request.Request 实例重用疑问？

request.Request 实例重用时如何改变 URL 及 headers? PY3.5

下例中：翻页可以，但一旦选择书籍 id 就会抛出 HTTP Error 400: Bad Request 异常

比如：在书名输入时，输入 3 ，在结果中选择 0 （按 f 翻页不会）

将此时的网址提出来单独测试，却能正确打开！

问题出在哪里尚不知

from urllib import request,parse

import re

pat_books = re.compile() # 显示有问题见下面的回复

pat_home = re.compile() # 显示有问题见下面的回复

headers = {

        "Referer":"http://so.mianhuatang.la/",

        "User-Agent":"Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36\
        (KHTML, like Gecko) Chrome/42.0.2311.90 Safari/537.36"
       }

class Search_req: #

def __init__(self):

    self.url = r'http://so.mianhuatang.la/cse/search?q={}&p={}&s=7965856832468911224&entry=1'
    self.books = []          # 书籍(章节)列表 [(url,name),]
    self.bkname = ''
    self.req = request.Request(self.url,headers=headers)
    
def _search(self,url):
    self.req.full_url = url
    print(url)
    with request.urlopen(self.req) as htm:
        return htm.read()
    
def searchBooks(self,page): # 书目 可能抛异常
    global pat_books     
    url = self.url.format(parse.quote(self.bkname),page)            
    res = self._search(url).decode('utf-8','ignore')
    self.books = pat_books.findall(res)
    
def searchHome(self,homeUrl): # 搜索书籍主目录的章节列表
    global pat_home           # 章节列表[(url,name),]        
    res = self._search(homeUrl).decode('gbk','ignore')
    self.books = pat_home.findall(res)

def showMenu(books,page):

        print('\n\t 第 {} 页'.format(page+1))
        for i,book in enumerate(books):
            print('{}:  {}'.format(i,book[1]))

def main(): # 节选

s = Search_req()           
while True: 
    s.bkname = input('\n 网络小说搜索 [回车退出] ：').strip()  
    if not s.bkname: break
    for page in range(10):
        try:
            s.searchBooks(page)     # 获取书目
            if not s.books: break                
            showMenu(s.books,page)  # 显示书籍搜索结果
            id = input('请根据序号选择(f:翻页  q:退出):').lower()
            if id == 'q' : return
            if id == 'f': continue
            homeurl,s.bkname = s.books[int(id)] #获得所选书籍主页[url,name]
            s.searchHome(homeurl)               # 获取主页上的章节列表 
            print('test: END')
            break
        except Exception as err:
            print(str(err))
            break

if name == 'main': # 这里应为__main__

    url = r'http://www.mianhuatang.la/31/31605/'  # 提出来单独测试
    req = request.Request(url,headers=headers)
    with request.urlopen(req) as htm:
        print('ok')

    main()  # 主测试