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QtCore.SIGNAL and QtCore.SLOT macros allow Python to interface with Qt signal and slot delivery mechanisms. This is the old way of using signals and slots. The example below uses the well known clicked signal from a QPushButton. The connect method has a non python-friendly syntax. It is necessary to inform the object, its signal (via macro. An introduction to creating PySide/PyQt signals and slots, using QObject. How signals and slots are useful, and what they can do when developing in PySide/PyQt.

Unlike a console mode application, which is executed in a sequential manner, a GUI based application is event driven. Functions or methods are executed in response to user’s actions like clicking on a button, selecting an item from a collection or a mouse click etc., called events.

Widgets used to build the GUI interface act as the source of such events. Each PyQt widget, which is derived from QObject class, is designed to emit ‘signal’ in response to one or more events. The signal on its own does not perform any action. Instead, it is ‘connected’ to a ‘slot’. The slot can be any callable Python function.

In PyQt, connection between a signal and a slot can be achieved in different ways. Following are most commonly used techniques −

A more convenient way to call a slot_function, when a signal is emitted by a widget is as follows −

PyQt uses two mechanisms for event handlers: high-level signal and slot mechanisms and low-level event handlers. We introduce the use of the low-level event handler, the processEvents function, whose function is to process time, simply to refresh the page. PyQt Signals and slots, transcending classes. Saltwater Unladen Swallow. Posts: 3 Threads: 2 Joined: Sep 2019 Reputation: 0 Likes received: 0 #1.

Suppose if a function is to be called when a button is clicked. Here, the clicked signal is to be connected to a callable function. It can be achieved in any of the following two techniques −

or

Example

Pyqt Signals And Slots Tutorial Key

In the following example, two QPushButton objects (b1 and b2) are added in QDialog window. We want to call functions b1_clicked() and b2_clicked() on clicking b1 and b2 respectively.

When b1 is clicked, the clicked() signal is connected to b1_clicked() function

When b2 is clicked, the clicked() signal is connected to b2_clicked() function

Example

The above code produces the following output −

Output

Graphical applications (GUI) are event-driven, unlike console or terminal applications. A users action like clicks a button or selecting an item in a list is called an event.

If an event takes place, each PyQt5 widget can emit a signal. A signal does not execute any action, that is done by a slot.

Related course:
Create GUI Apps with PyQt5

Signals and slot introduction
Consider this example:

The button click (signal) is connected to the action (slot). In this example, the method slot_method will be called if the signal emits.

This principle of connecting slots methods or function to a widget, applies to all widgets,

or we can explicitly define the signal:

PyQt supports many type of signals, not just clicks.

Example
We can create a method (slot) that is connected to a widget. A slot is any callable function or method.

On running the application, we can click the button to execute the action (slot).

Pyqt Signals And Slots Tutorials

If you are new to programming Python PyQt, I highly recommend this book.