爬取大众点评网的火锅商家信息，结果又错误，照不出

想要爬取大众点评网上的火锅商家的信息，却被告诉有错误，下面的是代码：
import urllib2

from bs4 import BeautifulSoup

import xlwt

url='http://www.dianping.com/chengdu/ch10/g110px'

wbk=xlwt.Workbook()

sheet=wbk.add_sheet('sheet 1',cell_overwrite_ok=True)

sheet.write(0,0,'title')

sheet.write(0,1,'comment')

sheet.write(0,2,'tag-address')

sheet.write(0,3,'recommend')

webpage=urllib2.urlopen(url)

soup=BeautifulSoup(webpage,'lxml',from_encoding='utf-8')

m=1

i=1

while i<=5:

    a=list(url)

    a[-1]=i

    b=''.join('%s' %id for id in a)#这一步相当重要

    webpage=urllib2.urlopen(b)

    html_doc=webpage.read()

    soup=BeautifulSoup(html_doc,'html.parser',from_encoding='utf-8')

    links=soup.find_all('div',class_='txt')

    for link1 in links:

        for link2 in link1.find_all('div',class_='tit'):

            sheet.write(m,1,link2.get_text())

        for link3 in link1.find_all('div',class_='comment'):

            sheet.write(m,2,link3.get_text())

        for link4 in link1.find_all('div',class_='tag-address'):

            sheet.write(m,3,link4.get_text())

        for link5 in link1.find_all('div',class_='recommend'):

            sheet.write(m,4,link5.get_text())

        m=m+1

    i=i+1

wbk.save(r'C:\Users\123\Desktop/hotpot.xls')

Spyder上的错误信息是：

runfile('C:/Users/123/Desktop/爬虫练习/大众点评网.py', wdir='C:/Users/123/Desktop/爬虫练习')
Traceback (most recent call last):

  File "<ipython-input-8-456082ca6ef2>", line 1, in <module>
    runfile('C:/Users/123/Desktop/爬虫练习/大众点评网.py', wdir='C:/Users/123/Desktop/爬虫练习')

  File "C:\Users\123\Anaconda\lib\site-packages\spyderlib\widgets\externalshell\sitecustomize.py", line 585, in runfile
    execfile(filename, namespace)

  File "C:/Users/123/Desktop/爬虫练习/大众点评网.py", line 19, in <module>
    webpage=urllib2.urlopen(url)

  File "C:\Users\123\Anaconda\lib\urllib2.py", line 127, in urlopen
    return _opener.open(url, data, timeout)

  File "C:\Users\123\Anaconda\lib\urllib2.py", line 410, in open
    response = meth(req, response)

  File "C:\Users\123\Anaconda\lib\urllib2.py", line 523, in http_response
    'http', request, response, code, msg, hdrs)

  File "C:\Users\123\Anaconda\lib\urllib2.py", line 448, in error
    return self._call_chain(*args)

  File "C:\Users\123\Anaconda\lib\urllib2.py", line 382, in _call_chain
    result = func(*args)

  File "C:\Users\123\Anaconda\lib\urllib2.py", line 531, in http_error_default
    raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)

HTTPError: Forbidden

请大家看看哪有问题

url打不开