Selenium元素包含在Ifram以下

当用selenium定位方法定位的元素在Ifram以下时，需要先切换到目标Ifram，然后在执行元素定位。比如QQ问卷登陆窗口.

       driver.switch_to.frame("QQLoginIframe")
        driver.find_element_by_id("switcher_plogin").click()
        time.sleep(5)
        driver.find_element_by_id("u").clear()
        driver.find_element_by_id("u").send_keys("xxxx")
        driver.find_element_by_id("p").clear()
        driver.find_element_by_id("p").send_keys("xxxx")
        time.sleep(5)
        driver.find_element_by_id("login_button").click()
        time.sleep(5)
        driver.find_element_by_id("login_button").click()