New-style Signal and Slot Support — PyQt 4.9.1 Reference Guide

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# New-style Signal and Slot Support

This section describes the new style of connecting signals and slots introduced in PyQt 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 happens. A slot is a Python callable. If a signal is connected to a slot then the slot is called when 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 the signal 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 a sub-class of `QObject`. When a signal is referenced as an attribute of an instance of the class then PyQt automatically binds the instance to the signal in order to create a bound signal. This is the same mechanism that Python itself uses to create bound methods from class functions.

A bound signal has `connect()`, `disconnect()` and `emit()` methods that implement the associated functionality.

A signal may be overloaded, ie. a signal with a particular name may support more than one signature. A signal may be indexed with a signature in order to select the one required. A signature is a sequence of types. A type is either a 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 no index is given.

When a signal is emitted then any arguments are converted to C++ types if possible. If an argument doesn’t have a corresponding C++ type then it is wrapped in a special C++ type that allows it to be passed around Qt’s meta-type system 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 signals can 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 may be 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 case each 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 class attribute 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:

    from PyQt4.QtCore import QObject, pyqtSignal

    class Foo(QObject):

        # This defines a signal called 'closed' that takes no arguments.
        closed = pyqtSignal()

        # This defines a signal called 'rangeChanged' that takes two
        # integer arguments.
        range_changed = pyqtSignal(int, int, name='rangeChanged')

        # This defines a signal called 'valueChanged' that has two overloads,
        # one that takes an integer argument and one that takes a QString
        # argument.  Note that because we use a string to specify the type of
        # the QString argument then this code will run under Python v2 and v3.
        valueChanged = pyqtSignal([int], ['QString'])

New signals should only be defined in sub-classes of `QObject`.

New signals defined in this way will be automatically added to the class’s `QMetaObject`. This means that they will appear in Qt Designer and can be introspected using the `QMetaObject` API.

Overloaded signals should be used with care when an argument has a Python type that has no corresponding C++ type. PyQt uses the same internal C++ class to represent such objects and so it is possible to have overloaded signals with different Python signatures that are implemented with identical C++ signatures with unexpected results. The following is an example of this:

    class Foo(QObject):

        # This will cause problems because each has the same C++ signature.
        valueChanged = pyqtSignal([dict], [list])

## Connecting, Disconnecting and Emitting Signals

Signals are connected to slots using the `connect()` method of a bound signal.

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

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

  Parameters: | slot – the slot to connect to, either a Python callable or another bound signal. type – the type of the connection to make.

Signals are disconnected from slots using the `disconnect()` method of a bound signal.

`disconnect`([slot])

  Disconnect one or more slots from a signal. An exception will be raised if the slot is not connected to the signal or if the signal has no connections at all.

  Parameters: | slot – the optional slot to disconnect from, either a Python callable or another bound signal. If it is omitted then all slots connected to the signal 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.

The following code demonstrates the definition, connection and emit of a signal without arguments:

    from PyQt4.QtCore import QObject, pyqtSignal

    class Foo(QObject):

        # Define a new signal called 'trigger' that has no arguments.
        trigger = pyqtSignal()

        def connect_and_emit_trigger(self):
            # Connect the trigger signal to a slot.
            self.trigger.connect(self.handle_trigger)

            # Emit the signal.
            self.trigger.emit()

        def handle_trigger(self):
            # Show that the slot has been called.

            print "trigger signal received"

The following code demonstrates the connection of overloaded signals:

    from PyQt4.QtGui import QComboBox

    class Bar(QComboBox):

        def connect_activated(self):
            # The PyQt documentation will define what the default overload is.
            # In this case it is the overload with the single integer argument.
            self.activated.connect(self.handle_int)

            # For non-default overloads we have to specify which we want to
            # connect.  In this case the one with the single string argument.
            # (Note that we could also explicitly specify the default if we
            # wanted to.)
            self.activated[str].connect(self.handle_string)

        def handle_int(self, index):
            print "activated signal passed integer", index

        def handle_string(self, text):
            print "activated signal passed QString", text

## Connecting Signals Using Keyword Arguments

It is also possible to connect signals by passing a slot as a keyword argument corresponding to the name of the signal when creating an object, or using the `pyqtConfigure()` method of `QObject`. For example the following three fragments are equivalent:

    act = QtGui.QAction("Action", self)
    act.triggered.connect(self.on_triggered)

    act = QtGui.QAction("Action", self, triggered=self.on_triggered)

    act = QtGui.QAction("Action", self)
    act.pyqtConfigure(triggered=self.on_triggered)

## The `pyqtSlot()` Decorator

Although PyQt allows any Python callable to be used as a slot when connecting signals, it is sometimes necessary to explicitly mark a Python method as being a Qt slot and to provide a C++ signature for it. PyQt provides the `pyqtSlot()` function decorator to do this.

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

  Decorate a Python method to create a Qt slot.

  Parameters: | types – the types that define the C++ signature of the slot. Each type may be a 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 of the Python method being decorated will be used. This may only be given as a keyword argument. result – the type of the result and may be a Python type object or a string that specifies a C++ type. This may only be given as a keyword argument.

Connecting a signal to a decorated Python method also has the advantage of reducing the amount of memory used and is slightly faster.

For example:

    from PyQt4.QtCore import QObject, pyqtSlot

    class Foo(QObject):

        @pyqtSlot()
        def foo(self):
            """ C++: void foo() """

        @pyqtSlot(int, str)
        def foo(self, arg1, arg2):
            """ C++: void foo(int, QString) """

        @pyqtSlot(int, name='bar')
        def foo(self, arg1):
            """ C++: void bar(int) """

        @pyqtSlot(int, result=int)
        def foo(self, arg1):
            """ C++: int foo(int) """

        @pyqtSlot(int, QObject)
        def foo(self, arg1):
            """ C++: int foo(int, QObject *) """

It is also possible to chain the decorators in order to define a Python method several times with different signatures. For example:

    from PyQt4.QtCore import QObject, pyqtSlot

    class Foo(QObject):

        @pyqtSlot(int)
        @pyqtSlot('QString')
        def valueChanged(self, value):
            """ Two slots will be defined in the QMetaObject. """

## Connecting Slots By Name

PyQt supports the `QtCore.QMetaObject.connectSlotsByName()` function that is most commonly used by pyuic4 generated Python code to automatically connect signals to slots that conform to a simple naming convention. However, where a class has overloaded Qt signals (ie. with the same name but with different arguments) PyQt needs additional information in order to automatically connect the correct signal.

For example the `QtGui.QSpinBox` class has the following signals:

    void valueChanged(int i);
    void valueChanged(const QString &text);

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` (which assumes that you have given the `QSpinBox` instance the name `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 unicode or `QString` argument.

This also happens with signals that take optional arguments. Qt implements this using multiple signals. For example, `QtGui.QAbstractButton` has the following signal:

    void clicked(bool checked = false);

Qt implements this as the following:

    void clicked();
    void clicked(bool checked);

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

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

    @pyqtSlot(int)
    def on_spinbox_valueChanged(self, i):
        # i will be an integer.
        pass

If you wanted to handle both variants of the signal, but with different Python methods, then your slot definitions might look like the following:

    @pyqtSlot(int, name='on_spinbox_valueChanged')
    def spinbox_int_value(self, i):
        # i will be an integer.
        pass

    @pyqtSlot(str, name='on_spinbox_valueChanged')
    def spinbox_qstring_value(self, s):
        # s will be a Python string object (or a QString if they are enabled).
        pass

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

    @pyqtSlot()
    def on_button_clicked(self):
        pass

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