• Python Qt Signals And Slots 777
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Signals and Slots in PySide. From Qt Wiki (Redirected from Signals and slots in PySide) Redirect page. Redirect to: Qt for Python Signals and Slots. 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. For instance, one event activates its slot and related subsequent events trigger another signal and the code in its slot to be executed. General understanding of the python programming language. No prior knowledge of QT is required. Connecting signals and slots. We use the QObject.connect method to connect signals and slots. PyQt5 signals and slots 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. 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 syntax, with a slotSignature. PyQt checks to see whether the signal is a Qt signal, and if it is not it assumes it is a Python signal.

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Python Qt Signals And Slots 777

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.

Using Qt Designer's Signal/Slot Editor

First design a simple form with a LineEdit control and a PushButton.

It is desired that if button is pressed, contents of text box should be erased. The QLineEdit widget has a clear() method for this purpose. Hence, the button’s clicked signal is to be connected to clear() method of the text box.

To start with, choose Edit signals/slots from Edit menu (or press F4). Then highlight the button with mouse and drag the cursor towards the textbox

As the mouse is released, a dialog showing signals of button and methods of slot will be displayed. Select clicked signal and clear() method

The Signal/Slot Editor window at bottom right will show the result −

Save ui and Build and Python code from ui file as shown in the below code −

Generated Python code will have the connection between signal and slot by the following statement −

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Run signalslot.py and enter some text in the LineEdit. The text will be cleared if the button is pressed.

Building Signal-slot Connection

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Instead of using Designer, you can directly establish signal-slot connection by following syntax −

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 technique −

Pyqt Signals And Slots Across Threads

Example

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.

The above code produces the following output −

Output