main.py

from PyQt5.QtWidgets import QApplication, QMainWindow, QGraphicsScene, QDesktopWidget
from PyQt5.QtCore import QRectF, Qt
from ui import Ui_MainWindow
import sys

class Window(Ui_MainWindow, QMainWindow):
    def __init__(self):
        super().__init__()
        self.setupUi(self)

        self.wheight = self.height()
        self.wwidth = self.width()

        self.scene = QGraphicsScene()
        self.scene.setSceneRect(QRectF(0, 0, self.wwidth, self.wheight))
        self.gv.setScene(self.scene)

        screen = QDesktopWidget().screenGeometry()
        ww = int(screen.width() / 2)
        wh = int(screen.height() / 2)

        self.resize(ww, wh)
        self.show()

        self.gv.fitInView(0, 0, self.wwidth, self.wheight)

    def resizeEvent(self, e):
        self.gv.fitInView(0, 0, self.wwidth, self.wheight)

def main():
    app = QApplication(sys.argv)
    w = Window()
    w.show()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()