python code to change extension of all files in a folder

import os

for root,dirs,files in os.walk('./'):
   for file in files:
       if file.endswith('.oldext'):
          F=open(os.path.join(root,file),'r')
          W=open('./foldername/'+file.replace('.oldext','.newext'),'w')
          for line in F.readlines():
             W.write(line)
           W.close()
           F.close()
print 'All Extensions changed Successfully'

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change the file extensions as your need 

replace oldext with the extension your files are currently having

replace newext with the new extension you want to have for all of ur files

save the above code in a file with .py extension

create a folder named foldername in same directory where the python file is saved 

keep all the files whose extension you want to change in the same folder as code   file