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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.

• Qt Signal Slot With 2 Arguments
• Signal And Slot In Qt
• Qt Signals And Slots Tutorial

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.

• Since QRect and QImage objects can be serialized for transmission via the signals and slots mechanism, they can be sent between threads in this way, making it convenient to use threads in a wide range of situations where built-in types are used.
• When the signal/slot is actually executed it is done in the receiver object's thread. Qt::AutoConnection (the default parameter) is a bit smarter. When a signal is emitted Qt checks the connection type, if it's an auto connection it checks the sender and receiver's thread affinity (the threads they live in).

It makes sense to add it in the declaration of both signals and slots. Qt will take care automatically of the lifecycle for arguments passed across threads. SGNAL and SLOT all support both the versions with and without const & but it's faster at execu.

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:

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¶

Qt Signal Slot With 2 Arguments

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.

Signal And Slot In Qt

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

Qt Signals And Slots Tutorial

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: