Why does instantiating QPixmap in a second thread block/slow main?

nobru

While a secondary thread is instantiating a big (32 megapixels) image, the main is blocked/slow.
In this example code, main responsiveness is tested by logging mouseMoveEvent.
Left-click starts a second thread which instantiates QPixmap. While this is happening, only a few mouseMoveEvent are logged.
Right-click starts a cpu-bound thread. While the cpu-bound thread is running, many mouseMoveEvent are logged.
test log on pastebin.com
Why does this happen? How can I avoid it?

import time
import threading
import sys

from PyQt5 import Qt, QtCore

def instantiate_qpixmap():
    pic_data = open('32megapixels.jpg', 'rb').read()
    print('{} - START QPIXMAP (ident {})'.format(time.time(), threading.get_ident()))
    for i in range(25):
        Qt.QPixmap().loadFromData(pic_data)
    print('{} - END QPIXMAP'.format(time.time()))

def cpu_bound_work():
    print('{} - START CPU-BOUND (ident {})'.format(time.time(), threading.get_ident()))
    for i in range(100000000):
        a = i + 1
    print('{} - END CPU-BOUND'.format(time.time()))


class widget(Qt.QWidget):
    
    def mouseMoveEvent(self, event):
        print('{} - pos {}'.format(time.time(), event.screenPos()))

    def mousePressEvent(self, event):
        if event.buttons() == QtCore.Qt.LeftButton:
            print('{} - LEFT (ident {})'.format(time.time(), threading.get_ident()))
            threading.Thread(target=instantiate_qpixmap).start()
            print('{} - LEFT launched'.format(time.time()))
        else:
            print('{} - RIGHT (ident {})'.format(time.time(), threading.get_ident()))
            threading.Thread(target=cpu_bound_work).start()
            print('{} - RIGHT launched'.format(time.time()))


app = Qt.QApplication(sys.argv)

wid = widget()
wid.show()

sys.exit(app.exec_())

nobru

About "no QGui outside main", I have indeed found people saying that e.g. creating QPixmaps in a thread or It is not safe to use QPixmap outside the gui Thread
Where is the documentation about this? I don't even get the warning.
How do I speed up my software? I suppose instantiating a QPixmap does the heavy jpg -> raster transformation which I suppose is slowing down my software. Would it be possible to do the transformation in another object (QImage?) in a secondary thread?

nobru

The documentation is here Threads and QObjects
«Although QObject is reentrant, the GUI classes, notably QWidget and all its subclasses, are not reentrant. They can only be used from the main thread.»
I suppose QPaintDevice (inherite by both QPixmap and QImage) is one of the "GUI classes".
So maybe I could do the jpg->raster transformation with something non-GUI (non-Qt) and pass the data to main.