selenium 无法找到element

今天玩了一下python 下的selenium，感觉蛮好玩的。试试看能不能模拟新浪微博登录。
模仿了一下官网给的例子试了一下，代码如下：

from selenium import webdriver
from selenium.webdriver.common.keys import Keys

driver = webdriver.Firefox()
driver.get("")

elem=driver.find_element_by_name("username")
elem.send_keys("name")

elem2 = driver.find_element_by_name("password")
elem2.send_keys("passwd")
elem2.send_keys(Keys.RETURN)

结果发现出现如下错误：
selenium.common.exceptions.ElementNotVisibleException: Message: u'Element is not currently visible and so may not be interacted with' ; Stacktrace: 
    at fxdriver.preconditions.visible (file:///tmp/tmpoOAZdm/extensions/[e-mail]/components/command_processor.js:8791)
    at DelayedCommand.prototype.checkPreconditions_ (file:///tmp/tmpoOAZdm/extensions/[e-mail]/components/command_processor.js:11438)
    at DelayedCommand.prototype.executeInternal_/h (file:///tmp/tmpoOAZdm/extensions/[e-mail]/components/command_processor.js:11455)
    at DelayedCommand.prototype.executeInternal_ (file:///tmp/tmpoOAZdm/extensions/[e-mail]/components/command_processor.js:11460)
    at DelayedCommand.prototype.execute/< (file:///tmp/tmpoOAZdm/extensions/[e-mail]/components/command_processor.js:11402) 

原因：

elem=driver.find_element_by_name("username")

这行代码得到的elem并不是唯一的，
改成如下就好：

elem = driver.find_element_by_xpath("//div[@class='W_login_form']/div[@class='info_list']/div/input")