Python for循环中无法定位元素的原因是什么？

import unittestimport timeimport xlrdfrom selenium import webdriverdef open_excel(file='file.xls'):    try:        data = xlrd.open_workbook(file)        return data    except Exception as e:        print(str(e))def excel_table_byindex(file='file.xls', colnameindex=0, by_index=0):    data = open_excel(file)    table = data.sheets()[by_index]    nrows = table.nrows #行数    colnames = table.row_values(colnameindex) #某一行数据    list = []    for rownum in range(1, nrows):        row = table.row_values(rownum)        if row:            app = {}            for i in range(len(colnames)):                app[colnames[i]] = row[i]            list.append(app)    return listclass login(unittest.TestCase):    @classmethod    def setUpClass(cls):        # 将浏览器驱动置于for循环外        cls.dr = webdriver.Firefox()        cls.dr.maximize_window()        cls.dr.implicitly_wait(10)    def test(self):        tabls = excel_table_byindex(file='./data/meit.xlsx')        # 遍历数据表        for i in range(0, len(tabls)):            self.dr.get("https://passport.meituan.com/account/unitivelogin?service=www&continue=http%3A%2F%2Fwww.meituan.com%2Faccount%2Fsettoken%3Fcontinue%3Dhttps%253A%252F%252Fwww.meituan.com%252F")            time.sleep(5)            # 输入登录信息            self.dr.find_element_by_id('login-email').send_keys(tabls[i]['username'])            self.dr.find_element_by_id("login-password").send_keys(tabls[i]['password'])            self.dr.find_element_by_class_name('btn').click()            time.sleep(3)    @classmethod    def tearDownClass(cls):        time.sleep(3)        cls.dr.quit()if __name__ == '__main__':    unittest.main()