appium切换Native_app和webview_1

self.driver.find_element_by_name('分享样式1').click()  
        self.driver.find_element_by_name('UMS sina icon').click()  
        sleep(2)  
        print self.driver.contexts  
        context_name = 'WEBVIEW_1'  
        self.driver.switch_to.context(context_name)  
        sleep(2)  
        self.driver.find_elements_by_xpath('//input[@id="userId"]')[0].send_keys('test@126.com')  
        self.driver.find_elements_by_xpath('//input[@id="passwd"]')[0].send_keys('test')  
        self.driver.find_elements_by_xpath('//a[@class="btnP"]')[0].click()  
        sleep(2)  
        self.driver.switch_to.context('NATIVE_APP')  
        self.driver.find_element_by_name('UMS nav button send').click()