# Support for Signals and Slots

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 a class attribute. When a signal is referenced as an attribute of an instance of the class then PyQt5 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. It also has a `signal` attribute that 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 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. The name of a C++ type is automatically normalised so that, for example, `QVariant` can be used instead of the non-normalised `const QVariant &`.

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()`

PyQt5 automatically defines signals for all Qt’s built-in signals. New signals can be defined as class attributes using the `pyqtSignal()` factory.

`PyQt5.QtCore.``pyqtSignal`(types[, name[, revision=0[, arguments=[]]]])

  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. revision – the revision of the signal that is exported to QML. This may only be given as a keyword argument. arguments – the sequence of the names of the signal’s arguments that is exported to QML. 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 PyQt5.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`. They must be part of the class definition and cannot be dynamically added as class attributes after the class has been defined.

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. PyQt5 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=PyQt5.QtCore.Qt.AutoConnection[, no_receiver_check=False]])

  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. no_receiver_check – suppress the check that the underlying C++ receiver instance still exists and deliver the signal anyway.

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 PyQt5.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 PyQt5.QtWidgets import QComboBox

    class Bar(QComboBox):

        def connect_activated(self):
            # The PyQt5 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. For example the following three fragments are equivalent:

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

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

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

## The `pyqtSlot()` Decorator

Although PyQt5 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. PyQt5 provides the `pyqtSlot()` function decorator to do this.

`PyQt5.QtCore.``pyqtSlot`(types[, name[, result[, revision=0]]])

  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. revision – the revision of the slot that is exported to QML. 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 PyQt5.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 PyQt5.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

PyQt5 supports the `connectSlotsByName()` function that is most commonly used by pyuic5 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) PyQt5 needs additional information in order to automatically connect the correct signal.

For example the `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 string argument.

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