined.

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. PyQt4 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:

Python Qt Designer

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 PyQt4 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. PyQt4 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¶

PyQt4 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) PyQt4 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:

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

Mixing New-style and Old-style Connections¶

Python Qt Signal Slot

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.

Python Qt Slot Example

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

PyQt connects the Qt C++ cross-platform framework with the Python language, it is a GUI module.

Qt is more than a GUI toolkit, which is why it features abstractions of network sockets or threads, along with Unicode, SQL, databases, SVG, OpenGL, XML, an operational we browser, a service system and a vast array of GUI widgets.

The principle on which a Qt class functions is related to a slot mechanism responsible for offering communication between items with the purpose of designing re-usable software components with ease.

Also, Qt comes with Qt Designer, a tool that acts as a graphical user interface. PyQt can design Python code from Qt Designer, while adding new GUI controls when both Qt Designer and Python programming language are used.

Book: Create Desktop Apps with Python PyQt5

PyQt classes

PyQt’s classes are classified into distinct modules, as it follows:

• QtCore: The QtCore setting features the core non-GUI functionality, being employed for items such as time, files, directories, distinct data types, threads or processes.

• QtGui: The QtGui features graphical components and related classes, such as buttons, windows, status bars, bitmaps, colours or fonts.

• QtNetwork: The QtNetwork is meant to be used for network programming purposes and it eases the coding of TCP/ IP and UDP clients or servers.

• QtXml: The QtXml servers the purposes of working with XML files, offering implementation for SAX and DOM APIs.

• QtSvg: The QtSvg features classes for revealing the contents of SVG files, SVG being a language meant to describe two-dimenl graphics or graphical applications in XML

• QtOpenGL: The QtOpenGL is meant for rendering both 3D and 2D graphics via the OpenGL library.

• QtSql: The QtSql offers means for working with databases.