pyqt5画线

Question

下面这段代码总是有问题，麻烦帮我改改
画的线没有保存下来
'''
简单的画板1.0
功能：在定点和移动中的鼠标所在处画一条线
作者：PyLearn
最后修改日期: 2017/10/18
'''
import sys
from PyQt5.QtWidgets import (QApplication, QWidget)
from PyQt5.QtGui import (QPainter, QPen)
from PyQt5.QtCore import Qt

class Example(QWidget):

def __init__(self):
    super(Example, self).__init__()
    
    #resize设置宽高，move设置位置
    self.resize(400, 300)
    self.move(100, 100)
    self.setWindowTitle("简单的画板1.0")
    
    #setMouseTracking设置为False，否则不按下鼠标时也会跟踪鼠标事件
    self.setMouseTracking(False)
    
    #设置两个变量接收移动中的点的x、y坐标
    self.pos_x = 20
    self.pos_y = 20
    self.pos_x0 = 20
    self.pos_y0 = 20
    self.flag=0

def paintEvent(self, event):
    if self.flag==1:
        painter = QPainter()
        painter.begin(self)
        pen = QPen(Qt.black, 2, Qt.SolidLine)
        painter.setPen(pen)
        
        #定点(20, 20) 到 (self.pos_x, self.pos_y)之间画线
        painter.drawLine(self.pos_x0, self.pos_y0, self.pos_x, self.pos_y)
        
        painter.end()

def mouseMoveEvent(self, event):
    '''
     按住鼠标移动事件：更新pos_x和pos_y的值
     调用update()函数在这里相当于调用paintEvent()函数
     每次update()时，之前调用的paintEvent()留下的痕迹都会清空
    '''
    self.pos_x = event.pos().x()
    self.pos_y = event.pos().y()
   
    self.update()

def mousePressEvent(self, event):
    self.flag=(self.flag==0)
    if self.flag==1:
         self.pos_x0 = event.pos().x()
         self.pos_y0 = event.pos().y()
    print(self.flag)

if name == "__main__":

app = QApplication(sys.argv)
pyqt_learn = Example()
pyqt_learn.show()
app.exec_()