old_contents: #!/usr/bin/env python
  # -*- coding: utf-8 -*-
  
  # Copyright (c) 2012 Jérémie DECOCK (http://www.jdhp.org)
  
  # Permission is hereby granted, free of charge, to any person obtaining a copy
  # of this software and associated documentation files (the "Software"), to deal
  # in the Software without restriction, including without limitation the rights
  # to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  # copies of the Software, and to permit persons to whom the Software is
  # furnished to do so, subject to the following conditions:
   
  # The above copyright notice and this permission notice shall be included in
  # all copies or substantial portions of the Software.
   
  # THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  # IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  # FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  # AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  # LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  # THE SOFTWARE.
  
  import argparse
  import hashlib
  
  CHUNK_SIZE = 2**12
  
  def main():
      """Main function"""
  
      # PARSE OPTIONS ###########################################################
  
      parser = argparse.ArgumentParser(description='Compute the md5sum of a file with "hashlib".')
      parser.add_argument("filenames", nargs='+', type=file, metavar="FILE", help="file to read")
      args = parser.parse_args()
  
      # COMPUTE HASHS ###########################################################
  
      for file_descriptor in args.filenames:
          hash = hashlib.md5()
  
          try:
              data = file_descriptor.read(CHUNK_SIZE)
              while data:
                  hash.update(data)
                  data = file_descriptor.read(CHUNK_SIZE)
          finally:
              file_descriptor.close()
  
          print hash.hexdigest()        # str
  
  if __name__ == '__main__': main()
  
  
new_contents: #!/usr/bin/env python3
  # -*- coding: utf-8 -*-
  
  # Copyright (c) 2012 Jérémie DECOCK (http://www.jdhp.org)
  
  # Permission is hereby granted, free of charge, to any person obtaining a copy
  # of this software and associated documentation files (the "Software"), to deal
  # in the Software without restriction, including without limitation the rights
  # to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  # copies of the Software, and to permit persons to whom the Software is
  # furnished to do so, subject to the following conditions:
   
  # The above copyright notice and this permission notice shall be included in
  # all copies or substantial portions of the Software.
   
  # THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  # IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  # FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  # AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  # LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  # THE SOFTWARE.
  
  import argparse
  import hashlib
  import os
  
  CHUNK_SIZE = 2**12
  
  def main():
      """Main function"""
  
      # PARSE OPTIONS ###########################################################
  
      parser = argparse.ArgumentParser(description='Print or check MD5 checksums.')
      parser.add_argument("filepaths", nargs='+', metavar="FILE", help="file to read")
      args = parser.parse_args()
  
      # COMPUTE HASHS ###########################################################
  
      for file_path in args.filepaths:
          if os.path.isfile(file_path):
              with open(file_path, 'rb') as fd:
                  try:
                      hash_generator = hashlib.md5()
                      data = fd.read(CHUNK_SIZE)
                      while len(data) > 0:
                          hash_generator.update(data)
                          data = fd.read(CHUNK_SIZE)
                  except:
                      print("{}: unknown error".format(file_path))  # TODO
                  finally:
                      fd.close()
  
                  hash_str = hash_generator.hexdigest()
                  print("{}  {}".format(hash_str, file_path))
          else:
              if os.path.isdir(file_path):
                  print('"{}" is a directory'.format(file_path))
              else:
                  print("unable to read {}".format(file_path))
  
  if __name__ == '__main__':
      main()
  
  
subject: Convert a snippet to Python3 + improve it (a lot).