从http://www.pm25.com/shenzhen.html抓取北京，深圳，上海，广州，成都的pm2.5指数，并按照空气质量从优到差排序，保存在txt文档里

代码如下：

#coding=utf-8
from selenium import webdriver
from time import sleep

class PM:

    def __init__(self):
        self.dr = webdriver.Chrome()
        self.pm25_info = self.get_pm25_info()

    def get_pm25_info(self):
        city_list = ['beijing','shenzhen','shanghai','guangzhou','chengdu']
        pm_info_list = []
        i = 0
        while i < len(city_list):
            self.dr.get('http://www.pm25.com/'+city_list[i]+'.html')
            sleep(3)
            city_name = self.dr.find_element_by_css_selector('.bi_loaction_city').text #定位城市名字
            pm_exp = self.dr.find_element_by_css_selector('.pm25_span').text #定位PM2.5指数
            air_num = self.dr.find_element_by_css_selector('.bi_aqiarea_num').text #定位AQI指数
            air_quality = self.dr.find_element_by_css_selector('.bi_aqiarea_right span').text #定位空气质量描述
            pm_info_list.append((city_name, pm_exp, air_num, air_quality))
            i += 1
        pm_info_list.sort(key=lambda x:float(x[2])) #按AQI指数排序(AQI指数决定了空气质量的优良)
        return pm_info_list

    def get_pm_info_file(self):
        self.file_title = '北上广深成五地PM2.5数据信息'
        self.file = open(self.file_title + '.txt', 'wb')
        self.floor = 1
        for item in self.pm25_info:
            separate_line = '**********' + 'TOP' + str(self.floor) + '**********\n'
            self.file.write(separate_line.encode('utf-8'))
            self.file.write(('城市：' + item[0] + '\n'
                             'PM2.5指数：' + item[1] + ' '+ '微克/立方米' + '\n'
                             'AQI指数：' + item[2] + '\n'
                             '空气质量：' + item[3]+ '\n').encode('utf-8'))
            self.floor += 1
        self.file.close()

    def quit(self):
        self.dr.quit()

if __name__ == '__main__':
    pm_info = PM()
    pm_info.get_pm_info_file()
    pm_info.quit()

网页如下：

生成txt如下：