Selenium提供了8种定位方式：
> id
> name
> class name
> tag name
> link text
> partial link text
> xpath
> css selector
在Python selenium中所对应的方法为：
> find_element_by_id()
> find_element_by_name()
> find_element_by_class_name()
> find_element_by_tag_name()
> find_element_by_link_text()
> find_element_by_partial_link_text()
> find_element_by_xpath()
> find_element_by_css_selector()