python multithreading separates large files after downloading in merged instances

Without further ado, let's go to the code:

import threading
import requests
import time
import os


class Mythread(threading.Thread):
  def __init__(self,url,startpos,endpos,f):
    super(Mythread,self).__init__()
    self.url=url
    self.startpos=startpos
    self.endpos=endpos
    self.fd=f
  def download(self):
    print('start thread:%s at %s'%(self.getName(),time.time()))
    headers={'Range':'bytes=%s-%s'%(self.startpos,self.endpos)}
    res=requests.get(self.url,headers=headers)
    self.fd.seek(self.startpos)
    self.fd.write(res.content)
    print('Stop thread:%s at%s'%(self.getName(),time.time()))
    self.fd.close()
  def run(self):
    self.download()
if __name__=="__main__":
  url='http://www.wendangxiazai.com/word/b-cfbdc77931b765ce050814a9-1.doc'
  filename=url.split('/')[-1]
  filesize=int(requests.head(url).headers['Content-Length'])
  print('%s filesize:%s'%(filename,filesize))


  threadnum=3
  threading.BoundedSemaphore(threadnum)# Number of threads allowed 
  step=filesize//threadnum
  mtd_list=[]
  start=0
  end=-1
  
  tempf = open('E:\Python\py\web'+filename,'w')
  tempf.close()
  mtd_list=[]
  with open('E:\Python\py\web'+filename,'rb+')as f:
    # Get the file handle 
    fileno=f.fileno()# return 1 An integer file descriptor that can be used by the underlying operating system  I/O  operation 
    while end<filesize-1:
      start=end+1
      end=start+step-1
      if end>filesize:
        end=filesize
      print ('Start:%s,end:%s'%(start,end))
      dup=os.dup(fileno)# Copy the file handle 
      fd=os.fdopen(dup,'rb+',-1)
      t=Mythread(url,start,end,fd)
      t.start()
      mtd_list.append(t)
    for i in mtd_list:
      i.join()
  f.close()