Python for循环中无法定位元素的原因是什么？

import unittest import time import xlrd from selenium import webdriver  def 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 list  class 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()