为什么我想爬取所以得页面 最后只打印出来最后一页得

import requests
from bs4 import BeautifulSoup
import pandas as pd


def method_name():
    # url = 'http://www.tianqihoubao.com/aqi/xingtai-202102.html'
    years = ['2013','2014', '2015', '2026','2017','2018', '2019', '2020']
    months = ['1', '2', '3', '4', '5', '6', '7', '8', '9', '10', '11', '12']
    # for i in years:
    #     for j in months:
    #         a = 'http://www.tianqihoubao.com/aqi/xingtai-'+str(i)+str(j)+'.html'
    #         print(a)
    #         pass
    #     pass
    # for i in years:
    #     for j in months:
    #         a = 'http://www.tianqihoubao.com/aqi/xingtai-{}{}.html' .format(i,j)
    #         print(a)
    #         pass
    #     pass


    for i in months:
        a = 'http://www.tianqihoubao.com/aqi/xingtai-2019{}.html' .format(i)
        print(a)

    global tr_list
    resp = requests.get(a)
    html = resp.content.decode('gbk')
    # 数据提取
    soup = BeautifulSoup(html, 'html.parser')
    tr_list = soup.find_all('tr')
    print(tr_list)


#获取网页源代码
method_name()

dates,conditions,temp,paiming= [],[],[],[]
pm10,So2,No2,Co,O3 = [],[],[],[],[]
for data in tr_list[1:]:
    sub_data = data.text.split()
    dates.append(sub_data[0])
    conditions.append(''.join(sub_data[1]))
    temp.append(''.join(sub_data[2]))
    paiming.append(''.join(sub_data[3]))
    pm10.append(''.join(sub_data[4]))
    So2.append(''.join(sub_data[5]))
    No2.append(''.join(sub_data[6]))
    Co.append(''.join(sub_data[7]))
    O3.append(''.join(sub_data[8]))
_data = pd.DataFrame()
_data['日期'] = dates
_data['质量等级'] = conditions
_data['AQI指数'] = temp
_data['当天AQI排名'] = paiming
_data['PM10'] = pm10
_data['So2'] = So2
_data['No2'] = No2
_data['Co'] = Co
_data['O3'] = O3
print(_data)