[FIXED] Why do Qt timers fire less frequently when not updating the GUI?

Issue

Below is a simple Qt for Python (PySide6) application that tries to count the number of times a slot gets called in one second:

import time
from PySide6.QtCore import *
from PySide6.QtTest import *
from PySide6.QtWidgets import *


class MainWindow(QWidget):

    def __init__(self, *args, **kwargs):
        super(MainWindow, self).__init__(*args, **kwargs)
        self.resize(400, 300)
        self.layout = QVBoxLayout()
        self.setLayout(self.layout)
        self.sheep_number = 0
        self.timer = QTimer()
        self.timer.timeout.connect(self.count_sheep)
        self.counted_sheep_label = QLabel()
        self.layout.addWidget(self.counted_sheep_label)  # comment out this line to see the bug

    @Slot()
    def count_sheep(self):
        self.sheep_number += 1
        self.counted_sheep_label.setText(f"Counted {self.sheep_number} sheep.")


if __name__ == "__main__":

    app = QApplication([])
    main_window = MainWindow()
    main_window.show()
    main_window.timer.start()
    then = time.time()
    QTest.qWait(1000)
    main_window.timer.stop()
    t = time.time() - then
    print(t, main_window.sheep_number)

The code brings up a GUI and leaves it on the screen for 1 second. It then prints the time that elapsed (according to Python rather than Qt) and number of calls to the slot. When I run this on my laptop, I get 1.0010387897491455 and 7155

However, when I comment out the indicated line, I get 1.0011870861053467 88.

I do not understand this behavior. Why would not drawing the GUI cause QTimer to fire less often?

Solution

You have to be cautious about using a QTimer with a zero interval. If you don't call a waiting function in the signal handler, it will result in a high CPU usage due to a busy loop. (It can be nearly 100%.)

In MacOS, you can see that by calling the app.exec() without calling the main_window.timer.stop().

And why does the QTest.qWait() not result in a high CPU usage on MacOS? The QTest::qWait() calls the QCoreApplication::processEvents(), which runs a loop calling the QEventDispatcherCoreFoundation::processEvents(), which calls CFRunLoopRunInMode() one more time and returns false. So, the QTest.qWait() will call the QTestPrivate::qSleep() less frequently if you update the UI in the signal handler. (Updating the UI may cause new events are enqueued into the event queue.)

I think this behavior in MacOS is a side effect. What about reporting this here?

Answered By - relent95