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()