PyQt5 QTimer 10ms is not accurate with QEventLoop and qasync in GUI APP

Yves Jime

Hi,

Pyqt5+python3.9

I have a weird behaviour with QTimer in PyQT5 when I try to measure frames and frequency. If you set a timer with 10ms = 1 / 100 every signal message gives the time every time. It works with the following snippet but then with another app with QEventLoop/qasync has a big latency.

First of all I have this sample code snippet:

from PyQt5 import QtWidgets
from PyQt5.QtWidgets import QApplication, QMainWindow
from PyQt5 import QtCore
from PyQt5.QtCore import QElapsedTimer
import sys
count = 0

def display():
    global t1, count
    with open("output.txt", "a") as f:
        f.writelines(str(t1.elapsed() / 1000.) + " " + str(count) + "\n")
        count = count + 1

app = QApplication([])
win = QMainWindow()
win.setGeometry(400,400,300,300)
win.setWindowTitle("CodersLegacy")


t1 = QElapsedTimer()
timer = QtCore.QTimer()
timer.timeout.connect(display)
t1.start()
timer.start(10)
count = 0

sys.exit(app.exec_())

It's pretty simple, write every time elapsed time and frame count. A sample output (tail of file):

20.799 2079
20.809 2080
20.82 2081
20.83 2082

You can divide 2082 / 20.83 = 100 (aprox.) then is the real frequency 10ms = 1/100 Good! But...

... I have a GUI PyQt5 app with EventLoop and Qasync. I put the main of the app:

def main():

    global splash

    QApplication.setAttribute(Qt.AA_UseDesktopOpenGL)
    QApplication.processEvents()

    app = QApplication([])
    w = MainWindow()
 
    w.resize(1600, 900)

  
    

    w.show()

    try:
        loop = qasync.QEventLoop(app)
        asyncio.set_event_loop(loop)
    except asyncio.InvalidStateError as e:
        print(e + " Error en Qasync")

    with loop:
        loop.run_forever()

if __name__ == "__main__":
    main()

And QMainWindow class (a snippet with important code), I have the timer signal:

class MainWindow(QMainWindow):
   ...
   self.timer = QTimer()
   self.timer.timeout.connect(self.timerFile)
   self.timer.start(10)
   
   def timerFile(self):
       global timer_count
       with open(current_file, "a") as f:

                f.writelines("TIME: " + str(t1.elapsed() / 1000.) + " " + str(timer_count) + "\n")
            timer_count = timer_count + 1

And the output file:

TIME: 4.357 228

So, total frequency is 228 / 4.357 = 52.32 = 1 / 52.32

It's double time. And I have tried timestamps, PreciseTimer, but always I have the same behaviour.

I don't know what happens with timer signals in PyQt5 with EventLoop or/and qasync. Someone could send me some tips to resolve this or investigate the solution.

I appreciate your help Kind Regards

SGaist

Hi and welcome to devnet,

What happens if you run your application without qasync ?
Shouldn't you use loop.exec() ?