import struct
from PyQt5.QtWidgets import (QMessageBox, QLineEdit)
from PyQt5.QtGui import QPixmap
{...}

class LoginWindow(QtWidgets.QWidget):
{...}
  def __init__(self):
    super(LoginWindow, self).__init__()
    # 加载ui文件
    loadUi('login.ui', self)
    # 设置背景图片
    background = QPixmap("img/login_background.jpeg")
    background = background.scaled(self.background_label.width(),self.background_label.width())
    self.background_label.setPixmap(background)
    # 设置输入密码框
    self.passwd_lineEdit.setEchoMode(QLineEdit.Password)
{.......}

def mainpage():
  app = QtWidgets.QApplication(sys.argv)
  login_window = LoginWindow()
  login_window.show()
  sys.exit(app.exec_())