appium运行python脚本时出现 cannot import name webdriver

这是我的代码
from appium import webdriver

desired_caps = {}

desired_caps['platformName'] = 'Android'

desired_caps['platformVersion'] = '5.0.2'

#desired_caps['app'] = os.path.abspath('/Users/a58/Downloads/zhuanzhuan_market_923.apk')

desired_caps['appPackage'] = 'com.sohu.inputmethod.sogou'
desired_caps['appActivity'] = '.com.sohu.inputmethod.sogou.SogouIMELauncher'

wd = webdriver.Remote('http://127.0.0.1:4723/wd/hub', desired_caps)