This section describes the new style of connecting signals and slotsintroduced in PyQt4 v4.5.

Signals And Slots Qt
Qt Signal Slot With Parameters
Qt Connect Action To Slot

By the way, it doesn’t matter whether we specify the argument in a connect call as const Copy& or Copy. Qt normalises the type to Copy any way. This normalisation does not imply, however, that arguments of signals and slots are always copied – no matter whether they are passed by const reference or by value. The old method allows you to connect that slot to a signal that does not have arguments. But I cannot know with template code if a function has default arguments or not. So this feature is disabled. There was an implementation that falls back to the old method if there are more arguments in the slot than in the signal. I connect the signal to a method in the usual way. To use the argument, I merely need to specify the argument in the method definition. What often confuses me is that I don't need to specify arguments in the connect statement. The example below emits a signal didSomething and passes two arguments, 'important' and 'information' to the update.

New-style Signal and Slot Support¶ This section describes the new style of connecting signals and slots introduced in PyQt4 v4.5. One of the key features of Qt is its use of signals and slots to communicate between objects. Their use encourages the development of reusable components. A signal is emitted when something of potential interest. SLOT can have any number of arguments as long as they can be marshalled back to scheme, the types currently are: int, string, bool, uint and double. Dispatch is handled entirely by Qt using a fake object per call to qt:connect meaning that all of the rules for matching types and numbers of arguments still apply.

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.

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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.

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

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

Parameters:	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

Parameters:

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.

Parameters:	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.

disconnect([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(*args)¶

Emit a signal.

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

Unbound and Bound Signals¶

Defining New Signals with pyqtSignal()¶

Connecting, Disconnecting and Emitting Signals¶

Defining New Signals with `pyqtSignal()`¶

Qt Connect Slot With Arguments