driver = webdriver.Firefox()
driver.get("https://www.baidu.com/")
driver.find_element_by_xpath().click()

<select id="nr" name="NR">
<option value="10" selected="">每頁顯示10條</option>
<option value="20">每頁顯示20條</option>
<option value="50">每頁顯示50條</option>
</select>

# coding:utf-8
from selenium import webdriver
from selenium.webdriver.common.action_chains import ActionChains
from selenium.webdriver.support.select import Select
import time
 
driver = webdriver.Chrome()
driver.get("https://www.baidu.com/")
driver.implicitly_wait(20)
 
mouse = driver.find_element_by_link_text("設置")
ActionChains(driver).move_to_element(mouse).perform()
driver.find_element_by_link_text("搜索設置").click()
time.sleep(2)
# 實例化select
s = Select(driver.find_element_by_id("nr"))
# 定位選項
s.select_by_value("20") # 選擇value="20"的項：通過value屬性
time.sleep(2) #為了明顯的看出變化
s.select_by_index(0) # 選擇第一項選項：通過選項的順序選擇，第一個為 0
time.sleep(2) #為了明顯的看出變化
s.select_by_visible_text("每頁顯示50條") # 選擇text="每頁顯示50條"的值，即在下拉時我們可以看到的文本

deselect_by_index(index) 
deselect_by_value(value) 
deselect_by_visible_text(text) 
deselect_all()