• One should first look how Signals/Slots work within only one Python process: If there is only one running QThread, they just call the slots directly. If the signal is emitted on a different thread it has to find the target thread of the signal and put a message/ post an event in the thread queue of this thread.
• (A Python signal is one that is emitted in Python code; a Qt signal is one emitted from an underlying C object.) We can use this syntax both to emit Python and Qt signals, and to connect to them. These signals can be connected to any callable, that is, to any function or method, including Qt slots; they can also be connected using the SLOT.
• The worker thread is implemented as a PyQt thread rather than a Python thread since we want to take advantage of the signals and slots mechanism to communicate with the main application. Class Worker(QThread): output = pyqtSignal(QRect, QImage) def init(self, parent = None): QThread.init(self, parent) self.exiting = False self.size = QSize(0, 0) self.stars = 0.

(6 replies) Hi, I'm just starting to get to grips with PyQt, and I'm having a bit of trouble connecting slots / signals, or understanding how I should do so to achieve what I am after. I am trying to write an application that will display a sequence of dialogs, with back / next / cancel buttons to step through the dialogs.

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Simple Signal/Slot implementation

Project description

signalslot: simple Signal/Slot implementation for Python

This package provides a simple and stupid implementation of the Signal/Slotpattern for Python.Wikipedia has a nice introduction:

Signals and slots is a language construct introduced in Qt forcommunication between objects[1] which makes it easy to implement theObserver pattern while avoiding boilerplate code.

Rationale against Signal/Slot is detailed in the “Pattern”section of the documentation.

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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.

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.

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.

Signal Slot Python Qt

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¶

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:

Use Signals And Slots Python

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.

Signals Slots Python Cheat

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

Python Signal Kill

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

Python Signal Module

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