python selenium 定位iframe中的元素，进入iframe，需要返回

from selenium import webdriver
from selenium.webdriver.common.by import By
from selenium.webdriver.support.ui import WebDriverWait
from selenium.webdriver.common.keys import Keys
from selenium.webdriver.common.action_chains import ActionChains
import time

driver=webdriver.Firefox()

driver.get("https://www.126.com/") #126邮箱登陆测试
time.sleep(4)
driver.switch_to_frame("x-URS-iframe")
e=driver.find_element_by_xpath("//input[@name='email']")
e.clear()
e.send_keys("********")
driver.find_element_by_xpath("//input[@name='password']").send_keys('*****')  
driver.find_element_by_id("dologin").click()
time.sleep(3)
#fa=driver.find_element_by_class_name('cnt')
#print(fa.id)
#driver.switch_to_default_content()  #切出
#e2=driver.find_element_by_id("normalLoginTab")
#driver.switch_to_frame("x-URS-iframe")
#driver.find_element_by_id('ismyphonebox').click()
#driver.find_element_by_class_name('btnbox').find_element_by_link_text(u'登录').click()

driver.switch_to_default_content() #进入一个iframe操作完成后，返回原来的页面需跳出iframe
driver.find_element_by_id('_mail_tabitem_1_39').click()