multithread.py

from PyQt5.QtGui import *
from PyQt5.QtWidgets import *
from PyQt5.QtCore import *

import time

class MainWindow(QMainWindow):

    def __init__(self, *args, **kwargs):
        super(MainWindow, self).__init__(*args, **kwargs)

        self.counter = 0

        layout = QVBoxLayout()

        self.l = QLabel("Start")
        b = QPushButton("DANGER!")
        b.pressed.connect(self.oh_no)

        c = QPushButton("?")
        c.pressed.connect(self.change_message)

        layout.addWidget(self.l)
        layout.addWidget(b)
        layout.addWidget(c)

        w = QWidget()
        w.setLayout(layout)

        self.setCentralWidget(w)

        self.show()

    def change_message(self):
        self.message = "OH NO"

    def oh_no(self):
        self.message = "Pressed"

        for n in range(100):
            time.sleep(0.1)
            self.l.setText(self.message)
            QApplication.processEvents()


app = QApplication([])
window = MainWindow()
app.exec_()