如何在pyqt5中更改圆的大小

import sys
from PyQt5.QtWidgets import QApplication, QWidget
from PyQt5.QtGui import QPainter, QColor, QPen
from PyQt5.QtCore import Qt

class MyWidget(QWidget):
    def __init__(self):
        super().__init__()
        self.setGeometry(100, 100, 400, 400)
        self.setWindowTitle('Change Circle Size')

    def paintEvent(self, event):
        painter = QPainter(self)
        painter.setRenderHint(QPainter.Antialiasing)
        pen = QPen(Qt.black, 2, Qt.SolidLine)
        painter.setPen(pen)
        brush = QColor(255, 0, 0)
        painter.setBrush(brush)
        radius = 50  # 初始圆的半径
        painter.drawEllipse(self.width() / 2 - radius, self.height() / 2 - radius, radius * 2, radius * 2)

    def mousePressEvent(self, event):
        radius = 100  # 新的圆的半径
        self.update()

if __name__ == '__main__':
    app = QApplication(sys.argv)
    widget = MyWidget()
    widget.show()
    sys.exit(app.exec_())