python+判断文件是否存在+从一个文件夹中copy文件到另一个文件夹

import os
import shutil
Adir = './trainA0/'
Bdir = './trainB0/'
alldir = './all/'
if not os.path.exists(Adir):
      print ("The source path trainA0 is not exist")
if not os.path.exists(Bdir):
      print ("The source path trainB0 is not exist")
if not os.path.exists(alldir):
      os.mkdir(alldir)

num=0
for fi in os.listdir(Adir):
      im_name = '0' + fi[1:]
      shutil.copyfile(Adir+fi,alldir+str(num)+'_'+fi) #把文件夹Adir中的文件fi，复制到文件夹alldir中，并重新命名为str(num)+'_'+fi
      shutil.copyfile(Bdir+im_name,alldir+str(num)+'_'+im_name)
      num = num + 1
      print (num)