最近在学习unittest单元测试框架，之前调试好的测试脚本，发现放在这个框架中，无法正常switch_to.window，总是报错无法定位到元素，个人感觉是因为窗口梅正常切换过去，新人求指教，谢谢，代码如下：

main.py

import time
import unittest
import auto_test
suite = unittest.TestSuite()
suite.addTest(auto_test.Python_autotest('get_browser'))
suite.addTest(auto_test.Python_autotest('search'))
suite.addTest(auto_test.Python_autotest('user_login'))
suite.addTest(auto_test.Python_autotest('user_logout'))
if __name__ == '__main__':
    runner = unittest.TextTestRunner()
    runner.run(suite)

auto_test.py

from selenium import webdriver
import time
import unittest
class Python_autotest(unittest.TestCase):
    def setUp(self,driver = webdriver.Firefox()):
        print('Test is start')
        self.driver = driver
    def get_browser(self):
        self.driver.get("https://www.baidu.com/")
    def search(self):
        try:
            self.driver.find_element_by_xpath('//*[@id="kw"]').send_keys('segmentfault')
            self.driver.find_element_by_xpath('//*[@id="su"]').click()
        except Exception:
            print('No found everything, sorry!')
        else:
            time.sleep(1)
            self.driver.find_element_by_xpath("/html/body/div[1]/div[5]/div[1]/div[3]/div[1]/h3/a").click()
    def user_login(self):
        self.driver.switch_to.window(self.driver.window_handles[-1])
        time.sleep(3)
        self.driver.find_element_by_xpath('/html/body/div[4]/div/div/div[1]/div[2]/a[1]').click()#点击页面中的前端菜单栏
    def user_logout(self):
        self.driver.quit()
    def tearDown(self):
        print('Test is finish')
if __name__ == '__main__':
    unittest.main()