RobotFramework框架执行UI自动化用例，都调用一个公共测试库，怎么只打开一个浏览器

WebOpLib.py测试库

from selenium import webdriver
from  cfg  import *


class WebOpLib():

    def openwebsite(self):
        self.driver=webdriver.Chrome()
        self.driver.maximize_window()
        self.driver.implicitly_wait(10)
    def TeacherLogin(self,uname,pwd):
        self.driver.get(g_teacher_login_url)
        self.driver.find_element_by_id('username').send_keys(uname)
        self.driver.find_element_by_id('password').send_keys(pwd)
        self.driver.find_element_by_id('submit').click()
        time.sleep(1)
    def closewebsite(self):
        self.driver.quit()

遇到的问题：

  对登陆这个模块的用例进行自动化，每执行一个用例，打开一个谷歌，显然是可以优化的！

from selenium import webdriver
from  cfg  import *
class WebOpLib():
    ROBOT_LIBRARY_SCOPE = 'GLOBAL'   #调用这个库的用例都打开同一个浏览器，设置全局的参数

    def openwebsite(self):
        self.driver=webdriver.Chrome()
        self.driver.maximize_window()
        self.driver.implicitly_wait(10)
    def TeacherLogin(self,uname,pwd):
        self.driver.get(g_teacher_login_url)
        self.driver.find_element_by_id('username').send_keys(uname)
        self.driver.find_element_by_id('password').send_keys(pwd)
        self.driver.find_element_by_id('submit').click()
        time.sleep(1)
    def closewebsite(self):
        self.driver.quit()