python 打开页面页面的链接，为什么总是报错呀？

打开页面页面的链接，为什么总是报错呀？代码如下

driver = webdriver.Firefox()
driver.get("http://www.baidu.com")  # 加载页面
links = driver.find_element_by_xpath("//a")
print (links)

length = len(driver.find_elements_by_tag_name("a"))
print (length)

for i in range(0, length):
    links = driver.find_elements_by_tag_name("a")
    link = links[i]
    print("1",link)
    if not ("_blank" in link.get_attribute("target") or "http" in link.get_attribute("href") or "javascript" in link.get_attribute("href")):
        link.click()
        sleep(2)
        print(link)
        driver.back()
    else:
        print("1")