selenium为网页中选择框元素的获取特别引入了一个Select对象，
引入对象的方式：
from selenium.webdriver.support.ui import Select
查询文档可以知道 Select 所支持的方法：
class selenium.webdriver.support.select.Select(webelement)[source] Constructor. A check is made that the given element is, indeed, a SELECT tag. If it is not, then an UnexpectedTagNameException is thrown. Args : webelement - element SELECT element to wrap Example: from selenium.webdriver.support.ui import Select # 引入 Select(driver.find_element_by_tag_name(“select”)).select_by_index(2) # 获取select元素 all_selected_options[source] Returns a list of all selected options belonging to this select tag deselect_all()[source] Clear all selected entries. This is only valid when the SELECT supports multiple selections. throws NotImplementedError If the SELECT does not support multiple selections deselect_by_index(index)[source] Deselect the option at the given index. This is done by examing the “index” attribute of an element, and not merely by counting. Args : index - The option at this index will be deselected throws NoSuchElementException If there is no option with specisied index in SELECT deselect_by_value(value)[source] Deselect all options that have a value matching the argument. That is, when given “foo” this would deselect an option like: <option value=”foo”>Bar</option> Args : value - The value to match against throws NoSuchElementException If there is no option with specisied value in SELECT deselect_by_visible_text(text)[source] Deselect all options that display text matching the argument. That is, when given “Bar” this would deselect an option like: <option value=”foo”>Bar</option> Args : text - The visible text to match against first_selected_option[source] The first selected option in this select tag (or the currently selected option in a normal select) options[source] Returns a list of all options belonging to this select tag select_by_index(index)[source] Select the option at the given index. This is done by examing the “index” attribute of an element, and not merely by counting. Args : index - The option at this index will be selected throws NoSuchElementException If there is no option with specisied index in SELECT select_by_value(value)[source] Select all options that have a value matching the argument. That is, when given “foo” this would select an option like: <option value=”foo”>Bar</option> Args : value - The value to match against throws NoSuchElementException If there is no option with specisied value in SELECT select_by_visible_text(text)[source] Select all options that display text matching the argument. That is, when given “Bar” this would select an option like: <option value=”foo”>Bar</option> Args : text - The visible text to match against throws NoSuchElementException If there is no option with specisied text in SELECT