先定位到一组frame：ele = dr.find_elements_by_tag_name('iframe')此时获得一组frame
再通过index取需要切进去的frame并取到该frame的id：r = ele[1].get_attribute('id')
接下来就可以直接切换进frame：dr.switch_to.frame(r)
from selenium import webdriver
url = 'http://mail.xxxxxx.cn/cgi-bin/frame_html?sid=1t7ez3_A400vGDBp,2&r=306fe5cbf6d3911c1d24bc1f32025c01'
dr = webdriver.Chrome()
dr.get(url)
dr.implicitly_wait(10)
dr.find_element_by_id('qquin').clear
dr.find_element_by_id('qquin').send_keys('xxxxx')
dr.find_element_by_id('pp').clear
dr.find_element_by_id('pp').send_keys('xxxxxx')
dr.find_element_by_id('btlogin').click()
ele = dr.find_elements_by_tag_name('iframe')
r = ele[1].get_attribute('id')
print r
dr.quit()