Qt Python Signal Slot

It would be possible to have the slots to which the resized and moved signals are connected check the new position or size of the circle and respond accordingly, but it's more convenient and requires less knowledge of circles by the slot functions if the signal that is sent can include that information. 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.

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.

Qt for python signal slot

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.

Python Qt Signal Slot

PyQt4.QtCore.pyqtSignal(types[, name])

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

  • types – the types that define the C++ signature of the signal. Each type maybe a Python type object or a string that is the name of a C++ type.Alternatively each may be a sequence of type arguments. In this caseeach sequence defines the signature of a different signal overload.The first overload will be the default.
  • name – the name of the signal. If it is omitted then the name of the classattribute is used. This may only be given as a keyword argument.
Return type:

an unbound signal

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:

Connecting, Disconnecting and Emitting Signals¶

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

connect(slot[, type=PyQt4.QtCore.Qt.AutoConnection[, no_receiver_check=False]])

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

  • slot – the slot to connect to, either a Python callable or another boundsignal.
  • type – the type of the connection to make.
  • no_receiver_check – suppress the check that the underlying C++ receiver instance stillexists and deliver the signal anyway.

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

Qt Python Signal Slot

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.

Parameters:slot – the optional slot to disconnect from, either a Python callable oranother bound signal. If it is omitted then all slots connected to thesignal are disconnected.

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


Emit a signal.

Parameters:args – the optional sequence of arguments to pass to any connected slots.

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.

PyQt4.QtCore.pyqtSlot(types[, name[, result]])

Decorate a Python method to create a Qt slot.

  • types – the types that define the C++ signature of the slot. Each type may bea Python type object or a string that is the name of a C++ type.
  • name – the name of the slot that will be seen by C++. If omitted the name ofthe Python method being decorated will be used. This may only be givenas a keyword argument.
  • result – the type of the result and may be a Python type object or a string thatspecifies a C++ type. This may only be given as a keyword argument.

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.

Qt Python Signal Slot Machine

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

Python Qt Signal Slot Example

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:

Qt Python Signal Slot

Qt For Python Signal Slot

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.