无法使用Selenium Web驱动程序（Python）拖放元素

Question

尝试使用selenium webdriver执行拖放操作时，会抛出以下回溯。

File
"/Library/Python/2.7/site-packages/selenium-2.15.0-py2.7.egg/selenium/webdriver/common/action_chains.py",
line 44, in perform
    action()   File "/Library/Python/2.7/site-packages/selenium-2.15.0-py2.7.egg/selenium/webdriver/common/action_chains.py",
line 201, in <lambda>
    self._driver.execute(Command.MOVE_TO, {'element': to_element.id})) AttributeError: 'str' object has no attribute 'id'`

代码段：

from selenium import webdriver
from selenium.webdriver import ActionChains
driver = webdriver.Firefox()
ac = ActionChains()
source = driver.get_element_by_xpath(<xpath>)
target = driver.get_element_by_xpath(<xpath>)
ac.drag_and_drop(source, target).perfrom()

即使click_and_hold也会抛出相同的错误。感谢帮助。