问题：
python+seleniu做自动化用 driver.find_element_by_link_text('退出').click()做qq邮箱退出操作程序异常，求大神解答一下哪里有问题!
异常情况如下：
File "D:/PycharmProjects/untitled/mainTest.py", line 13, in
login().user_logout(driver)
File "D:\PycharmProjects\untitled\LoginPublic.py", line 24, in user_logout
driver.find_element_by_link_text('退出').click()
File "C:\Python27\lib\site-packages\selenium\webdriver\remote\webdriver.py", line 282, in find_element_by_link_text
return self.find_element(by=By.LINK_TEXT, value=link_text)
File "C:\Python27\lib\site-packages\selenium\webdriver\remote\webdriver.py", line 712, in find_element
{'using': by, 'value': value})['value']
File "C:\Python27\lib\site-packages\selenium\webdriver\remote\webdriver.py", line 201, in execute
self.error_handler.check_response(response)
File "C:\Python27\lib\site-packages\selenium\webdriver\remote\errorhandler.py", line 194, in check_response
raise exception_class(message, screen, stacktrace)
selenium.common.exceptions.NoSuchElementException
Process finished with exit code 1
