main.py

import sys, random

from PyQt6 import uic
from PyQt6.QtCore import QSize, QRectF
from PyQt6.QtGui import QPainter, QPixmap, QColor
from PyQt6.QtWidgets import QApplication, QMainWindow, QWidget, QPushButton, QLabel


class Main_widget(QMainWindow):
    def __init__(self):
        super().__init__()

        uic.loadUi('Circle.ui', self)

        self.do_paint = False
        self.buttonCircle.clicked.connect(self.paint)
        self.sizes = QSize(self.buttonCircle.size().width(), self.buttonCircle.size().height() - 70)

    def paintEvent(self, event):
        if self.do_paint:
            qp = QPainter()
            qp.begin(self)
            self.draw_flag(qp)
            qp.end()
        self.do_paint = False

    def paint(self):
        self.do_paint = True
        self.update()

    def draw_flag(self, qp):
        r = random.randint(0, 100)

        qp.setBrush(QColor(255, 255, 0))
        qp.drawEllipse(150, 100, r, r)


if __name__ == '__main__':
    app = QApplication(sys.argv)
    ex = Main_widget()
    ex.show()
    sys.exit(app.exec())