如何在PyQt中使用QPainter在已存在的UI窗口上绘图？

<code class="python">from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.QtGui import QPainter, QPen, QColor


class Ui_Form(object):
    def setupUi(self, Form):
        Form.setObjectName("Form")
        Form.resize(400, 300)
        # ... (rest of your UI setup code) ...

    def retranslateUi(self, Form):
        _translate = QtCore.QCoreApplication.translate
        Form.setWindowTitle(_translate("Form", "Form"))

    def paintEvent(self, event):
        qp = QPainter(self)
        self.drawMyShape(qp)

    def drawMyShape(self, qp):
        pen = QPen(QColor(0, 0, 255), 3, QtCore.Qt.SolidLine) # 蓝色，粗细3像素，实线
        qp.setPen(pen)
        qp.drawLine(50, 50, 350, 250) # 绘制一条线

        pen.setColor(QColor(255, 0, 0)) # 红色
        pen.setWidth(5) # 粗细5像素
        qp.setPen(pen)
        qp.drawRect(100, 100, 100, 100) # 绘制一个矩形

        pen.setColor(QColor(0, 255, 0)) # 绿色
        pen.setWidth(2) # 粗细2像素
        pen.setStyle(QtCore.Qt.DashLine) # 虚线
        qp.setPen(pen)
        qp.drawEllipse(250, 50, 50, 50) # 绘制一个椭圆


if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    Form = QtWidgets.QWidget()
    ui = Ui_Form()
    ui.setupUi(Form)
    Form.show()
    sys.exit(app.exec_())</code>