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last modified July 16, 2020
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In this part of the PyQt5 programming tutorial, we explore events and signalsoccurring in applications.
In the above example, our slot shows a message box. The term slot is important when using Qt from C, because slots must be declared in a special way in C. In Python however, any function can be a slot – we saw this above. For this reason, the distinction between slots. Can you give an example of when it would be necessary? Already addressed in your previous question, but I'll re-iterate again. Although PyQt4 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. Python, pylint, pyreverse, code analysis, checker, logilab, pep8. Read the doc Install it Contribute Get support. Coding Standard. Checking line-code's length, checking if variable names are well-formed according to your coding standard; checking if imported modules are used.
As mentioned previously, when using threads execution of Python is limited to a single thread at one time. The Python code that handles signals from your threads can be blocked by your workers and vice versa. Since blocking your slot functions blocks the event loop, this. The datatype may be any Python type name or a string identifying a C datatype. Since this tutorial presupposes no C knowledge, we'll stick to Python types. A PySide/PyQt Signal-Sending Circle. Let's define a Circle with properties x, y, and r, denoting the x and y position of the center of the circle, and its radius, respectively.
Events in PyQt5
GUI applications are event-driven. Events are generated mainly by theuser of an application. But they can be generated by other means as well; e.g. anInternet connection, a window manager, or a timer.When we call the application's exec_() method, the application entersthe main loop. The main loop fetches events and sends them to the objects.
In the event model, there are three participants:
- event source
- event object
- event target
The event source is the object whose state changes. It generates events.The event object (event) encapsulates the state changes in the event source.The event target is the object that wants to be notified. Event source objectdelegates the task of handling an event to the event target.
PyQt5 has a unique signal and slot mechanism to deal with events.Signals and slots are used for communication between objects. A signalis emitted when a particular event occurs. A slot can be any Python callable.A slot is called when its connected signal is emitted.
PyQt5 signals and slots
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This is a simple example demonstrating signals and slots in PyQt5.
In our example, we display a QtGui.QLCDNumberand a QtGui.QSlider. We change the lcdnumber by dragging the slider knob.
Here we connect a valueChanged signal of the slider to thedisplay slot of the lcd number.
The sender is an object that sends a signal. The receiveris the object that receives the signal. The slot is the method thatreacts to the signal.
PyQt5 reimplementing event handler
Events in PyQt5 are processed often by reimplementing event handlers.
In our example, we reimplement the keyPressEvent() event handler.
If we click the Escape button, the application terminates.
Event object in PyQt5
Event object is a Python object that contains a number of attributesdescribing the event. Event object is specific to the generated eventtype.
In this example, we display the x and ycoordinates of a mouse pointer in a label widget.
The x and y coordinates are displayd in a QLabelwidget.
Mouse tracking is disabled by default, so the widget only receives mouse moveevents when at least one mouse button is pressed while the mouse is being moved.If mouse tracking is enabled, the widget receives mouse move events evenif no buttons are pressed.
The e is the event object; it contains data about the eventthat was triggered; in our case, a mouse move event. With the x()and y() methods we determine the x and y coordinates ofthe mouse pointer. We build the string and set it to the label widget.
PyQt5 event sender
Sometimes it is convenient to know which widget is the sender of a signal.For this, PyQt5 has the sender method.
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We have two buttons in our example. In the buttonClicked methodwe determine which button we have clicked by calling thesender() method.
Both buttons are connected to the same slot.
We determine the signal source by calling the sender() method.In the statusbar of the application, we show the labelof the button being pressed.
PyQt5 emitting signals
Objects created from a QObject can emit signals.The following example shows how we to emit custom signals.
We create a new signal called closeApp. This signal isemitted during a mouse press event. The signal is connected to theclose() slot of the QMainWindow.
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A signal is created with the pyqtSignal() as a class attributeof the external Communicate class.
The custom closeApp signal is connected to the close()slot of the QMainWindow.
When we click on the window with a mouse pointer, the closeApp signalis emitted. The application terminates.
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In this part of the PyQt5 tutorial, we have covered signals and slots.